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k-omega SST near-wall mesh

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Old   April 16, 2014, 07:56
Default k-omega SST near-wall mesh
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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 pressure-based solver, steady, k-omega SST for turbulence.

I have doubts concerning the mesh I should generate: I have noticed that Fluent doesn't offer the choice between wall-law/low-Re concerning the near-wall modeling, and I assumed that it picked the appropriate model, according to the Y+ of the near-wall layer.

This extract from the Fluent User's Guide seems to confirm my hypothesis:

"The wall boundary conditions for the k equation in the k-omega models are treated in the same way as the equation is treated when enhanced wall treatments are used with the k-epsilon models. This means that all boundary conditions for wall-function meshes will correspond to the wall function approach, while for the fine meshes, the appropriate low-Reynolds number boundary conditions will be applied."

In order to generate lighter meshes, I chose the wall-law 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 k-omega SST model is supposed to be used only with low-Re meshes, and that If I wanted to keep the cell count low, I had to switch to the k-epsilon or standard k-omega models.

I'd like to stick with the k-omega SST since it should combine the strengths of the k-epsilon and std k-omega models... could someone clarify the point concerning the meshes? Am I obliged to generate low Re grids if I want to use the K-omega SST model?

Thanks!

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Old   April 17, 2014, 18:19
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Not necessarily. The thing is that k-omega is good for near-wall modeling with a y+ of around 1 while k-epsilon is good for fully turbulent free stream flow. Thus, k-omega lacks accuracy when applied to fully turbulent farfield flow whereas k-epsilon models the near-wall region with wall functions.
The k-w SST model combines the advantages of both approaches as it includes a blending function (F1) that switches from k-w in the nearfield to k-e 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 k-w SST model, you won't take advantage of the accurate boundary layer resolution of the k-w model as you would effectively only make use of the k-e model.
What I want to say is: If you are using the k-w SST model with a y+ of 30, you could also just use the realizable k-e with enhanced wall treatment as it wouldn't really make any difference.
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Old   April 18, 2014, 03:54
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I understand, thanks a lot!
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