# maintaining a logarithmic velocity distribution

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 January 8, 2007, 04:57 maintaining a logarithmic velocity distribution #1 Morten Andersen Guest   Posts: n/a Hello cfx-users I am reasonably inexperienced cfx user and I have a problem that I hope somebody can help me with. I would like to maintain a given logarithmic velocity distribution through a wind tunnel. My problem consists of four things 1: The velocity distribution at the inlet after the simulation does not match the distribution I have defined. 2: I have a problem maintaining a logarithmic velocity profile through a wind tunnel. This is strange since the velocity distribution at the inlet is defined according to the law of the wall defined in the cfx manual (Theory)(Eqn. 400). 3: The distribution of k and epsilon in the domain does not match the distribution defined at the inlet. 4: Is it posible to define the friction velocity in the domain or is it always calculated according to Eqn. 387 (Theory manual) The wind tunnel: The width of the wind tunnel is 2.7m, the height is 9.1m and the lenght is 20m. I have used a structured net distribution with 4 nodes perpendicular to the wind direction 20 nodes parallel to the vind dicektion and 35 nodes on the vertical edges. The control function used on the vertical edges is hyperbolic with the spacing at the bottom chosen to 0.01m and the spacing at the top chosen to be free. The control functions used on the remaining edges are uniform. I have used a k-epsilon turbulence model. In the outfile it is posible to see the boundary conditions chosen. Regards Morten This run of the CFX-10.0 Solver started at 9:10:23 on 8 Jan 2007 by user Morten Andersen on QUISTGAARD (intel_pentium_winnt5.1) using the command: "C:\Program Files\Ansys Inc\CFX\CFX-10.0\bin\perllib\cfx5solve.pl" -stdout-comms -batch -ccl - Setting up CFX-5 Solver run ... +--------------------------------------------------------------------+ | | | CFX Command Language for Run | | | +--------------------------------------------------------------------+ LIBRARY: CEL: EXPRESSIONS: C = 5.0 Cmy = 0.09 Uf = 0.1011[m s^-1] my = 1.79e-5 [Pa s] rho = 1.2 [kg m^-3] yR = 0.01 [m] Kplus = yR*rho*Uf/my error = 0.0000001 [m] Ystjerne = rho*Uf*(y+error)/my karmans = 0.41 Uz = Uf/karmans*loge(Ystjerne/(1+0.3*Kplus)) kinl = Uf^2/sqrt(Cmy) epsiloninl = (Cmy*kinl^2)/(Uf*karmans*(y+error)) local = 5 physical = 50 [s] 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 ABSORPTION COEFFICIENT: Absorption Coefficient = 0.01 [m^-1] Option = Value END DYNAMIC VISCOSITY: Dynamic Viscosity = 1.831E-05 [kg m^-1 s^-1] Option = Value END EQUATION OF STATE: Molar Mass = 28.96 [kg kmol^-1] Option = Ideal Gas END REFRACTIVE INDEX: Option = Value Refractive Index = 1.0 [m m^-1] END SCATTERING COEFFICIENT: Option = Value Scattering Coefficient = 0.0 [m^-1] END SPECIFIC HEAT CAPACITY: Option = Value Reference Pressure = 1 [atm] Reference Specific Enthalpy = 0. [J/kg] Reference Specific Entropy = 0. [J/kg/K] Reference Temperature = 25 [C] Specific Heat Capacity = 1.0044E+03 [J kg^-1 K^-1] Specific Heat Type = Constant Pressure END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 2.61E-2 [W m^-1 K^-1] END END END END EXECUTION CONTROL: PARALLEL HOST LIBRARY: HOST DEFINITION: quistgaard Installation Root = C:\Program Files\ANSYS Inc\CFX\CFX-%v Host Architecture String = intel_pentium_winnt5.1 END END PARTITIONER STEP CONTROL: Multidomain Option = Independent Partitioning Runtime Priority = Standard MEMORY CONTROL: Memory Allocation Factor = 1.0 END PARTITIONING TYPE: MeTiS Type = k-way Option = MeTiS Partition Size Rule = Automatic END END RUN DEFINITION: Definition File = C:/Morten/uni/CFX/hastighedsprofil potens lang \ tunel/prefiles/pre.def Interpolate Initial Values = Off Run Mode = Full END SOLVER STEP CONTROL: Runtime Priority = Standard EXECUTABLE SELECTION: Double Precision = Off END MEMORY CONTROL: Memory Allocation Factor = 1.0 END PARALLEL ENVIRONMENT: Number of Processes = 1 Start Method = Serial END END END FLOW: DOMAIN: Domain 1 Coord Frame = Coord 0 Domain Type = Fluid Fluids List = Air Ideal Gas Location = Assembly BOUNDARY: inl Boundary Type = INLET Location = INL BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Normal Speed = Uz Option = Normal Speed END TURBULENCE: Epsilon = epsiloninl Option = k and Epsilon k = kinl END END END BOUNDARY: out Boundary Type = OUTLET Location = OUT BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Average Static Pressure Relative Pressure = 0 [Pa] END PRESSURE AVERAGING: Option = Average Over Whole Outlet END END END BOUNDARY: left Boundary Type = WALL Location = LEFT BOUNDARY CONDITIONS: WALL INFLUENCE ON FLOW: Option = Free Slip END END END BOUNDARY: right Boundary Type = WALL Location = RIGHT BOUNDARY CONDITIONS: WALL INFLUENCE ON FLOW: Option = Free Slip END END END BOUNDARY: bottom Boundary Type = WALL Location = BOTTOM BOUNDARY CONDITIONS: WALL INFLUENCE ON FLOW: Option = No Slip END WALL ROUGHNESS: Option = Rough Wall Roughness Height = yR END END END BOUNDARY: top Boundary Type = INLET Location = TOP BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Cartesian Velocity Components U = 0 [m s^-1] V = 0 [m s^-1] W = Uz END TURBULENCE: Epsilon = epsiloninl Option = k and Epsilon k = kinl END END END DOMAIN MODELS: BUOYANCY MODEL: Option = Non Buoyant END DOMAIN MOTION: Option = Stationary END REFERENCE PRESSURE: Reference Pressure = 1 [atm] END END FLUID MODELS: COMBUSTION MODEL: Option = None END HEAT TRANSFER MODEL: Fluid Temperature = 293 [K] Option = Isothermal END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = k epsilon END TURBULENT WALL FUNCTIONS: C Coefficient = C Option = Scalable END END INITIALISATION: Option = Automatic INITIAL CONDITIONS: Velocity Type = Cartesian CARTESIAN VELOCITY COMPONENTS: Option = Automatic END EPSILON: Epsilon = epsiloninl Option = Automatic with Value END K: Option = Automatic with Value k = kinl END STATIC PRESSURE: Option = Automatic END END END END OUTPUT CONTROL: RESULTS: File Compression Level = Default Option = Standard END END SIMULATION TYPE: Option = Steady State END SOLUTION UNITS: Angle Units = [rad] Length Units = [m] Mass Units = [kg] Solid Angle Units = [sr] Temperature Units = [K] Time Units = [s] END SOLVER CONTROL: ADVECTION SCHEME: Option = High Resolution END CONVERGENCE CONTROL: Maximum Number of Iterations = 100 Physical Timescale = physical Timescale Control = Physical Timescale END CONVERGENCE CRITERIA: Residual Target = 1e-10 Residual Type = RMS END DYNAMIC MODEL CONTROL: Global Dynamic Model Control = On END EQUATION CLASS: continuity ADVECTION SCHEME: Freestream Damping Option = First Order Option = High Resolution END CONVERGENCE CONTROL: Physical Timescale = physical Timescale Control = Physical Timescale END END EQUATION CLASS: momentum ADVECTION SCHEME: Freestream Damping Option = First Order Option = High Resolution END CONVERGENCE CONTROL: Physical Timescale = physical Timescale Control = Physical Timescale END END END END COMMAND FILE: Version = 10.0 Results Version = 10.0 END

 January 8, 2007, 12:37 Re: maintaining a logarithmic velocity distributio #2 Joe Guest   Posts: n/a My problem consists of four things 1: The velocity distribution at the inlet after the simulation does not match the distribution I have defined. 2: I have a problem maintaining a logarithmic velocity profile through a wind tunnel. This is strange since the velocity distribution at the inlet is defined according to the law of the wall defined in the cfx manual (Theory)(Eqn. 400). 3: The distribution of k and epsilon in the domain does not match the distribution defined at the inlet. This is a known problem and is probably what is causing 1 & 2. The wind tunnel: The width of the wind tunnel is 2.7m, the height is 9.1m and the lenght is 20m. I have used a structured net distribution with 4 nodes perpendicular to the wind direction 20 nodes parallel to the vind dicektion and 35 nodes on the vertical edges. The control function used on the vertical edges is hyperbolic with the spacing at the bottom chosen to 0.01m and the spacing at the top chosen to be free. The control functions used on the remaining edges are uniform. Your mesh is WAAAAAAAY too coarse. Do a mesh independance study. This is contributing to problems 1-3. Read the manual section on "near wall meshing".

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