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Inaccurate RANS simulations when using Wall-Functions at Re<= 4 10^6

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Old   April 22, 2019, 11:42
Default Inaccurate RANS simulations when using Wall-Functions at Re<= 4 10^6
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Hi Foamers!
This is the first time asking for help in FOAM community, I hope someone out-there could help me.

I' am currently simulating the flow over a submarine. The mesh is structured and is high quality. Re number, for the case #1, is 10 M. I used firstly wall-resolving approach. The results are pretty good and well-aligned with literature. In this case no problems.
I modified the mesh in order to use wall-functions and, at this Reynolds, I got very good results compared with wall-resolving approach. The only thing to mention is initial residuals of p-equations do not go down more than 4 10^-05.

I want to run LES on this geometry and for this reason I decided to move to lower Re numbers.

In the beginning I moved to Re= 1 10^6. The simulation "wall-resolved" converges fine and results are ok. When I use wall-functions p-equation residuals do not go down 10^-3. Wall functions seems to have problems to properly resolve the flow close to the sail (looking at y+ contours), and the forces differs from the case "wall-resolved" of 10-20%, both the pressure and viscous component. I've tried many many combinations of numerical schemes for the calculations of gradients without any improvement.
I though the problem was the low Reynolds: a different physics which could be more difficult to catch using wall-functions. I think it makes sense.

So I switched to Re 4 10^6. Same story.

So, for Re 10 10^6, no problem when using wall-functions. For Re 4 and 1 million I can't get a good solutions using wall-functions.

Let me give you some more details about the simulation. We are talking about steady-state RANS simulation. Angle of attach 0 deg.

When using wall functions I use Spalding's continuous formulation and <y+> is 25 on the submarine's hull; on sail and rudders has peaks of 40.

Turbulence is modeled using a classic k-omega SST. I use SIMPLEC (so consistent set to "yes" in SIMPLE dictionary), but I tried also SIMPLE or PISO p-v-coupling.

In attachments you can find fvSchemes, fvSolutions and checkMesh Log file.

I'm going to try some different schemes for convective terms, LUST or linearLimited 0.5 /1 , for instance.

Do you have suggestions? Is someone an expert in using wall-function for such flows and could correct me if I'm doing something wrong? Did someone already faced this problem?
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Old   April 24, 2019, 10:35
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An interesting situation.

I have done some submarine validation work with suboff and got very good agreement at Re 14e06 with a high Re mesh in the pasat. But I did not test it at lower Re values.

On the other hand, I think the wall functions must work also well at lower Re values unless yPlus distribution on the hull and sail is very bad. You said y+ was 25 on the hull, was it average or max value?

There may also be a problem on the boundary and the inital conditons for the wall-function case.

Could you try any other turbulence model, specially SpalartAllmaras and LienCubicKE?
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Old   April 24, 2019, 10:51
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Yes, I said <y+> is 25, which is the average. The max y+ is about 40. "There may also be a problem on the boundary and the initial conditions for the wall-function case." // They are the same (except at the wall) for wall-resolved case. There is any chance I need to change my BCs for lower Re and Wall-functions? I think not. I can try a SA. In the past I've done few simulation on this geometry using SA but it is less accurate compared to k-omega sst. I also tried a realizable k-epsilon without success (it means there was something wrong in setup for epsilon magnitude) and I should conclude that case. I'll let you know with SA if there is any improvement.

By the way, LUST scheme for convective term is unstable. I have switched on limiters (coefficient equal to 1) for gradients and also for laplacian schemes and in that case residuals of p-equation decreased till 5E-05 and solution converged. Forces were 35% bigger that WR. I would like to understand what's going on and how to properly set-up the case.
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numerical schemes, rans, reynolds number, wall-functions

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