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-   -   SST model with rough wall? (http://www.cfd-online.com/Forums/cfx/21877-sst-model-rough-wall.html)

David November 29, 2005 05:48

SST model with rough wall?
 
Hi, all

Through changing the CCL of wall roughness parameter, it could set the roughness height when SST model is employed, and the solver works normally. The results of simulated boundary layers with rough wall differ from the ones with smooth wall, and the former seem more reasonable.

But in some metarials it said that the representation of roughness in this model is not entirely consistent and straighforward. I think it maybe the reason why CFX could not set roughness under SST model. Who could give the internal explaining of this problem?

Mike November 29, 2005 12:07

Re: SST model with rough wall?
 
There's some extra parameters that need to be set in the CCL to make SST with rough walls consistent, which is why you cannot pick it in CFX-Pre. The CCL should be similar to the following:

BOUNDARY : MyWall Boundary Type = WALL Location = WALL BOUNDARY CONDITIONS :

WALL INFLUENCE ON FLOW :

Option = No Slip

END

WALL ROUGHNESS:

Option = Rough Wall

Roughness Height = 0.0002 [m]

END

TURBULENT WALL FUNCTIONS :

Option = Scalable

Minimum Yplus = 2.5

END END END

Be sure to include the Minimum Yplus parameter and use a value of 2.5, as shown. You will also need to set the following expert parameter:

FLOW: EXPERT PARAMETERS:

effective viscosity type = 2 END END

Details on why these are required would have to come from CFX Support. Regards, Mike


David November 29, 2005 20:26

Re: SST model with rough wall?
 
Mike, Thank you for your detail guide of setting the roughness parameter!

Sam November 30, 2005 00:37

Re: SST model with rough wall?
 
Hi Mike,

I thought the default value for min Y+ is about 11.06 if scalable wall function is used. How do you determine what value is appropriate anyway?

Mike November 30, 2005 12:22

Re: SST model with rough wall?
 
Hi Sam The default value is normally 11.06 as you say, and should usually be left alone. This value corresponds to the edge of the viscous sublayer for a smooth wall. However, rough walls break up the viscous sublayer so a different formulation is required for the min Y+. You shouldn't need to worry about determining what value is appropriate, but in this case the code doesn't automatically change the min Y+, which is one reason why you can't pick rough walls in CFX-Pre.


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