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April 16, 2014, 07:56 
komega SST nearwall mesh

#1 
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Join Date: Feb 2014
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Hello everyone,
I'm doing calculations on an air inlet (a pipe, basically), the objective being the evaluation of the total pressure losses at a certain distance. I use the pressurebased solver, steady, komega SST for turbulence. I have doubts concerning the mesh I should generate: I have noticed that Fluent doesn't offer the choice between walllaw/lowRe concerning the nearwall modeling, and I assumed that it picked the appropriate model, according to the Y+ of the nearwall layer. This extract from the Fluent User's Guide seems to confirm my hypothesis: "The wall boundary conditions for the k equation in the komega models are treated in the same way as the equation is treated when enhanced wall treatments are used with the kepsilon models. This means that all boundary conditions for wallfunction meshes will correspond to the wall function approach, while for the fine meshes, the appropriate lowReynolds number boundary conditions will be applied." In order to generate lighter meshes, I chose the walllaw approach, and I made sure that the Y+ was always between 30 and 200, as close as possible to 30. My professor, however, just told me that the komega SST model is supposed to be used only with lowRe meshes, and that If I wanted to keep the cell count low, I had to switch to the kepsilon or standard komega models. I'd like to stick with the komega SST since it should combine the strengths of the kepsilon and std komega models... could someone clarify the point concerning the meshes? Am I obliged to generate low Re grids if I want to use the Komega SST model? Thanks! 

April 17, 2014, 18:19 

#2 
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Alex
Join Date: Jan 2014
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Not necessarily. The thing is that komega is good for nearwall modeling with a y+ of around 1 while kepsilon is good for fully turbulent free stream flow. Thus, komega lacks accuracy when applied to fully turbulent farfield flow whereas kepsilon models the nearwall region with wall functions.
The kw SST model combines the advantages of both approaches as it includes a blending function (F1) that switches from kw in the nearfield to ke in the freestream flow. This is obviously also a function of the local y+ value. If you are using a y+ of 30 and the kw SST model, you won't take advantage of the accurate boundary layer resolution of the kw model as you would effectively only make use of the ke model. What I want to say is: If you are using the kw SST model with a y+ of 30, you could also just use the realizable ke with enhanced wall treatment as it wouldn't really make any difference. 

April 18, 2014, 03:54 

#3 
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I understand, thanks a lot!


November 7, 2014, 11:15 
OMGA on the wall in SST

#4 
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Ali.E
Join Date: Sep 2010
Location: Lisboa
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Dear all,
I am working with Fluent and I have simulated a flat plate and the mesh is enough refine (Y+<=1). The turbulence model that I use is kw SST and I uncheck LowReynoldsNumber in the turbulence panel. My goal is to obtain the equation that Fluent uses for OMEGA value on the wall surface. Therefore, I compared the omega value on the plate surface obtained by Fluent and mine; but they are not the same and they are exactly different. I used this equation: OMG=sqrt(OMG_vis^2+OMG_log^2) OMG_vis=6*ro*(U_tau^2)/(0.075*Mu*Yplus^2) OMG_log=ro*(U_tau^2)/(0.3*k*Mu*Yplus) ro=dinsity U_tau=friction velosity Mu=viscosity k=0.41 It would be appriciated if you help me about the equation. How can I get the same OMGA value with Fluent? 

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