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k-epsilon on very fine mesh.

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Old   May 15, 2009, 03:28
Default k-epsilon on very fine mesh.
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mehul
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Hey friends,

I just want to know that what will be the effect if I am using very fine mesh near wall, where y plus is less that 30. I am using k-epsilon model in parallel. What will be the effect on results? Will k-epsilon work? Because k-epsilon considers that, the first layer starts from y plus=30.
Kindly suggest the answer.:co nfused:
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Old   May 15, 2009, 15:12
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When your yplus for the first grid point is less than 30 and you are not using near wall damping with the k-epsilon model, the model might be stable due to the amount of dissipation in the numerical scheme but you are not going to get accurate results.
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Old   May 18, 2009, 04:57
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Hello harishg,

thank you for your prompt reply.

Did you mean to say that, dissipation in k-epsilon is always less. and it does not depend upon mesh??
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Old   May 18, 2009, 06:29
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Quote:
Originally Posted by mehn View Post
Hey friends,
I just want to know that what will be the effect if I am using very fine mesh near wall, where y plus is less that 30.
It depends on the wall treatment: some implementations will handle the transition into the laminar sublayer and some will not. Hopefully it is documented for your code possibly under "wall functions".
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Old   May 18, 2009, 10:25
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What happens is in the viscous region (yplus < 5), the laminar part dominates and the turbulent stresses should vanish. When you do not damp the k-e model, the turbulent stresses are still present in the near wall region and they change your solution and sometimes depending on the model and numerical method the solution can remain stable but produce wrong results. If it is mentioned in your code that yplus should be greater than 30, it dos not have wall damping inbuilt. You can add model dependent wall damping function to integrate into the viscous sublayer or just live with the wall function and have yplus >=30. The problem with wall function is that they cannot be applied for separated flows to obtain accurate results.
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Old   May 19, 2009, 05:10
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I see.

thank you harishg
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