Author Topic: Is there a way to specify surface roughness while using the SST model?  (Read 1275 times)

Offline pitney1

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Is there a way to specify surface roughness while using the SST model? Is it possible to use scalable wall functions with these turbulence models?


Offline william

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Wall roughness is a hidden beta feature for omega_based turbulence. To activate this, a manual CCL edit of the Fluid Models section and each wall boundary is required.

First, activate the CFX 11.0 beta Features under Edit à Options_ CFX-Pre (see attached). Then, set SST up the default way and write out the .def file. Finally, extract the ccl from the .def file using the cfx5cmds executable and modify the ccl as shown below to activate scalable wall functions (if desired) and wall roughness height. (For instructions on the use of cfx5cmds, type "cfx5cmds help" in a CFX Commandline window (in Windows) or at the command prompt (Linux/Unix).)

Below are CCL examples using automatic and scalable wall functions respectively.


Automatic wall treatment
This is the default. In order to specify the automatic near wall treatment explicitly, the Option parameter in `TURBULENT WALL FUNCTIONS (under the FLUID MODELS section see attached) must be set to `Rough Wall Omega Automatic.

In the following, an example is given for a heated wall with automatic wall treatment:

BOUNDARY : Bottom
Boundary Type =WALL
Location = RCT B
BOUNDARY CONDITIONS :
WALL INFLUENCE ON FLOW :
Option = No Slip
TURBULENT WALL FUNCTIONS:
Option = Rough Wall Omega Automatic
END
END
WALL ROUGHNESS:
Option = Rough Wall
Roughness Height = 0.0005 [m]
END # WALL ROUGHNESS
HEAT TRANSFER:
Option = Fixed Temperature
Fixed Temperature = 8.E1 [K]
TURBULENT WALL FUNCTIONS:
Option = Rough Wall Omega Automatic
END
END # HEAT TRANSFER
END


Scalable wall functions
In order to specify the scalable wall functions, the Option parameter in `TURBULENT WALL FUNCTIONS must be set to `Rough Wall Omega Scalable. The example shows the parameters for a heated wall with scalable wall functions. The default value of the lower - limit (Minimum Yplus) is 2.5 and can be changed via the ccl:

BOUNDARY : Bottom
Boundary Type = WALL
Location = RCT B
BOUNDARY CONDITIONS :
WALL INFLUENCE ON FLOW :
Option = No Slip
TURBULENT WALL FUNCTIONS :
Option = Rough Wall Omega Scalable
Rough Wall Minimum Yplus = 2.5
END
END
WALL ROUGHNESS:Option = Rough Wall
Roughness Height = 0.0005 [m]
END # WALL ROUGHNESS
HEAT TRANSFER:
Option = Fixed Temperature
Fixed Temperature = 8.E1 [K]
TURBULENT WALL FUNCTIONS :
Option = Rough Wall Omega Scalable
END
END # HEAT TRANSFER
END

In addition, an expert parameter must be set to use the max-function for the computation of the effective diffusion coefficients.

EXPERT PARAMETERS :
effective viscosity type = 2
END