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-   -   maintaining a logarithmic velocity distribution (https://www.cfd-online.com/Forums/cfx/23496-maintaining-logarithmic-velocity-distribution.html)

 Morten Andersen January 8, 2007 04:57

maintaining a logarithmic velocity distribution

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

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:

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

Freestream Damping Option = First Order

Option = High Resolution

END

CONVERGENCE CONTROL:

Physical Timescale = physical

Timescale Control = Physical Timescale

END

END

EQUATION CLASS: momentum

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

 Joe January 8, 2007 12:37

Re: maintaining a logarithmic velocity distributio

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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