[sidharath]

Hello all,

I was wondering if someone can share their views briefly on the nature of the Wall adopted local eddy viscosity model (LES WALE) in CFX.

Is my understanding right:
1. It is an SGS model (explicit LES); improvement over the Smagorinsky model.

2. It is wall modelled method, an in walls are still modelled by RANS ?? or all the subgrid is modelled using WALE, including walls?

3. Are wall modelled really LES or just a 'larger resolved spectrum' hybrid RANS-LES models?

Thank you,
Sid

[ghorrocks]

I am no expert in WALE, but here goes:

1) The WALE model is often an improvement over Smagorinsky model, but not always. That is why you have the option to choose either.

2) No, it is a full LES model. It uses LES right to the wall.

3) Yes, this class of LES models (which includes WALE) are full LES models. They are not hybrid RANS-LES systems.

[sidharath]

Thank you Glenn!

So WALE is a wall-resolved LES model and algebraic eddy viscosity based sub-grid model is only used inside viscous sublayer?

Also, I was wondering what kind of flows are better resolved by Smagorinsky model as compared to WALE. I'll look this up but I was wondering if you know some from experience.

Thank you again, I couldn't decipher this from Ansys's documents.

Sid

[ghorrocks]

So WALE is a wall-resolved LES model

WALE stands for Wall-Adapting Local Eddy-viscosity. Not wall resolved.

algebraic eddy viscosity based sub-grid model is only used inside viscous sublayer?

No. The algebraic SGS model is applied everywhere.

I was wondering what kind of flows are better resolved by Smagorinsky model as compared to WALE. I'll look this up but I was wondering if you know some from experience.

I have never used WALE so cannot say from experience. I can only refer you to the CFX documentation (which states WALE is the best choice for most LES models) and the literature (which I have no idea what it says).

[sidharath]

Thanks again.
Last few things:

1. When I said wall resolved, I meant WALE is either wall resolved or wall modelled. As you said before that it solves right till the wall so, i'll assume it is wall resolved. Correct?

2. Assuming the grid and time step is fine enough, the scales would be resolved by LES and only in the viscous sublayer, the SGS model would inject the viscosity to model the 'molecular' dissipation. Is that right?

3. I understand from the literature that WALE takes care of non zero viscosity problem in laminar shear flow seen in Smagorinsky model; which i suppose would better the laminar-turbulent transition and therefore, a 'universal' improvement. I'll dig up and see if I can find some cases where WALE or dynamic SGS models are relatively underperforming.

Thank you again,
Sid

[ghorrocks]

1) Be very careful here, as you may be getting confused here. WALE is "Wall adapting Local Eddy-viscosity". It gets this name because in wall bounded laminar flows it returns a zero eddy viscosity and therefore can degenerate to a laminar flow. The name does not say anything about how the wall integration is handled (eg wall functions vs integration to the wall). So make sure you do not think that WALE does anything different in the wall region compared to Smagorinsky.

2) I do not understand this question.

If you are saying does the SGS model inject the viscosity to model the molecular dissipation - then you have missed the fundamental point of LES. You should read some background theory on LES to understand it. (Molecular dissipation is just normal viscosity and needs no special model. The SGS model is used to model the dissipation of the small turbulence scales which are isotropic and can be modelled with a simple SGS model.)

If you are asking if the SGS model only models molecular dissipation in the viscous sublayer near walls - The SGS is not intended to do this, but it is possible in some cases it would happen. It does not sound ideal, however.

3) Yes, WALE does appear better than Smagorinsky in general. But in my experience there will always be some cases where seemingly inferior models perform better. But this is just a guess from experience, I have no evidence to back this claim up.