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What are the best settings for a channel flow simulation? 

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October 10, 2022, 23:15 
What are the best settings for a channel flow simulation?

#1 
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Ashkan Kashani
Join Date: Apr 2016
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Hello all,
I want to use CFX to study the transient flow around a rectangular bluff body placed in a 2D open channel as shown in Figure 1. For the simulation, I am using the Free Surface Homogenous Multiphase model to capture the waterair interface. The boundary conditions used are as follows (consult Figure 1 for labels): AE: water inlet (so Water Volume Fraction = 1 and Air Volume Fraction with a uniform Normal Speed of Uinlet = 35.2 cm/s. DE & AB: noslip wall BC: outlet boundary condition with the Average Static Pressure being set as a hydrostatic pressure for the water phase and zero for the air phase, which is realized by the following expression: PressureDist = WaterDensity * g * (yGlobal)*WaterVF DC: opening boundary condition, set by Opening Pres. and Dirc and a Relative Pressure of zero. And finally, the two side faces are set as Symmetry boundaries to realize a 2D simulation. Regarding initial conditions, I set the streamwise component of the velocity to Uinlet = 35.2 cm/s and the other two to zero. The top half of the channel is filled with the air phase (AirVF = step(yGlobal/1[m])) and the other half is filled with water at t = 0. The pressure field is also initialized by the same expression above. Note the content of the CCL file has been copied and pasted to the end of this thread for further details. I need to establish a 'uniform flow' (which, in my case, means constant depth and fully developed water velocity profile) within some certain distance upstream and downstream of the location where the bluff body is to be inserted in the model later. I need to ensure this location is not affected by the inlet and outlet boundaries. I was hoping that these settings would result in such a flow condition. However, as the transient solution continues, some strange waves, as shown in Figure 2, appear on the free surface and keep travelling back and forth with very slow attenuation. Apparently, it takes a very long simulation time for these waves to die down so that the desired uniform flow would take place. This makes the simulation very burdensome since I am not interested in this transient part of the solution. I believe the problem stems from the inconsistency between the boundary conditions and the flow physics in the domain, or it might be related to the initial conditions. However, I am unsure what could be a better setting for this problem. I would appreciate any comments. P.S. # State file created: 2022/10/10 20:58:47 # Build 18.2 20170714T23:24:21.554000 LIBRARY: CEL: EXPRESSIONS: AirVF = step((yGlobal)/1[m]) InletElev = (areaInt(Water.Volume \ Fraction)@Inlet_water+areaInt(Water.Volume Fraction)@Inlet_air)/0.0003 OutletElev = (areaInt(Water.Volume Fraction)@Outlet)/0.0003 PressureDist = WaterDensity *g*((yGlobal))*WaterVF Uinlet = 35.2[cm/s] WaterDensity = 997[kg m^3] WaterVF = 1AirVF 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: Air at 25 C Material Description = Air at 25 C and 1 atm (dry) Material Group = Air Data, Constant Property Gases Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Density = 1.185 [kg m^3] Molar Mass = 28.96 [kg kmol^1] Option = Value 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 = 2.61E02 [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 THERMAL EXPANSIVITY: Option = Value Thermal Expansivity = 0.003356 [K^1] END END END MATERIAL: Aluminium Material Group = CHT Solids, Particle Solids Option = Pure Substance Thermodynamic State = Solid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 2702 [kg m^3] Molar Mass = 26.98 [kg kmol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 9.03E+02 [J kg^1 K^1] END REFERENCE STATE: Option = Specified Point Reference Specific Enthalpy = 0 [J/kg] Reference Specific Entropy = 0 [J/kg/K] Reference Temperature = 25 [C] END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 237 [W m^1 K^1] END END END MATERIAL: Copper Material Group = CHT Solids, Particle Solids Option = Pure Substance Thermodynamic State = Solid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 8933 [kg m^3] Molar Mass = 63.55 [kg kmol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 3.85E+02 [J kg^1 K^1] END REFERENCE STATE: Option = Specified Point Reference Specific Enthalpy = 0 [J/kg] Reference Specific Entropy = 0 [J/kg/K] Reference Temperature = 25 [C] END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 401.0 [W m^1 K^1] END END END MATERIAL: Soot Material Group = Soot Option = Pure Substance Thermodynamic State = Solid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 2000 [kg m^3] Molar Mass = 12 [kg kmol^1] Option = Value END REFERENCE STATE: Option = Automatic END ABSORPTION COEFFICIENT: Absorption Coefficient = 0 [m^1] Option = Value END END END MATERIAL: Steel Material Group = CHT Solids, Particle Solids Option = Pure Substance Thermodynamic State = Solid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 7854 [kg m^3] Molar Mass = 55.85 [kg kmol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 4.34E+02 [J kg^1 K^1] END REFERENCE STATE: Option = Specified Point Reference Specific Enthalpy = 0 [J/kg] Reference Specific Entropy = 0 [J/kg/K] Reference Temperature = 25 [C] END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 60.5 [W m^1 K^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 MATERIAL: Water Ideal Gas Material Description = Water Vapour Ideal Gas (100 C and 1 atm) Material Group = Calorically Perfect Ideal Gases, Water Data Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Molar Mass = 18.02 [kg kmol^1] Option = Ideal Gas END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 2080.1 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1.014 [bar] Reference Specific Enthalpy = 0. [J/kg] Reference Specific Entropy = 0. [J/kg/K] Reference Temperature = 100 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 9.4E06 [kg m^1 s^1] Option = Value END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 193E04 [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 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: Option = None END INITIAL TIME: Option = Automatic with Value Time = 0 [s] END TIME DURATION: Option = Total Time Total Time = 40 [s] END TIME STEPS: Option = Timesteps Timesteps = 0.01 [s] END END DOMAIN: Default Domain Coord Frame = Coord 0 Domain Type = Fluid Location = B32 BOUNDARY: Bottom Boundary Type = WALL Location = Bottom BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: Inlet_air Boundary Type = WALL Location = Inlet_air BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: Inlet_water Boundary Type = INLET Location = Inlet_water BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Normal Speed = Uinlet Option = Normal Speed END TURBULENCE: Option = Medium Intensity and Eddy Viscosity Ratio END END FLUID: Air BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 0 END END END FLUID: Water BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 1 END END END END BOUNDARY: Outlet Boundary Type = OUTLET Location = Outlet BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Average Static Pressure Pressure Profile Blend = 0.05 Relative Pressure = PressureDist END PRESSURE AVERAGING: Option = Average Over Whole Outlet END END END BOUNDARY: Sym1 Boundary Type = SYMMETRY Location = Sym1 END BOUNDARY: Sym2 Boundary Type = SYMMETRY Location = Sym2 END BOUNDARY: Top Boundary Type = OPENING Location = Top BOUNDARY CONDITIONS: FLOW DIRECTION: Option = Normal to Boundary Condition END FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Opening Pressure and Direction Relative Pressure = 0 [atm] END TURBULENCE: Option = Medium Intensity and Eddy Viscosity Ratio 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 DOMAIN MODELS: BUOYANCY MODEL: Buoyancy Reference Density = 1.125 [kg m^3] Gravity X Component = 0 [m s^2] Gravity Y Component = g Gravity Z Component = 0 [m s^2] Option = Buoyant BUOYANCY REFERENCE LOCATION: Option = Automatic END END DOMAIN MOTION: Option = Stationary END MESH DEFORMATION: Option = None END REFERENCE PRESSURE: Reference Pressure = 1 [atm] END END FLUID DEFINITION: Air Material = Air at 25 C 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 = Off Option = Isothermal END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = k epsilon BUOYANCY TURBULENCE: Option = None END END TURBULENT WALL FUNCTIONS: Option = Scalable END END FLUID PAIR: Air  Water INTERPHASE TRANSFER MODEL: Option = Free Surface 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 = AirVF END END END FLUID: Water INITIAL CONDITIONS: VOLUME FRACTION: Option = Automatic with Value Volume Fraction = WaterVF END END END INITIAL CONDITIONS: Velocity Type = Cartesian CARTESIAN VELOCITY COMPONENTS: Option = Automatic with Value U = Uinlet V = 0 [m s^1] W = 0 [m s^1] END STATIC PRESSURE: Option = Automatic with Value Relative Pressure = PressureDist END TURBULENCE INITIAL CONDITIONS: Option = Medium Intensity and Eddy Viscosity Ratio 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 Coord Frame = Coord 0 Expression Value = OutletElev Option = Expression END MONITOR POINT: Monitor Point 2 Coord Frame = Coord 0 Expression Value = InletElev Option = Expression END MONITOR RESIDUALS: Option = Full END MONITOR TOTALS: Option = Full END END RESULTS: File Compression Level = Default Option = Standard Output Equation Residuals = All END TRANSIENT RESULTS: Transient Results 1 File Compression Level = Default Option = Standard OUTPUT FREQUENCY: Option = Time Interval Time Interval = 0.1 [s] END END END SOLVER CONTROL: Turbulence Numerics = First Order ADVECTION SCHEME: Option = High Resolution END CONVERGENCE CONTROL: Maximum Number of Coefficient Loops = 25 Minimum Number of Coefficient Loops = 1 Timescale Control = Coefficient Loops END CONVERGENCE CRITERIA: Residual Target = 0.00001 Residual Type = RMS END TRANSIENT SCHEME: Option = Second Order Backward Euler TIMESTEP INITIALISATION: Option = Automatic END END END END COMMAND FILE: Version = 18.2 END 

October 11, 2022, 00:07 

#2 
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Glenn Horrocks
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Where did you get the time step size of 0.1s from? I would use adaptive timestepping, homing in on 35 coeff loops pre iteration and with max and min time step sizes wide enough you do not hit them. Then CFX will find its own time step size.
In my experience in free surface models boundaries with variable pressure applied are problematic. If you can replace these with simpler boundaries that will make things a lot easier.
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October 13, 2022, 11:37 

#3 
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Ashkan Kashani
Join Date: Apr 2016
Posts: 42
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Thank you for the reply!
I took your advice. However, after many trials, I concluded that there is no way to establish a flat free surface anywhere along an openchannel using the multiphase model; no matter how far the inlet and outlet are moved away from the section of interest, there are always some sorts of waves that never stop to affect the region of interest. I am not sure if these waves are physical or numerical, but it seems it takes so many timesteps for them to die down if they do so at all. That's why I decided to replace the multiphase model with a simpler singlephase model by treating the free surface as a freeslip wall condition, as shown in Figure 1 along with other boundary conditions. Figure 2 shows the experimental setup with which I am trying to compare the CFD model results for verification. As shown in Figure 2, pressure distribution underneath the block has been measured using manometer tubes in the lab. Now here is my question: Under what boundary conditions, are the results from the CFD model comparable to the experiments? What CFD output variable should be compared to the pressure values measured using manometer tubes in the lab, as shown in Figure 2? 

October 13, 2022, 21:36 

#4  
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Glenn Horrocks
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Quote:
This is too broad a question to be useful. It is a bit like asking a builder "How do you build a house?" Quote:
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