Sloshing Motion in a Real-Scale Water Storage Tank under Nonlinear Ground Motion

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Sloshing Motion in a Real-Scale Water Storage Tank under Nonlinear Ground Motion
water
Article
Sloshing Motion in a Real-Scale Water Storage Tank
under Nonlinear Ground Motion
Heng Jin 1,2 , Ruiyin Song 1, *           and Yi Liu 1,3
 1   Ningbo Institute of Technology, Zhejiang University, Ningbo 315000, China; jinheng@nit.zju.edu.cn (H.J.);
     liuyilulu@nit.zju.edu.cn (Y.L.)
 2   Ningbo Institute of Dalian University of Technology, Ningbo 315016, China
 3   Ningbo Shenglong Group Company Limited, Ningbo 315105, China
 *   Correspondence: sry@nit.zju.edu.cn
                                                                                                       
 Received: 9 June 2020; Accepted: 18 July 2020; Published: 24 July 2020                                

 Abstract: Water storage tanks in cities are usually large and are occasionally affected by earthquakes.
 A sudden earthquake can cause pressure pulses that damage water containers severely. In this study,
 the sloshing motion in a high filling level tank caused by seismic excitation is investigated by
 the numerical method in a 2D model. Two well-studied strong earthquakes are used to analyze
 the broadband frequency nonlinear displacement of the tank both in the longitudinal and vertical
 directions. Based on careful experimental verification, the free surface motion and the elevations at
 the side wall are captured, and the sloshing pressure response is examined. The results show that the
 2D section of the cylindrical tank can be used to estimate the maximum response of the 3D sloshing,
 and the water motions under the seismic excitations are consistent with the modal characteristics of
 the sloshing. The time histories response of the water motion reflected that the sloshing response is
 hysteretic compared with the seismic excitation. The anti-seismic ability of the damping baffle shows
 that its effect on sloshing pressure suppression is limited, and further study on the seismic design of
 water tanks in earthquake-prone regions is needed.

 Keywords: sloshing; ground motion; water storage tank; damping baffle; response analysis

1. Introduction
       Water is the source of life. It is an essential element in human production and life. Almost all
fields including agricultural irrigation, hydropower, climate, etc., are inextricably linked to water;
therefore, the management of the water resources is necessary. The utilization, environmental health,
and storage safety of the water resources all are part of the management of water. The water tanks,
as the container for water storage, are usually large and have higher safety standards. Therefore, it is
very important to ensure the safety of the water tank under external environmental loads such as an
earthquakes or wind.
       In high seismic regions, the seismic design of water tanks has attracted more attention from
structural engineers as the urban water storage demand increases [1,2]. The design of a liquid container
is of particular concern when the sloshing frequency is close to the seismic excitation frequency
(i.e., resonance), because pressure pulses from the dynamic load may seriously damage the system [3,4].
Sloshing has been widely studied as a typical fluid motion phenomenon [5]. The free surface motion
and the resonance frequency location are the key factors determining the hydrodynamic load in the
container, and the movement of the water body in the tank is closely related to external excitations.
Therefore, a more careful evaluation of the hydrodynamics performance of a sloshing water storage
tank under external excitation is required.

Water 2020, 12, 2098; doi:10.3390/w12082098                                         www.mdpi.com/journal/water
Sloshing Motion in a Real-Scale Water Storage Tank under Nonlinear Ground Motion
Water 2020, 12, 2098                                                                                 2 of 14

     At first, researchers focused on the sloshing phenomena subject to a harmonic excitation [6–9].
As was shown, sloshing characteristics were influenced by tank parameters such as water depth [10],
tank shape [7,11,12], baffle type [9,11,13–15], external excitation [6], and so on [16]. A series of
analytical [15,17], numerical [10,18], and experimental [19,20] methods were established for solving the
sloshing problem under different conditions. However, Chen et al. [21] compared the response of liquid
tanks under harmonic excitations with those under the ground motion excitation, and they found that
simple harmonic excitation cannot replace nonlinear seismic excitation for the seismic analysis of liquid
tanks, because seismic excitation usually consists of a broadband frequency vibration, and harmonic
excitation only contains a single frequency. The primary frequencies of the ground motion decrease
with the increasing intensity of a seismic event. The frequencies of a strong earthquake are usually
distributed across a lower range, which is similar to the natural frequencies of many large industrial
tanks [22]. Therefore, the sloshing phenomena under various types of seismic excitation may differ.
     In order to improve the computational efficiency and reduce the costs, a small-scale geometric
model was usually used on a tank whose characteristic length in the excitation direction was between
0.6 and 1 m [23,24]. However, the ground motion has not been proportionally scaled, along with an
overestimated sloshing response estimated. Meanwhile, the filling levels of the tank in the previous
study were usually selected to be at a shallow depth [25–27] (depth/tank’s length < 0.1) or near critical
depth (depth/tank length ≈ 0.337) [25,28] to study the nonlinear sloshing phenomenon. The limited
cases considering high filling level conditions focused on the problem of the impact of water on the
tank roof under the large displacement [29]. Jin et al. [19] tested the sloshing response in a high filling
level tank with filling depth(d)/tank length(L) = 0.5 by the experiments, and they found that the water
motion tended to be linear in such a deep tank, which differed from the violent nonlinear sloshing
in the shallow water tank. For city water storage tanks, the size is large, and the filling level is high.
The load from the sudden ground movement will be the main threat to its safety. Unlike the long-term
loads such as wind or waves in tuned liquid damper systems on high-rise buildings or the liquid
tanks in ships, the seismic wave is a non-periodic impulse vibration with a broadband frequency
characteristic. Under this condition, the motion response of the water in the container may differ from
the previous studies.
     In this research, the liquid sloshing motion in a real-scale water storage tank was investigated under
several selected ground motions, using the computational fluid dynamic (CFD) software—OpenFOAM®
(Version: 1906, OpenCFD, Bracknell, UK) [30]. The high filling level, real-scale tank, and ground
motion were used to distinguish the present water motion from previous sloshing studies in other
fields. First, the numerical model and the governing equation were introduced. The problem of seismic
response of the water tank was described in detail. Then, the numerical model was used to discuss the
motion of the free surface in the water tank and the sloshing control effect by the damping plate after
rigorous numerical verification.

2. Numerical Method and Model

2.1. Governing Equation
     Navier–Stokes equations [30] were used to describe the incompressible, viscous, water flows in
the present numerical model. In the sloshing problem, the sloshing flow was usually assumed to be a
laminar flow, and it can give an acceptable result when the free surface motion was small [10,16,31].
In this study, the laminar model was used to solve the governing equations below:

                                                 ∂ui
                                                     =0                                                 (1)
                                                 ∂xi

                               ∂ui     ∂u     1 ∂p    µ ∂ ∂ui
                                   + ui i = −       +             + fbody                               (2)
                               ∂t      ∂x j   ρ ∂xi   ρ ∂x j ∂x j
Sloshing Motion in a Real-Scale Water Storage Tank under Nonlinear Ground Motion
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      2020, 12,
            12, 2098
                x FOR PEER REVIEW                                                                                 33 of
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      The meaning of the variables in the above equation is as follows: ρ is the density of the water, ui
      The meaning
represents           of thecomponents
             the velocity    variables ininthethe
                                                above  equation
                                                  i direction,   xi is
                                                                     (i as follows:
                                                                        = x,            ρ is the densityand
                                                                              y in two-dimensions          of the  water,
                                                                                                               x, y,  z in
u i represents  the
three-dimensions),  velocity  components     in the i direction,  x
                      p is the pressure, μ is the dynamic viscosity, i (i = x,  y in  two-dimensions       and  x,
                                                                             and fbody represents the body forces, y, z in
such as gravity. Thep Finite
three-dimensions),      is the pressure,  µ is the dynamic
                                Volume Method      (FVM) was   viscosity,
                                                                  used for  and    f bodydiscretization
                                                                               spatial    represents theofbody     forces,
                                                                                                             the partial
such   as gravity. The  Finite  Volume   Method    (FVM)    was  used     for  spatial   discretization
differential equations. The quantities of interest, e.g., the pressure and velocity of fluid, were stored of the  partial
differential equations.
at the centroid           The volumes.
                  of control   quantitiesThe
                                          of interest, e.g., the pressure
                                               volume-of-fluid                and velocity
                                                                   (VoF) method                of fluid, to
                                                                                         was selected    were  storedthe
                                                                                                            capture     at
the
freecentroid
      surface. of
               Thecontrol
                    volumevolumes.
                             fractionThe   volume-of-fluid
                                      of the  fluid was defined(VoF) in method      was selected
                                                                         the transport     equation:to capture the free
surface. The volume fraction of the fluid was defined in the transport equation:
             ρ = ρα=w ater ρ ρ + +(1(1−−ααw ater ))ρρ air, ,v v==α α w ater
                       α w ater
                                                                            v    + (1 − α w) ater ) v air,
                                                                              + (1 − α
                                                                        v w ater            v ,
                                                                                                                     (3)
                                                                                                                      (3)
                          water water            water   air        water water            water   air

       water is
where αwater  isthe
                 thevolume
                    volumefraction
                           fractionof
                                   of water
                                      water and
                                            and vv is
                                                   is the kinematic viscosity. The volume fraction of
                            following equation:
water was expressed by the following  equation:
                                                  ∂ α∂α        ∂ui
                                                        ++ αα ∂ui =   =0                                             (4)
                                                  ∂ t∂t        ∂ xi i 0
                                                              ∂x
                                                                                                                      (4)

   The variable
   The  variable α represents water
                 α represents water when
                                    when its value is
                                         its value    equal to
                                                   is equal    1 and
                                                            to 1 and the
                                                                     the interface
                                                                         interface of
                                                                                   of water
                                                                                      water and
                                                                                            and air
                                                                                                air
when α ≈ 0.5.
when α ≈ 0.5.

2.2. Numerical Model

     The sloshing model was established based on OpenFOAM (Version: 1906), and the results were
solved through the interFoam solver. The solution process of the new interFoam solver is shown in
Figure 1.1.The
             ThePIMPLE
                  PIMPLE     algorithm,
                          algorithm,      a combination
                                      a combination            of (Pressure
                                                         of PISO   PISO (Pressure
                                                                              ImplicitImplicit    with of
                                                                                        with Splitting  Splitting of
                                                                                                          Operator)
Operator)    and  SIMPLE     (Semi-Implicit     Method    for  Pressure-Linked      Equations),   in
and SIMPLE (Semi-Implicit Method for Pressure-Linked Equations), in Figure 1 was applied to solve the Figure  1  was
applied to solve the
pressure–velocity      pressure–velocity
                    coupling   relationship.coupling    relationship.
                                               More detail              Morethe
                                                              of the solver,    detail of the conditions,
                                                                                  boundary    solver, the boundary
                                                                                                          and other
conditions,
setups        and other
        can refer  to thesetups  can study
                           previous   refer to[18].
                                                the previous
                                                    The meshstudy near [18]. Thesurface
                                                                       the free   mesh nearwasthe  free surface
                                                                                                refined          was
                                                                                                        to improve
refined  to  improve    the  accuracy   of  free   surface   capture.   The   6  degree-of-freedom
the accuracy of free surface capture. The 6 degree-of-freedom (6DoF) tank movements were imposed       (6DoF)   tank
movements
through        were imposed
          a formatted          throughofa time
                        displacement       formatted
                                                  series displacement
                                                         data.           of time series data.

                                 Figure 1.
                                 Figure    Solution process
                                        1. Solution process of
                                                            of the
                                                               the interFoam
                                                                   interFoam solver.
                                                                             solver.

2.3. Problem Description
2.3. Problem Description
      To investigate the water motion in a real-scale tank, which was subjected to the ground motions,
      To investigate the water motion in a real-scale tank, which was subjected to the ground
the present numerical model was validated first with experimental tests. The experimental water tank
motions, the present numerical model was validated first with experimental tests. The experimental
was 1 m (L) × 0.8 m (H) × 0.1 m (B) as shown in Figure 2, and the filling depth d/L was 0.5. The amplitude
water tank was 1 m (L) × 0.8 m (H) × 0.1 m (B) as shown in Figure 2, and the filling depth d/L was 0.5.
of horizontal displacement A/L was 0.0025, and the excitation frequency ω/ω1 = 0.996 in Figure 2b
The amplitude of horizontal displacement A/L was 0.0025, and the excitation frequency ω/ω1 = 0.996
and ω/ω1 = 0.603 in Figure 2c (ω1 is the first-order natural frequency of the liquid tank). The detailed
in Figure 2b and ω/ω1 = 0.603 in Figure 2c (ω1 is the first-order natural frequency of the liquid tank).
experimental setup and the arrangement of the experimental system were the same as the author’s
The detailed experimental setup and the arrangement of the experimental system were the same as
previous experimental device, and there is no baffle installation in the validation test [19]. Then, the free
the author’s previous experimental device, and there is no baffle installation in the validation test
surface shape and the flow motion were compared carefully in Figure 2b,c. The heights of the free
[19]. Then, the free surface shape and the flow motion were compared carefully in Figure 2b,c. The
surface curves and the time history of the free surface elevation along the left tank wall are also shown
heights of the free surface curves and the time history of the free surface elevation along the left tank
in Figure 3 and validated based on the experimental result and theory model [32]. The uncertainty
wall are also shown in Figure 3 and validated based on the experimental result and theory model
analysis was conducted to obtain the standard error and confidence level of the experimental result.
[32]. The uncertainty analysis was conducted to obtain the standard error and confidence level of the
The numerical result fell within the 95% confidence interval. The theoretical results were a bit biased
experimental result. The numerical result fell within the 95% confidence interval. The theoretical
results were a bit biased after 14 s, which may be because it did not consider the viscosity and
Sloshing Motion in a Real-Scale Water Storage Tank under Nonlinear Ground Motion
Water 2020, 12, 2098                                                                                                                 4 of 14

   Water 2020, 12, x FOR PEER REVIEW                                                                                              4 of 14
afterWater
      14 s,2020,
             which      may
                 12, x FOR    beREVIEW
                           PEER  because it did not consider the viscosity and compressibility of                       the
                                                                                                                      4 of 14 fluid.
In general,   the   error  of the  experimental     tests was    acceptable,    and   the   result   of
   compressibility of the fluid. In general, the error of the experimental tests was acceptable, and the
                                                                                                        the  numerical      model
     compressibility
wasresult
     credible.           of the fluid. In general,  the  error  of the experimental      tests  was  acceptable,    and  the
           of theFurthermore,
                   numerical model the ability  of the present
                                        was credible.              modelthe
                                                         Furthermore,      to ability
                                                                               simulate     strong
                                                                                       of the        nonlinear
                                                                                                present   modelwater       motion
                                                                                                                   to simulate
was  result
     also    of the numerical
            verified    with  themodel  was credible.
                                   experimental     testFurthermore,
                                                          of  Faltinsen the
                                                                          etability
                                                                              al.    of the
                                                                                  [33]  in    present4.
                                                                                             Figure    model
                                                                                                          The  to simulate
                                                                                                                tank’s    internal
   strong nonlinear water motion was also verified with the experimental test of Faltinsen et al. [33] in
     strong nonlinear water motion was also verified with the experimental test of Faltinsen et al. [33] in
dimensions
   Figure       were
            4. The      1.0 m
                     tank’s    × 0.98dimensions
                             internal   m × 0.1 m were      1.0 ×
                                                      (length       × 0.98 m× ×breadth),
                                                                 m height        0.1 m (lengththe d/L   was 0.4,
                                                                                                    × height        and thethe
                                                                                                              × breadth),       A/L
     Figure 4. The tank’s internal dimensions were 1.0 m × 0.98 m × 0.1 m (length × height × breadth), the
was  0.01. The
   d/L             goodthe agreements     wereTheobtained     in two sets were
                                                                             of tests.    The present      numerical       using a
     d/Lwas
         was0.4,0.4,and
                      and the A/L
                               A/L was
                                     was 0.01.
                                          0.01. The  good agreements
                                                     good    agreements             obtained
                                                                          were obtained        in in two
                                                                                                   two     sets
                                                                                                        sets of of   tests.
                                                                                                                 tests. TheThe
laminar
   presentassumption
     presentnumerical
                           can
               numericalusing
                                be  used
                            usingaalaminar
                                          to analyze
                                     laminar assumption
                                                        the
                                              assumption can
                                                            sloshing
                                                             canbebeusedproblem.
                                                                      usedtotoanalyze
                                                                                analyze thethe  sloshing
                                                                                             sloshing      problem.
                                                                                                        problem.

           Figure   2. Free
              2. 2.Free     surface shapeofofsloshing
                                               sloshing water in  a rectangular tank  under    horizontal harmonic
      Figure
         Figure      Freesurface
                           surfaceshape
                                    shape of sloshing water
                                                         water in
                                                               in aa rectangular   tank
                                                                     rectangular tank     under
                                                                                         under    horizontal
                                                                                                 horizontal   harmonic
                                                                                                             harmonic
           excitation, (a) data acquisition system, (b) Resonant sloshing, ω/ω1 = 0.996, (c) non-resonant sloshing,
      excitation,  (a)
         excitation,   data  acquisition  system,  (b) Resonant   sloshing, ω/ω 1 =  0.996,  (c) non-resonant  sloshing,
                      (a) data acquisition system, (b) Resonant sloshing, ω/ω1 = 0.996, (c) non-resonant sloshing,
           ω/ω1 = 0.603.
      ω/ωω/ω= 0.603.
          1 1 = 0.603.

      Figure  3. The
           Figure       validation
                    3. The  validationofof
                                         thethepresent
                                                presentnumerical      modelwith
                                                        numerical model      withexperimental
                                                                                     experimental teststests
                                                                                                         [19] [19]
                                                                                                              and and    theoretical
                                                                                                                    theoretical
      solution
           solution    ω/ωω/ω
                [32],[32],  1 = 10.996,
                                  = 0.996, A/L==0.0025,
                                         A/L     0.0025, d/L =  0.5.(a)
                                                             = 0.5.  (a)Uncertainty
                                                                         Uncertainty     analysis
                                                                                      analysis      of three
                                                                                               of three        experimental
                                                                                                          experimental    tests, tests,
      theFigure
          error  bars
           the error   indicated
                       bars         a
                             indicated Standard
                                         a        Error
                                           Standard      of
                                                     Error  time
                                                            of     (X-error)
                                                                time          and
                                                                      (X-error)  andheight (Y-error)
                                                                                      height (Y-error)  of the
                                                                                                          of the
                  3. The validation of the present numerical model with experimental tests [19] and theoretical free
                                                                                                                  free surface
                                                                                                                        surface  wave.
           wave.
      (b)solution
          Red      (b)
               shallow Red   shallow
                            area       area  represented
                                   represented    the     the  prediction
                                                       prediction     limitlimit
                                                                            of    of
                                                                                the  the average
                                                                                     average       experimental
                                                                                               experimental          result
                                                                                                                   result
                    [32], ω/ω1 = 0.996, A/L = 0.0025, d/L = 0.5. (a) Uncertainty analysis of three experimental tests,      at
                                                                                                                            at aa 95%
           95%
      confidenceconfidence
         the errorlevel.       level.
                     bars indicated a Standard Error of time (X-error) and height (Y-error) of the free surface
         wave. (b) Red shallow area represented the prediction limit of the average experimental result at a
         95% confidence level.
Sloshing Motion in a Real-Scale Water Storage Tank under Nonlinear Ground Motion
Water 2020, 12, x FOR PEER REVIEW                                                                                                                          5 of 14
Water 2020, 12, 2098                                                                                                                                         5 of 14
 Water 2020, 12, x FOR PEER REVIEW                                                                                                                          5 of 14

       Figure 4. Flow motion through the vertical slot baffle in Faltinsen’s experimental tests [33], τ is the
       porosity of the baffle, (a) τ = 0.085, ω/ω1 = 1.328, Time = 14 s, (b) τ = 0.527, ω/ω1 = 0.96, Time = 46 s.
        Figure 4.4. Flow
        Figure      Flow motion
                          motion through
                                    through the
                                              the vertical
                                                   vertical slot
                                                            slot baffle
                                                                 baffle in
                                                                         in Faltinsen’s
                                                                            Faltinsen’s experimental
                                                                                            experimental tests
                                                                                                           tests [33],
                                                                                                                  [33], ττ is
                                                                                                                            is the
                                                                                                                                the
        porosity
        porosityof ofthe
                      thebaffle,  (a)ττ== 0.085,
                          baffle,(a)      0.085, ω/ω
                                                 ω/ω11 =
                                                       = 1.328, Time =    14 s,
                                                                       = 14     (b) ττ ==0.527,
                                                                             s, (b)             ω/ω1 1= =
                                                                                          0.527,ω/ω       0.96,
                                                                                                        0.96,   Time= =
                                                                                                              Time      4646 s. s.
       Then, a real-scale water tank in Haroun [34] was selected as the baseline structure for this study.
This tankThen,  was located in            Los Angeles,           which is a city           with potential    the earthquakes           risk [35].       The study.
                                                                                                                                                               tank
          Then, aa real-scale
                      real-scale waterwater tank  tank in  in Haroun
                                                               Haroun [34]   [34] was
                                                                                   was selected
                                                                                            selected as  as the    baseline structure
                                                                                                                   baseline      structure       for this
                                                                                                                                                 for   this   study.
geometry
 This           is  shown       in   Figure       5,   with    18   m   (Diameter        (D))    ×  18.25   m   (Height′(H′)).        The     filling     depth     d
  This tank
          tank waswas located
                         located in    inLosLosAngeles,
                                                   Angeles, which  which isisaacity  citywithwithpotential
                                                                                                     potentialearthquakes
                                                                                                                    earthquakes risk     risk[35].
                                                                                                                                                 [35]. TheThe tank
                                                                                                                                                                 tank
was    10.8 m (60%         D), and        then the tank            can be regarded             as a slender       [36] or deep
                                                                                                                            0 (H )).water
                                                                                                                                  0               tank (2d/D >d
 geometry
  geometry is     is shown
                      shown in    inFigure
                                       Figure5,5,with   with18   18mm(Diameter
                                                                          (Diameter(D))    (D))××18.2518.25mm(Height
                                                                                                                  (Height′(H′)).        The
                                                                                                                                        The filling
                                                                                                                                                filling depth
                                                                                                                                                            depth d
1).  This    filling   level    was     larger      than     the   value     of  d/D    =  0.4,   and   therefore,      the    wave     motion        resembles  > 1).>
 was
  was10.810.8m       (60%D),
                 m(60%       D),and andthen thenthe   thetank
                                                            tankcan  canbebe regarded
                                                                                regarded     as as
                                                                                                 a slender
                                                                                                     a slender [36][36]
                                                                                                                      or deep
                                                                                                                           or deep  water
                                                                                                                                        water tank     (2d/D
                                                                                                                                                    tank    (2d/D
aThis
   standing
         filling  wave
                    level  with
                            was     the
                                     largerlargest
                                                 than   free
                                                          the   surface
                                                                 value    ofelevations
                                                                               d/D    =   0.4, atand
                                                                                                  the tank     walls the
                                                                                                        therefore,       [37].wave
                                                                                                                                The Froude
                                                                                                                                        motion       number
                                                                                                                                                       resembles   is
  1). This filling level was larger than the value of d/D = 0.4, and therefore, the wave motion resembles
the   only dimensionless
 aa standing                            number
                                             largesttofree  be considered           if thereatisthe   a scaled      model[37]. used. TheIn     CFD analysis,
    standing wave  wave with with the the    largest      free    surface elevations
                                                                 surface       elevations                 tankwalls
                                                                                                at the tank        walls  [37]. The         Froude
                                                                                                                                          Froude           number
                                                                                                                                                        number       is
simplified
 is the    only    assumptions
                   dimensionless           and
                                            number   models to  be  are     usually
                                                                     considered         if required
                                                                                            there    is a  to
                                                                                                          scaled makemodel  the     calculation
                                                                                                                                 used.     In   CFD       process
                                                                                                                                                          analysis,
  the only dimensionless number to be considered if there is a scaled model used. In CFD analysis,
economically
 simplified            viable. Therefore,               a two-dimensional                 (2D)to Oxymakesection       of the cylindrical               tank was
  simplifiedassumptions
                     assumptions        andand  models models are usually       required
                                                                      are usually             required      the
                                                                                                             to calculation
                                                                                                                   make the process   calculation economically
                                                                                                                                                            process
extracted
 viable.        to  build
             Therefore,      a   numerical
                             a two-dimensional       water      tank.    The     size
                                                                (2D) Oxy section (2D)   of   the    numerical
                                                                                            of theOxy cylindrical   tank    was     the   same       as   the   reala
  economically          viable.       Therefore, a two-dimensional                                          section tank of thewascylindrical
                                                                                                                                      extracted to          build
                                                                                                                                                         tank     was
water
 numericaltank    in  X
                  water  and    Y
                            tank.   directions.
                                      The      size     The
                                                       of  the  Z  direction
                                                                 numerical        in
                                                                                  tankthe    numerical
                                                                                           was     the  same model
                                                                                                                 as  thewasreala   symmetry
                                                                                                                                   water     tank    plane.
                                                                                                                                                      in   X    The
                                                                                                                                                              and   Y
  extracted to build a numerical water tank. The size of the numerical tank was the same as the real
mesh      dependence
 directions.       The   Z    was tested
                             direction        in  to ensure
                                                  the    numerical  thatmodel
                                                                           the present
                                                                                    was      a  mesh
                                                                                                symmetry size can
                                                                                                                plane.capture
                                                                                                                           The      the sloshing
                                                                                                                                  mesh      dependence     surface
                                                                                                                                                                 was
  water tank in X and Y directions. The Z direction in the numerical model was a symmetry plane. The
well
 testedin toTable
                ensure1. Finally,
                           that     thethe     medium
                                          present              mesh size        with Δx =the      0.15   m, Δy =surface
                                                                                                                      0.05 mwell  wasin  selected,
                                                                                                                                            Table 1.and          the
  mesh dependence               was      tested      to mesh
                                                         ensuresize   thatcan thecapture
                                                                                   present mesh      sloshing
                                                                                                          size can capture           the sloshing           Finally,
                                                                                                                                                            surface
mesh      near    the   free    water       surface       was     refined      locally.      Two     well-studied         ground       motions,          the   1940
 the
  wellmedium
          in Tablemesh         size with
                        1. Finally,       the ∆x medium= 0.15 mesh
                                                                 m, ∆ysize  = 0.05withm Δx  was= selected,
                                                                                                    0.15 m, Δy  and     the mesh
                                                                                                                     = 0.05    m was   near     the freeand
                                                                                                                                           selected,          waterthe
Imperial
 surface       Valley
              was         (IV) and
                      refined             1971Two
                                   locally.         San well-studied
                                                            Fernando (SF)ground     earthquakes motions, [38],thewere
                                                                                                                    1940  selected
                                                                                                                             Imperial    forValley
                                                                                                                                               the following
                                                                                                                                                          (IV)   and
  mesh near the free water surface was refined locally. Two well-studied ground motions, the 1940
analysis.
 1971    San They
                Fernandotransferred
                                (SF)         a nonlinear
                                        earthquakes                vibration
                                                               [38],   were(SF)    with the
                                                                                selected       for broadband
                                                                                                    the following    frequency
                                                                                                                         analysis.     to the transferred
                                                                                                                                                   water tank.a
  Imperial       Valley (IV)        and     1971 San Fernando                         earthquakes          [38], were       selectedThey   for the following
The    strongvibration
 nonlinear         vibrationwith   of the  theIV     earthquake          lasted a to    long time,       while
                                                                                                            tank.the      SFstrong
                                                                                                                               earthquake           hadofstrong
  analysis. They          transferred           a broadband
                                                   nonlinear vibrationfrequency     withthe    thewater
                                                                                                     broadband       The
                                                                                                                       frequency        vibration
                                                                                                                                         to the water        the   IV
                                                                                                                                                                tank.
instantaneous
 earthquake            vertical      displacement             and    short    duration.        Both    the  ground       motions      in   the   longitudinal
  The strong lasted          a long
                     vibration       of time,
                                          the IV   while      the SF earthquake
                                                       earthquake          lasted a long    had time,
                                                                                                   strongwhile
                                                                                                             instantaneous          vertical displacement
                                                                                                                     the SF earthquake                had strong
(X
 and direction)
       short         and vertical
                 duration.       Both     (Y direction)
                                          the    ground          directions
                                                              motions       in  thewere      considered,
                                                                                     longitudinal         (X   instead ofand
                                                                                                              direction)        the vertical
                                                                                                                                      condition    (Y   in  which
                                                                                                                                                         direction)
  instantaneous vertical displacement and short duration. Both the ground motions in the longitudinal
only    longitudinal
 directions       were       displacement
                           considered,                 was applied          in previous          studiesonly[23,24].     By the way,          the locationwas     of
  (X direction)        and    vertical (Y instead direction)    of directions
                                                                    the condition   wereinconsidered,
                                                                                                which              longitudinal
                                                                                                                 instead                displacement
                                                                                                                              of the condition            in which
the   SF
 applied    earthquake         is  close
              in previousdisplacement         to
                                 studies [23,24].  the    water      tank     in  Haroun.         Therefore,       the    water     motion        under      these
  only longitudinal                                     wasByapplied
                                                                  the way,    inthe    location
                                                                                  previous           of the [23,24].
                                                                                                  studies     SF earthquake
                                                                                                                          By the way, is close  thetolocation
                                                                                                                                                        the water   of
different
 tank    in    excitations
             Haroun.             might the
                          Therefore,         performwater    differently.
                                                               motion     under  Each
                                                                                    these of the     ground
                                                                                               different        motionsmight
                                                                                                            excitations        was scaled
                                                                                                                                        perform    to differently.
                                                                                                                                                        the same
  the SF earthquake is close to the water tank in Haroun. Therefore, the water motion under these
peak
 Each   ground
          of the      acceleration
                    ground       motions   (PGA) wasofscaled0.5 g so to that
                                                                        the samethey represented            the   same earthquake              intensity.       The
  different      excitations        might perform              differently.       Eachpeak  of theground
                                                                                                      ground  acceleration
                                                                                                                   motions was     (PGA)scaled of  0.5
                                                                                                                                                     to g   sosame
                                                                                                                                                          the    that
information
 they               of  the   two     scaled       earthquake          events     is  listed     in  Table    2,  and    the   scaled     ground         motions
  peakrepresented           the same (PGA)
           ground acceleration               earthquake  of 0.5intensity.
                                                                   g so that The  theyinformation
                                                                                           representedofthe     thesame
                                                                                                                      two earthquake
                                                                                                                              scaled earthquake  intensity.  events
                                                                                                                                                                  The
are
 is   shown
    listed    in in   Figure
                  Table    2,    6. the scaled ground motions are shown in Figure 6.
                               and
  information of the two scaled earthquake events is listed in Table 2, and the scaled ground motions
  are shown in Figure 6.

                                                         Thegeometry
                                              Figure5.5.The
                                             Figure          geometryofofthe
                                                                          thewater
                                                                             waterstorage
                                                                                   storagetank.
                                                                                           tank.

                                               Figure 5. The geometry of the water storage tank.
Sloshing Motion in a Real-Scale Water Storage Tank under Nonlinear Ground Motion
Water 2020, 12, x FOR PEER REVIEW                                                                                6 of 14

      Table 1. Grid dependence of the free surface elevation at the right side wall under the Imperial
      Valley (IV) earthquake.
Water 2020, 12, 2098                                                                                        6 of 14
                               Maximum          Calculation       Averaged Free Surface Peaks at      Relative
   Mesh       Description
                               Size (x × y)      Time (s)            Right Side Wall (x = 9 m)       Difference
    1Table 1. Grid
                 Fine         0.075 × 0.025        3391                      0.1839 m                     -
                   dependence of the free surface elevation at the right side wall under the Imperial Valley
    2(IV) earthquake.
             Medium (used)     0.15 × 0.05          464                      0.1746 m                    5%
    3           Coarse          0.3 × 0.1            88                      0.1580 m                   14%
                                     Maximum            Calculation    Averaged Free Surface Peaks        Relative
      Mesh         Description
      Table 2. Information of twoSize (x × y)groundTime
                                   strong               (s)
                                                    motions.  at Right
                                                             PGA:  peakSide
                                                                         ground     = 9 m)
                                                                            Wall (xacceleration,
                                                                                               Difference
                                                                                                 SF: San
      Fernando.
        1           Fine        0.075 × 0.025        3391               0.1839 m                    -
      2      Medium (used)     0.15 × 0.05         464                         0.1746 m                     5%
      3          Coarse         0.3 × 0.1           88                         0.1580      Strong Motion Duration
Earthquake Event      Scaled PGA (g)       Scale Factor               Scaled PGD* (m) m                14%
                                                                                                    (s)
                          Longitudinal:                          Longitudinal:
      Table 2. Information of two strong ground motions. PGA: peak ground acceleration, SF: San Fernando.
         IV             0.5zhiweiVertical:         1.78      0.154zhiweiVertical:                30
     Earthquake Event          0.317
                            Scaled PGA (g)     Scale Factor          0.048
                                                             Scaled PGD* (m)     Strong Motion Duration (s)
                          Longitudinal:
                           Longitudinal: 0.5                     Longitudinal:
                                                            Longitudinal:  0.154
         SF IV          0.5zhiweiVertical:         1.78
                                                   0.41      0.160zhiweiVertical:            30 18
                             Vertical: 0.317                  Vertical: 0.048
                               0.282
                           Longitudinal:   0.5                       0.120
                                                            Longitudinal:  0.160
             SF                                        0.41                                          18
                            Vertical: 0.282                    Vertical: 0.120
                                         PGD*: peak ground displacement.
                                              PGD*: peak ground displacement.

      Figure 6. Acceleration and displacement of two selected earthquakes, (a) the scaled acceleration and the
      displacement of the Imperial Valley 1940 earthquake, (b) the scaled acceleration and the displacement
      of the San Fernando 1971 earthquake.
Sloshing Motion in a Real-Scale Water Storage Tank under Nonlinear Ground Motion
Water 2020, 12, x FOR PEER REVIEW                                                                                                 7 of 14
        Figure 6. Acceleration and displacement of two selected earthquakes, (a) the scaled acceleration and
        the displacement
      Figure                of and
              6. Acceleration   the displacement
                                     Imperial Valley  1940
                                                 of two    earthquake,
                                                        selected        (b) the
                                                                 earthquakes, (a) scaled acceleration
                                                                                  the scaled           andand
                                                                                             acceleration   the
        displacement  of the San  Fernando  1971 earthquake.
      the displacement of the Imperial Valley 1940 earthquake, (b) the scaled acceleration and the
Water 2020, 12, 2098 of the San Fernando 1971 earthquake.
      displacement                                                                                                                 7 of 14
  3. Results and Discussion
3. Results and Discussion
3.3.1.
   Results and Discussion
       Comparison of Water Motion between Two-Dimensional and Three-Dimensional Results
3.1. Comparison
3.1.     In order to
      Comparison      of  Water
                       of make
                          Waterthe Motion  betweenprocess
                                      calculation
                                   Motion  between     Two-Dimensional
                                                                economical,
                                                       Two-Dimensional       andthe
                                                                             and    Three-Dimensional
                                                                                         Oxy section wasResults
                                                                                   Three-Dimensional           Results
                                                                                                                 extracted to study the
  water    sloshing
       In order
          order         in thethe
                   to make
                       make      tank  under seismic
                                     calculation    process excitation.
                                                               economical,However,
                                                                                the Oxy
                                                                                      Oxy in section
                                                                                             case thewas 2D extracted
                                                                                                               simulationtocan     be used
                                                                                                                               study   the
      In          to            the calculation     process    economical,      the          section was      extracted to     study   the
  instead
water        of the
         sloshing     real
                      in  the3D   status,
                               tank       the
                                      under    comparison
                                              seismic           of  water
                                                          excitation.        motion
                                                                        However,         between
                                                                                        in case   the the
                                                                                                       2D  2D   and
                                                                                                             simulation3D  problems
                                                                                                                            can  be     was
                                                                                                                                     used
water sloshing in the tank under seismic excitation. However, in case the 2D simulation can be used
  conducted.
instead        theAreal
           of the     similar
                    real          trend the
                           3D status,
                                status, can comparison
                                              be seen in Figure       7. The
                                                               of water
                                                                  water         free between
                                                                           motion      surface atthe  the2DOxyand(Z3D3D= 0) section in
                                                                                                                         problems     wasthe
instead    of              3D           the comparison         of         motion      between the        2D and          problems     was
  3D   simulation
conducted. A           coincided      with the   2D  result.   Meanwhile,       it can    be  seen   that  the   free   surface  elevation
conducted.       A similar
                    similar trend
                                trend can   be seen
                                       can be   seen in in Figure
                                                            Figure 7.
                                                                    7. The
                                                                        The free
                                                                             free surface
                                                                                     surface at at the
                                                                                                    the Oxy
                                                                                                         Oxy (Z (Z = = 0) section in
                                                                                                                       0) section   in the
                                                                                                                                       the
3Ddecreased
     simulation  along     the Z direction
                     coincided      with  the  from
                                               2D      0 to ±9
                                                    result.      m (Oyz section
                                                              Meanwhile,      it can    of the
                                                                                        be  seen tank)
                                                                                                   that   in the
                                                                                                         the   free 3D   model,
                                                                                                                      surface     which is
                                                                                                                               elevation
3D simulation coincided with the 2D result. Meanwhile, it can be seen that the free surface elevation
  different from
decreased      alongthe  theprevious
                              Z directionresearch
                                  direction  from 0in0 toa cube
                                                        to  ±9 m  tank [10].    They     showed      a similar     sloshing
                                                                                                                       model,response    isin
decreased      along     the  Z              from           ±9  m (Oyz    section
                                                                   (Oyz section       of  the tank)
                                                                                      of the   tank)    in the
                                                                                                        in  the 3D3D   model,   which is
                                                                                                                                which
  the  Z  direction.
different from
             from the    This   is
                     the previous  because
                           previous research we
                                       research inused    a cylindrical
                                                   in aa cube
                                                          cube tank
                                                                tank [10]. tank,
                                                                       [10]. They  and
                                                                             They showed  the  side  wall
                                                                                       showed aa similar     of the
                                                                                                     similar sloshing tank  was  a
                                                                                                                 sloshing response  curved
                                                                                                                             response inin
different
  surface.
the  Z        Further
        direction.        investigation
                       This   is because   of the
                                           we   usedtwo a  models is supplied
                                                           cylindrical   tank,   and  inthe
                                                                                          Figure
                                                                                             side   8. of the tank was a curved
                                                                                                   wall
the Z direction. This is because we used a cylindrical tank, and the side wall of the tank was a curved
surface. Further
surface.    Further investigation
                        investigation of  of the
                                             the two
                                                  two models
                                                         models is is supplied
                                                                      supplied in   in Figure
                                                                                        Figure 8. 8.

        Figure 7. Free surface shape of the sloshing in the water tank under the IV earthquake, (a) 2D case, t =
       Figure s,
        10.25
      Figure   7. (b)
               7.  Free
                   Free3D  case, tshape
                         surface
                         surface   = 10.25
                                   shape     s.the
                                          ofof  thesloshing
                                                    sloshingininthe
                                                                  thewater
                                                                      water  tank
                                                                           tank   under
                                                                                under thethe
                                                                                           IVIV earthquake,
                                                                                              earthquake, (a) (a)
                                                                                                               2D2D  case,
                                                                                                                  case, t=
        = 10.25
      t10.25      s, (b)
             s, (b)  3D 3D   case,
                         case,      t = 10.25
                               t = 10.25  s. s.

                                   Figure 8. Free surface elevations at the right side wall.
                                    Figure 8. Free surface elevations at the right side wall.
      The maximum free surface   Figure elevation     appeared
                                          8. Free surface        at the
                                                          elevations     Oxyright
                                                                      at the   section
                                                                                   side of the tank, as seen in Figure 8.
                                                                                        wall.
In thisThe   maximum
        study,  the freefree    surface
                            surface     on elevation   appeared
                                            the Oxy section     wasat extracted
                                                                      the Oxy section
                                                                                  to showof the
                                                                                              thetank,
                                                                                                   water  assloshing
                                                                                                             seen in Figure
                                                                                                                       in the8.
  In this
tank The  study,
      under      the  free   surface
              seismic excitation.
           maximum      free surfaceThison  the Oxy   section
                                               resultappeared
                                         elevation             was  extracted
                                                       usually indicated
                                                                 at the Oxythe  to show
                                                                                 most of
                                                                               section     the  water
                                                                                        dangerous        sloshing
                                                                                           the tank, section       in the
                                                                                                                 of Figure
                                                                                                        as seen in         tank
                                                                                                                    the tank.8.
  under
Therefore,seismic
             in     excitation.
                order  to   save    This
                                   time     result
                                           and      usually
                                                calculation   indicated
                                                              costs,  the  the
                                                                           watermost   dangerous
                                                                                  movement        in   section
                                                                                                      the
In this study, the free surface on the Oxy section was extracted to show the water sloshing in the tank    most  of  the
                                                                                                                 dangeroustank.
  Therefore,
section
under     was in
         seismic  order
               calculated to here
                  excitation. saveThistime
                                      mainly and
                                          result   calculation
                                                based   on the
                                                   usually      costs,  the
                                                                          thewater
                                                                 2D simulation.
                                                            indicated          most movement
                                                                                      The           insection
                                                                                            free surface
                                                                                      dangerous         the most
                                                                                                               of dangerous
                                                                                                             shape   andtank.
                                                                                                                   the     the
  section
time       was
      history
Therefore,      calculated
             inof  elevation
                order   to savehere   mainly
                                between
                                    time and    based   on the
                                              thecalculation    2D
                                                   2D and 3Dcosts,  simulation.
                                                                simulation
                                                                      the water    The  free
                                                                               are movement   surface
                                                                                   further shown  in the shape
                                                                                                        in most and
                                                                                                           Figures   8the
                                                                                                                       andtime
                                                                                                                 dangerous   9.
  history
They       of elevation
       indicated  that  thebetween
                              2D        the
                                   results   2D
                                             were and  3D  simulation
                                                    acceptable.          are
                                                                  Therefore,  further
                                                                               the     shown
                                                                                    motion       in  Figures
                                                                                              response
section was calculated here mainly based on the 2D simulation. The free surface shape and the time         of  8 and
                                                                                                              water    9.
                                                                                                                     and  They
                                                                                                                           the
conclusions   drawn from
history of elevation           it should
                         between      the 2D be and
                                                meaningful.
                                                      3D simulation are further shown in Figures 8 and 9. They
Water 2020, 12, x FOR PEER REVIEW                                                                                                8 of 14
Water 2020, 12, x FOR PEER REVIEW                                                                                                8 of 14
indicated that the 2D results were acceptable. Therefore, the motion response of water and the
Water 2020, 12,
indicated
conclusions     2098
             that   the 2D
                 drawn     results
                        from       were
                             it should    meaningful.Therefore, the motion response of water and8 ofthe
                                       beacceptable.                                                 14

conclusions drawn from it should be meaningful.

                                    Figure 9. Free surface response at selected moments.
                                    Figure 9. Free surface response at selected moments.
                                    Figure 9. Free surface response at selected moments.
3.2. Free
3.2. Free Surface
            Surface Motion
                    Motion in  in the
                                   the High
                                       High Filling
                                               Filling Depth
                                                       Depth Water
                                                                 Water Tank
                                                                         Tankunder
                                                                                 underSeismic
                                                                                        SeismicExcitation
                                                                                                  Excitation
3.2. Free Surface Motion in the High Filling Depth Water Tank under Seismic Excitation
       From Figure
       From     Figure6,6,the
                            the   largest
                               largest       longitudinal
                                         longitudinal     andand      vertical
                                                                 vertical   PGDs  PGDs    appeared
                                                                                    appeared     in thein    the10first
                                                                                                          first      s of 10
                                                                                                                           the sground
                                                                                                                                  of the
ground From    Figure
           motions.     6,
                      Duringthe   largest
                                  this       longitudinal
                                        stage,    the shapes   and
                                                                of    vertical
                                                                    the  free     PGDs
                                                                                surface   appeared
                                                                                         at  the
motions. During this stage, the shapes of the free surface at the 2 s, 4 s, 6 s, 8 s, and 10 s and2 s,  4in
                                                                                                          s, the
                                                                                                             6  s, first
                                                                                                                   8 s,    10
                                                                                                                         and   s
                                                                                                                               10 of  the
                                                                                                                                   s and
                                                                                                                                      the
ground
the Fast
Fast      motions.
      FourierFourier  During
                      Transfer
                 Transfer    (FFT)this stage,
                                    (FFT)        the shapes
                                                      of twoofof
                                             amplitudes
                                     amplitudes                     the ground
                                                                  two
                                                                ground  free   surface
                                                                            motions      at shown
                                                                                    motions
                                                                                       are  theare
                                                                                                 2 s,shown
                                                                                                       4 s,
                                                                                                      in     6 s,in8 10.
                                                                                                          Figure     s,  and
                                                                                                                     FigureThe1010.s and
                                                                                                                                      The
                                                                                                                                 results
the  Fast
results     Fourier
          showed      Transfer
                     that   the     (FFT)
                                   response  amplitudes
                                                 of the       of
                                                          water  two    ground
                                                                    reached     the motions
                                                                                     maximum   are
showed that the response of the water reached the maximum displacement of the tank. Although the     shown
                                                                                                    displacement in  Figure
                                                                                                                          of    10.
                                                                                                                              the    The
                                                                                                                                    tank.
results
Although
maximum   showed
              the    that the instantaneous
                   maximum
               instantaneous      response
                                   displacement  of the
                                                      of  water
                                                      displacement
                                                         the       reachedof the
                                                               SF earthquake    theSFmaximum
                                                                                      earthquake
                                                                                    was  larger (seedisplacement
                                                                                                       was
                                                                                                         Tablelarger      offree
                                                                                                                        (see
                                                                                                                  1), the     the
                                                                                                                               Tabletank.
                                                                                                                                       1),
                                                                                                                                  liquid
Although
the  free     the
           liquid  maximum
                   motion        instantaneous
                              caused    by   it was   displacement
                                                      not  significantly of   the
                                                                             more  SF earthquake
                                                                                    violent   than
motion caused by it was not significantly more violent than that of the IV earthquake. The maximum     was
                                                                                                     that   oflarger
                                                                                                                the  IV (see   Table
                                                                                                                          earthquake.  1),
the
Thefree
free      liquid
       maximum
      surface      motion
                elevations    caused
                     free surface
                              of the twoby  itearthquakes
                                                was notofsignificantly
                                         elevations          the             more
                                                                   two earthquakes
                                                              reached      11.20  mviolent    thanm,that
                                                                                            reached
                                                                                     and 11.19              of the
                                                                                                         11.20     m IV
                                                                                                       respectively.   andearthquake.
                                                                                                                          The11.19
                                                                                                                                ratio m,
                                                                                                                                       of
The    maximum
respectively.     Thefree
                       ratiosurface
                               of  the  elevations
                                        free   surface of   the
                                                         elevation twoto  earthquakes
                                                                           the  original   reached
                                                                                          water    level
the free surface elevation to the original water level was only 3.7% for the IV earthquake and 3.6% for  11.20
                                                                                                           was    m
                                                                                                                  onlyand 3.7%11.19
                                                                                                                                 for   m,
                                                                                                                                      the
respectively.
IV earthquake
the               The
     SF earthquake.andratio
                         3.6%offor
                         There     the
                                  is   free
                                      the
                                     no    SFsurface    elevation
                                                earthquake.
                                          significant           There
                                                        difference    to isthe
                                                                       here. nooriginal   water
                                                                                 significant       level was
                                                                                               difference         only 3.7% for the
                                                                                                              here.
IV earthquake and 3.6% for the SF earthquake. There is no significant difference here.

      Figure 10. Free surface response at selected moments, (a) the free surface response under the Imperial
      Valley 1940 earthquake, (b) the free surface response under the San Fernando 1971 earthquake.
Imperial Valley 1940 earthquake, (b) the free surface response under the San Fernando 1971
      earthquake.

     The free surface shapes are shown in Figure 10, indicating that the location of the largest node
of water vibration is different. The nodes in IV are located around 1/3D and 2/3D, and the nodes in
     Water 2020, 12, 2098                                                                                                          9 of 14
SF are located at the side wall. They corresponded to the first and third modes sloshing in the water
tank, respectively [39]. Based on the Fast Fourier Transfer (FFT), the seismic excitations were
transferredTheto free
                    thesurface   shapes domain,
                           frequency       are shownand  in Figure  10, indicating
                                                              the first              that thefrequencies
                                                                          three natural           location of the (ωi,largest
                                                                                                                        i = 1,node
                                                                                                                                 2, 3)ofwere
marked. The results showed that the primary frequencies of SF and IV were close to ωSF
     water  vibration     is different.   The  nodes   in  IV are located  around    1/3D     and   2/3D,  and  the  nodes    in      are
                                                                                                                                   1 and ω3,
     located at the side wall. They corresponded to the first and third modes sloshing in the water tank,
respectively. Therefore, the corresponding sloshing modes were excited. On the other hand, the odd
     respectively [39]. Based on the Fast Fourier Transfer (FFT), the seismic excitations were transferred to
resonance modes of sloshing are (n = 1, 3, …) usually activated by the anti-symmetric motion (sway
     the frequency domain, and the first three natural frequencies (ωi, i = 1, 2, 3) were marked. The results
in the   horizontal
     showed     that the direction)        of the container,
                            primary frequencies        of SF and IVand  wereeven
                                                                              close the
                                                                                     to ω1resonance           modes (nTherefore,
                                                                                               and ω3 , respectively.          = 2, 4, …)
corresponded       to  the   symmetric       motion     (heavy    in the  vertical   direction).
     the corresponding sloshing modes were excited. On the other hand, the odd resonance modes         The    FFT   results    in  Figure
                                                                                                                                        of 10
indicated   thatare
     sloshing      the(nresonance
                          = 1, 3, . . . )sloshing    is more likely
                                          usually activated     by the to   occur under motion
                                                                         anti-symmetric         the IV earthquake.
                                                                                                          (sway in the horizontal
     The envelopes
     direction)             of the maximum
                  of the container,     and even thefreeresonance
                                                           surface modes
                                                                     revealed(n =that    . . . )effects
                                                                                   2, 4,the               of water
                                                                                                 corresponded      to on
                                                                                                                       the the   tank walls
                                                                                                                            symmetric
were similar (Figure 11), although the excitations of the ground motion were irregular and
     motion    (heavy     in the  vertical   direction).    The  FFT   results  in Figure      10  indicated    that   the  resonance
     sloshing is
asymmetric.        more likely to
                 Meanwhile,         theoccur   under the
                                          maximum           IV surface
                                                          free earthquake.elevation appeared near the side wall, and the
           The   envelopes     of  the  maximum       free  surface
minimum point was located at the center of the still water level.    revealed   that the The effects  of water level
                                                                                                 high-water       on theattank
                                                                                                                             the walls
                                                                                                                                   tank wall
     were  similar    (Figure   11),  although    the excitations   of the ground   motion
of the water tank is also usually the cause of the destruction and overturning of the tank. The  were   irregular   and   asymmetric.
     Meanwhile, the maximum free surface elevation appeared near the side wall, and the minimum point
maximum rate of free surface elevation was 6.58% for the IV earthquake and 8.20% for the SF
     was located at the center of the still water level. The high-water level at the tank wall of the water
earthquake. These results mean that earthquakes with large instantaneous displacements, such as
     tank is also usually the cause of the destruction and overturning of the tank. The maximum rate of
the SF  earthquake,
     free                 may lead
          surface elevation              to more
                                 was 6.58%      for dangerous      situations.
                                                    the IV earthquake            This for
                                                                           and 8.20%     wasthe  inconsistent
                                                                                                    SF earthquake. with    the conclusion
                                                                                                                         These   results
reflected
     meanin    theearthquakes
            that     response of   withthelarge
                                            water    motion during
                                                 instantaneous           the first 10
                                                                   displacements,    suchs when
                                                                                              as the aSFlarge   groundmay
                                                                                                          earthquake,        displacement
                                                                                                                                 lead to
occurred.   Therefore,situations.
     more dangerous          the time This history
                                                wasof     the free surface
                                                      inconsistent   with theelevation
                                                                                conclusionatreflected
                                                                                                   the right    sideresponse
                                                                                                             in the     wall is of shown
                                                                                                                                      the in
Figure   12 motion
     water   to helpduring
                         understand
                                  the firstthe
                                             10 sabove
                                                   when aphenomenon.          Figure 12 shows
                                                             large ground displacement                    thatTherefore,
                                                                                                  occurred.     the sloshing         was not
                                                                                                                              the time
fullyhistory
      excitedofinthe thefree
                           early  stageelevation
                              surface      of the earthquake.       The wall
                                                     at the right side    maximum
                                                                               is shown  free in surface
                                                                                                  Figure 12 response      did not appear
                                                                                                               to help understand
     thestage
at the   abovewhenphenomenon.
                            the maximumFigure 12ground
                                                   shows that    the sloshing occurred,
                                                              displacement       was not fully    andexcited   in the early
                                                                                                         the sloshing          stage of
                                                                                                                             phenomenon
     the earthquake.       The  maximum       free  surface   response   did  not  appear
continued and kept growing, despite the large ground displacement having passed. The elevation at the  stage   when    the  maximum
     groundafter
decreased      displacement
                      10 s in theoccurred,    and the sloshing
                                      SF earthquake        event.phenomenon
                                                                   The water kept  continued
                                                                                         sloshing  and stably
                                                                                                        kept growing,
                                                                                                                 in the IV despite    the
                                                                                                                                earthquake
     large ground displacement having passed. The elevation decreased after 10 s in the SF earthquake
event until a new seismic pulse peak appeared, and the elevation increased again. The above results
     event. The water kept sloshing stably in the IV earthquake event until a new seismic pulse peak
indicated that the sloshing excited by the seismic motion was hysteretic compared with the ground
     appeared, and the elevation increased again. The above results indicated that the sloshing excited by
motion.   Therefore, the water storage tank still faced the threat of collapse, although the violent
     the seismic motion was hysteretic compared with the ground motion. Therefore, the water storage
ground
     tankmotion
           still facedpassed.
                         the threat of collapse, although the violent ground motion passed.

                     Figure 11. The maximum free surface envelope under two different ground motions.
                Figure 11. The maximum free surface envelope under two different ground motions.
Water 2020, 12, 2098                                                                                              10 of 14
            Water 2020, 12, x FOR PEER REVIEW                                                                                 10 of 14

                                       Figure 12. Free surface elevations at right side wall.

3.3. Seismic Design of the Water Storage Tank and the Anti-Sloshing Ability of the Damping Baffle
                                                    12. Free surface elevations at right side wall.
     Even a short-term seismicFigure         pulse
                                            Figure  12.will
                                                        Free cause     continuous
                                                             surface elevations          sloshing
                                                                                  at right  side wall. in the water storage tank and
bring a sloshing load, which delayed more than the ground motion.
        3.3.  Seismic   Design   of the Water   Storage  Tank   and the  Anti-Sloshing    Ability   of the For
                                                                                                           Dampingthe Baffle
                                                                                                                        safety design of the
         3.3. Seismic Design of the Water Storage Tank and the Anti-Sloshing Ability of the Damping Baffle
water tank in  Evenearthquake-prone
                      a short-term seismicareas,          a well-studied
                                                 pulse will    cause continuous  vertical
                                                                                      sloshingdamping         baffle
                                                                                                   in the water         (seetank
                                                                                                                    storage     Figure
                                                                                                                                     and 13) was
used to test   Even
        bringits       a short-term
                    suppression
                a sloshing              seismic   pulse
                                        on sloshing,
                              load, which    delayed more will
                                                           such cause
                                                               thanasthecontinuous
                                                                        [15,20,40,41]. sloshing
                                                                          ground motion.The         in  the
                                                                                               For height    water
                                                                                                     the safety      storage
                                                                                                               ofdesign
                                                                                                                    the baffle tanka and
                                                                                                                                      = 2/3d. The
                                                                                                                           of the water
mesh around the baffle was refined. Figure 14 gives the free surface elevations at the righttheside wall
         bring
        tank   ina  sloshing    load,
                   earthquake-prone     which
                                         areas, delayed
                                                 a         more
                                                    well-studied   than   the
                                                                    vertical   ground
                                                                              damping    motion.
                                                                                          baffle  (seeFor  the
                                                                                                         Figure safety
                                                                                                                 13)  wasdesign
                                                                                                                            used   of
                                                                                                                                  to  test
         water
        its       tank in earthquake-prone
            suppression      on sloshing, such as   areas,  a well-studied
                                                       [15,20,40,41].           vertical
                                                                        The height        damping
                                                                                      of the   baffle abaffle
                                                                                                          = 2/3d.(see
                                                                                                                   TheFigure
                                                                                                                         mesh 13)     was
                                                                                                                                 around
in the cases    of   tanks     with and on    without      the    damping        baffle.The The      results   showed        that    the   vertical
        the baffle was refined. Figure 14 gives the free surface elevations at the right side wall in the casesThe
         used   to  test its suppression          sloshing,   such   as  [15,20,40,41].        height     of the  baffle  a =  2/3d.    of
dampingtanks
           baffle
         mesh   with  had
                 aroundandthea baffle
                                 better
                             without    was
                                       the effect
                                            damping  onbaffle.
                                              refined.    the free
                                                         Figure         surface
                                                                  14 gives
                                                                 The        the
                                                                      results      elevations
                                                                                 free
                                                                               showed  surface
                                                                                         that the     at thedamping
                                                                                                 elevations
                                                                                                     vertical  atside    wall,
                                                                                                                   the right      under
                                                                                                                               side
                                                                                                                           baffle    wall
                                                                                                                                   had   a the SF
earthquake.
         in theeffect
        better     casesonofthe
                              tanks
                                 freewith    andelevations
                                       surface     without at thethe
                                                                   damping
                                                                      side wall,baffle.
                                                                                  underThe the results    showed that the vertical
                                                                                                SF earthquake.
            damping baffle had a better effect on the free surface elevations at the side wall, under the SF
            earthquake.

                             Figure 13. Location of the damping baffle in the water tank and mesh geometry.
                                  13. Location of the damping baffle in the water tank and mesh geometry.
                    Figure Figure
                           13. Location   of the damping baffle in the water tank and mesh geometry.
                  On the other hand, the sloshing pressure on the two measuring points with and without the
            damping baffle was compared. The two measuring points were located near the bottom and the
            free surface of the water tank, which were 1 m and 10 m away from the bottom, respectively. Then,
            the pressure variation at these two points is shown in Figure 15, and the sloshing pressures were
            normalized by p/ρgh-1 to express the fluctuation of sloshing pressure. Here, the ρgh was the hydrostatic
            pressure at the selected points. The results showed that the pressure variation ratio near the water
            surface was larger than that in the bottom area. The damping baffle had a significant suppression effect
            on the sloshing pressure near the free liquid surface, but this only occurred after the violent ground
            motion passed. The pressure and free surface elevation had similar trends due to the seismic excitation.
                  With the damping baffle, the maximum response of the sloshing surface and the pressure at the
            selected points P1 and P2 were suppressed by 16.8%, 5.0%, and 3.5% under the IV earthquake and
            37.2%, 0.5%, and 4.2% for the SF earthquake. Then, the conclusion can be drawn that the suppression
            of sloshing pressure by the vertical damping baffle was limited under the nonlinear seismic excitation,
            although it can suppress the water motion well. That is probably because the vertical baffle suppressed
            the violent sloshing through its function of resonant frequency tuning. Since the nonlinear seismic
            excitation and the structural differences in size and the filling level may cause the different pressure
            response and the water motion, the results of the baffle configuration in the previous sloshing study
Water 2020, 12, 2098                                                                                        11 of 14

cannot be applied directly to the seismic design of the tank. Therefore, analysis of the water motion
Water 2020, 12, x FOR PEER REVIEW                                                              11 of 14
and pressure within a water storage tank under an earthquake load is still needed.

      Figure 14. The suppression effect of the vertical damping baffle on sloshing water, (a) the free surface
      elevation under the Imperial Valley 1940 earthquake, (b) the free surface elevation under the San
      Fernando 1971 earthquake.

     On the other hand, the sloshing pressure on the two measuring points with and without the
damping baffle was compared. The two measuring points were located near the bottom and the free
surface of the water tank, which were 1 m and 10 m away from the bottom, respectively. Then, the
pressure variation at these two points is shown in Figure 15, and the sloshing pressures were
normalized by p/ρgh-1 to express the fluctuation of sloshing pressure. Here, the ρgh was the
hydrostatic pressure at the selected points. The results showed that the pressure variation ratio near
the water surface was larger than that in the bottom area. The damping baffle had a significant
     Figure 14.
suppression      Theon
              effect  suppression   effect
                         the sloshing      of the vertical
                                        pressure  near thedamping    baffle
                                                            free liquid     on sloshing
                                                                         surface,         water,
                                                                                   but this  only(a) the free after
                                                                                                   occurred
     Figure 14. The suppression effect of the vertical damping baffle on sloshing water, (a) the free surface
the violent ground motion passed. The pressure and free surface elevation had similar trendsthe
     surface elevation under  the Imperial Valley 1940  earthquake, (b) the free surface elevation under   due to
     elevation under the Imperial Valley 1940 earthquake, (b) the free surface elevation under the San
     San Fernando
the seismic         1971 earthquake.
             excitation.
     Fernando 1971 earthquake.

     On the other hand, the sloshing pressure on the two measuring points with and without the
damping baffle was compared. The two measuring points were located near the bottom and the free
surface of the water tank, which were 1 m and 10 m away from the bottom, respectively. Then, the
pressure variation at these two points is shown in Figure 15, and the sloshing pressures were
normalized by p/ρgh-1 to express the fluctuation of sloshing pressure. Here, the ρgh was the
hydrostatic pressure at the selected points. The results showed that the pressure variation ratio near
the water surface was larger than that in the bottom area. The damping baffle had a significant
suppression effect on the sloshing pressure near the free liquid surface, but this only occurred after
the violent ground motion passed. The pressure and free surface elevation had similar trends due to
the seismic excitation.

                        Figure 15.
                        Figure     The fluctuation
                               15. The fluctuation of
                                                   of sloshing
                                                      sloshing pressure
                                                               pressure at
                                                                        at selected
                                                                           selected points.
                                                                                    points.

4. Conclusions
     The water motion in a real-scale water storage tank under two well-studied ground motions
were investigated based on the OpenFOAM in this study. The experimental tests were conducted for
validation of the present numerical model. The larger tank size, the high filling level, and the nonlinear
seismic excitation were considered in this research. Other than the previous studies on sloshing in

                        Figure 15. The fluctuation of sloshing pressure at selected points.
Water 2020, 12, 2098                                                                                            12 of 14

a rectangular water tank, the results under the current conditions were slightly different in terms of
baffle configuration design, pressure response, and water sloshing.
      (1) A comparison between 2D and 3D results of the water sloshing in a cylindrical tank was
conducted. The 2D analysis could be used to predict the water sloshing response in a cylindrical tank.
The Oxy section gave the maximum free surface elevation of the sloshing, and the most dangerous
condition can be covered.
      (2) Due to the huge size of the water tank and the large mass of the filled water, the free surface
motion was small, even under the strong ground motion. The maximum free surface elevation did not
exceed 10%.
      (3) Similar to the response under the harmonic excitation, the free surface motion still satisfied the
sloshing modal characteristic under seismic excitation. The maximum free surface elevation appeared
at the side wall, and there were two nodes when the seismic frequency was close to the third-order
natural frequency of the water tank.
      (4) In addition, the water motion was hysteretic compared with the ground motion. The maximum
free surface elevation appeared after the seismic wave passed instead of the seismic wave was passing.
Meanwhile, the pressure response was different, which kept decreasing after the strong seismic
wave passed.
      (5) The effect of the well-used vertical damping baffle on the suppression of sloshing pressure was
limited when the strong ground motion was passing. Further study about the seismic design of the
water storage tank is still needed.

Author Contributions: Writing—original draft, H.J.; Writing—review and editing, R.S. and Y.L. All authors have
read and agreed to the published version of the manuscript.
Funding: This research was funded by the Natural Science Foundation of Zhejiang, grant number LQ19E090005,
the Natural Science Foundation of Ningbo, grant number 2019A610164 and National Natural Science Foundation
of China, grant number 51605431.
Conflicts of Interest: The authors declare no conflict of interest.

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