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August 21, 2012, 14:40 
FSI: Pressure and Normal Force don't match with expected values

#1 
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Hello everyone,
I'm currently working on slosh damping in a cylindrical tank. I try to set a twoways FSI model to study the impact of a vibrating latex membrane located on the bottom of the tank on freesurface waves. To start progressively, I am trying to run a simulation without any forced motion for the membrane. I want to check the deformation of this 1mm thick membrane due to the water weight. The problem is that the pressure in the bottom of the tank and the normal force on the interface are not what could be expected with a simple hydrostatic model. When I do the math, I obtain an expected 22N force and a +1545Pa relative pressure. But the simulation gives me: a 0.05N force and a 2Pa. I already performed a simulation with just the CFD and I obtained the expected values, and I keep the same CFD setup for the FSI simulation so I really don't know what is wrong. Any ideas are welcomed! Thank you, Geraud. 

August 21, 2012, 14:58 

#2 
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Edmund Singer P.E.
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Do you have gravity vector defined? And are you pulling out Absolute Pressure as your pressure?


August 21, 2012, 15:06 

#3 
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Yes, I have selected the buoyancy model for the fluid and I monitor the pressure (not the absolute pressure) with CFDPost, on the interface and in the fluid.
CEL: EXPRESSIONS: DenWater = 997 [kg m^3] InitPressure = DenWater*g*(UpHz)*VFWater UpH = 0.158[m] VFAir = 1VFWater VFWater = step((zUpH)/1[m]) END END MATERIAL: Air Ideal Gas Material Description = Air Ideal Gas (constant Cp) Material Group = Air Data, Calorically Perfect Ideal Gases Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Molar Mass = 28.96 [kg kmol^1] Option = Ideal Gas END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 1.0044E+03 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Specific Enthalpy = 0. [J/kg] Reference Specific Entropy = 0. [J/kg/K] Reference Temperature = 25 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 1.831E05 [kg m^1 s^1] Option = Value END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 2.61E2 [W m^1 K^1] END ABSORPTION COEFFICIENT: Absorption Coefficient = 0.01 [m^1] Option = Value END SCATTERING COEFFICIENT: Option = Value Scattering Coefficient = 0.0 [m^1] END REFRACTIVE INDEX: Option = Value Refractive Index = 1.0 [m m^1] END END END MATERIAL: Water Material Description = Water (liquid) Material Group = Water Data, Constant Property Liquids Option = Pure Substance Thermodynamic State = Liquid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 997.0 [kg m^3] Molar Mass = 18.02 [kg kmol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 4181.7 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Specific Enthalpy = 0.0 [J/kg] Reference Specific Entropy = 0.0 [J/kg/K] Reference Temperature = 25 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 8.899E4 [kg m^1 s^1] Option = Value END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 0.6069 [W m^1 K^1] END ABSORPTION COEFFICIENT: Absorption Coefficient = 1.0 [m^1] Option = Value END SCATTERING COEFFICIENT: Option = Value Scattering Coefficient = 0.0 [m^1] END REFRACTIVE INDEX: Option = Value Refractive Index = 1.0 [m m^1] END THERMAL EXPANSIVITY: Option = Value Thermal Expansivity = 2.57E04 [K^1] END END END END FLOW: Flow Analysis 1 SOLUTION UNITS: Angle Units = [rad] Length Units = [m] Mass Units = [kg] Solid Angle Units = [sr] Temperature Units = [K] Time Units = [s] END ANALYSIS TYPE: Option = Transient EXTERNAL SOLVER COUPLING: ANSYS Input File = ds.dat Option = ANSYS MultiField COUPLING TIME CONTROL: COUPLING INITIAL TIME: Option = Automatic END COUPLING TIME DURATION: Option = Total Time Total Time = 1 [s] END COUPLING TIME STEPS: Option = Timesteps Timesteps = 0.00001 [s] END END END INITIAL TIME: Option = Coupling Initial Time END TIME DURATION: Option = Coupling Time Duration END TIME STEPS: Option = Coupling Timesteps END END DOMAIN: Fluid Coord Frame = Coord 0 Domain Type = Fluid Location = part fluid BOUNDARY: InterfaceMembrane Boundary Type = WALL Location = interfacemembrane Shadow BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END MESH MOTION: ANSYS Interface = FSIN_1 Option = ANSYS MultiField Receive from ANSYS = Total Mesh Displacement Send to ANSYS = Total Force END END END BOUNDARY: Opening Boundary Type = OPENING Location = opening BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Entrainment Relative Pressure = 0 [Pa] END MESH MOTION: Option = Stationary END END FLUID: Air BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 1 END END END FLUID: Water BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 0 END END END END BOUNDARY: Wall Boundary Type = WALL Location = wall BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END MESH MOTION: Option = Stationary END END END DOMAIN MODELS: BUOYANCY MODEL: Buoyancy Reference Density = 1.185 [kg m^3] Gravity X Component = 0 [m s^2] Gravity Y Component = 0 [m s^2] Gravity Z Component = g Option = Buoyant BUOYANCY REFERENCE LOCATION: Option = Automatic END END DOMAIN MOTION: Option = Stationary END MESH DEFORMATION: Option = Regions of Motion Specified MESH MOTION MODEL: Option = Displacement Diffusion MESH STIFFNESS: Option = Increase near Small Volumes Stiffness Model Exponent = 10 END END END REFERENCE PRESSURE: Reference Pressure = 1 [atm] END END FLUID DEFINITION: Air Material = Air Ideal Gas Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID DEFINITION: Water Material = Water Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID MODELS: COMBUSTION MODEL: Option = None END FLUID: Air FLUID BUOYANCY MODEL: Option = Density Difference END END FLUID: Water FLUID BUOYANCY MODEL: Option = Density Difference END END HEAT TRANSFER MODEL: Fluid Temperature = 25 [C] Homogeneous Model = True Option = Isothermal END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = Laminar END END FLUID PAIR: Air  Water INTERPHASE TRANSFER MODEL: Option = None END MASS TRANSFER: Option = None END SURFACE TENSION MODEL: Option = None END END MULTIPHASE MODELS: Homogeneous Model = On FREE SURFACE MODEL: Option = Standard END END END INITIALISATION: Option = Automatic FLUID: Air INITIAL CONDITIONS: VOLUME FRACTION: Option = Automatic with Value Volume Fraction = VFAir END END END FLUID: Water INITIAL CONDITIONS: VOLUME FRACTION: Option = Automatic with Value Volume Fraction = VFWater END END END INITIAL CONDITIONS: Velocity Type = Cartesian CARTESIAN VELOCITY COMPONENTS: Option = Automatic with Value U = 0 [m s^1] V = 0 [m s^1] W = 0 [m s^1] END STATIC PRESSURE: Option = Automatic with Value Relative Pressure = InitPressure END END END OUTPUT CONTROL: MONITOR OBJECTS: MONITOR BALANCES: Option = Full END MONITOR FORCES: Option = Full END MONITOR PARTICLES: Option = Full END MONITOR POINT: Monitor Point 1 Cartesian Coordinates = 0 [m], 0 [m], 0.001 [m] Domain Name = Fluid Option = Cartesian Coordinates Output Variables List = Absolute Pressure END MONITOR RESIDUALS: Option = Full END MONITOR TOTALS: Option = Full END END RESULTS: File Compression Level = Default Option = Standard END TRANSIENT RESULTS: Transient Results 1 File Compression Level = Default Option = Standard OUTPUT FREQUENCY: Option = Every Timestep END END END SOLVER CONTROL: ADVECTION SCHEME: Option = High Resolution END CONVERGENCE CONTROL: Maximum Number of Coefficient Loops = 5 Minimum Number of Coefficient Loops = 1 Timescale Control = Coefficient Loops END CONVERGENCE CRITERIA: Residual Target = 1.E4 Residual Type = RMS END EXTERNAL SOLVER COUPLING CONTROL: COUPLING DATA TRANSFER CONTROL: Convergence Target = 1e2 Under Relaxation Factor = 0.75 END COUPLING STEP CONTROL: Maximum Number of Coupling Iterations = 10 Minimum Number of Coupling Iterations = 1 SOLUTION SEQUENCE CONTROL: Solve ANSYS Fields = Before CFX Fields END END END MULTIPHASE CONTROL: Volume Fraction Coupling = Coupled END TRANSIENT SCHEME: Option = Second Order Backward Euler TIMESTEP INITIALISATION: Option = Automatic END END END END COMMAND FILE: Version = 14.0 Results Version = 14.0 END SIMULATION CONTROL: EXECUTION CONTROL: EXECUTABLE SELECTION: Double Precision = Off END INTERPOLATOR STEP CONTROL: Runtime Priority = Standard MEMORY CONTROL: Memory Allocation Factor = 1.0 END END MFX RUN CONTROL: MFX RUN DEFINITION: MFX Run Mode = Start ANSYS and CFX Process ANSYS Input File = On Restart ANSYS Run = Off END MFX SOLVER CONTROL: ANSYS Installation Root = C:\Program Files\ANSYS Inc\v140\ansys END END PARALLEL HOST LIBRARY: HOST DEFINITION: lb13207 Remote Host Name = LB13207 Host Architecture String = winntamd64 Installation Root = C:\Program Files\ANSYS Inc\v%v\CFX END END PARTITIONER STEP CONTROL: Multidomain Option = Independent Partitioning Runtime Priority = Standard EXECUTABLE SELECTION: Use Large Problem Partitioner = Off END MEMORY CONTROL: Memory Allocation Factor = 1.0 END PARTITIONING TYPE: MeTiS Type = kway Option = MeTiS Partition Size Rule = Automatic Partition Weight Factors = 0.25000, 0.25000, 0.25000, 0.25000 END END RUN DEFINITION: Run Mode = Full Solver Input File = CFX.def END SOLVER STEP CONTROL: Runtime Priority = Standard MEMORY CONTROL: Memory Allocation Factor = 1.0 END PARALLEL ENVIRONMENT: Number of Processes = 4 Start Method = Platform MPI Local Parallel Parallel Host List = lb13207*4 END PROCESS COUPLING: Process Name = CFX Host Port = 50037 Host Name = LB13207 END END END END 

August 21, 2012, 15:11 

#4 
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Edmund Singer P.E.
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Concerning Pressure difference:
I believe the hyrdostatic portion of the pressure in CFX is carried in Absolute Pressure and not in Pressure. 

August 21, 2012, 15:14 

#5 
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Actually, I was just checking the absolute pressure instead and I obtain: 101328Pa at the interface and inside the fluid. So no sign of the water weight there too.


August 21, 2012, 15:25 

#6 
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Edmund Singer P.E.
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In my limited time I looked at this, I think:
VFAir = 1VFWater VFWater = step((zUpH)/1[m]) Based on VFWater the above CEL indicates to me that your z axis is pointing to the bottom of the screen. InitPressure = DenWater*g*(UpHz)*VFWater UpH = 0.158[m] and BUOYANCY MODEL: Buoyancy Reference Density = 1.185 [kg m^3] Gravity X Component = 0 [m s^2] Gravity Y Component = 0 [m s^2] Gravity Z Component = g Option = Buoyant BUOYANCY REFERENCE LOCATION: Option = Automatic END indicates that your z axis is pointing to the top of the screen. Fix them to be consistant. I suggest changing: VFWater = step((UpHz)/1[m]) 

August 21, 2012, 15:34 

#7 
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Oh! I really did that?!
Shame on me! I don't know how I could missed it. Thank you very much. Geraud. 

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