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Old   February 5, 2013, 10:52
Question Boundary Layer Low Re Turbulent Flow
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R. Silva
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Hi folks,

I'm implementing a Low Re k-ep model and, as mentioned in the forum on several occasions, I have to refine the mesh heavily near the wall.

I have a boundary layer, and I have used the CFD tools for accessing the distance for the first layer. In my case it's 5E-6 m, which I think is very close to the numeric minimum value of the software.

My question is this: as I refine the mesh near the wall, I get a problem in the convergence. The k and ep variables just stall at a certain residual value.

Has anyone experienced this?

Best Regards,

Rui Silva
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Old   February 7, 2013, 01:36
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Mohammed
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Maybe you have reached convergence if stall at high residual value.

At what residual value it stalls before refining and after refining?
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Old   February 7, 2013, 08:30
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R. Silva
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Thank you for your reply Mohd123,

Before refining I get to about 10^-3 residual (standard for COMSOL) and after refining the value is about 10^-2.

Could it be that the boundary layer mesh is to fine and causing numerical diffusion?

Best Regards,

Rui

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Maybe you have reached convergence if stall at high residual value.

At what residual value it stalls before refining and after refining?
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Old   February 8, 2013, 06:06
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Mohammed
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The best thing I suggest to do to make sure that mesh is totally independent of solution is to do a Grid Independence Study, so that you keep refining the mesh (from course to fine) until you reach a contant solution and the more you refine the mesh the solution will be constant.


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Originally Posted by RSilva View Post
Thank you for your reply Mohd123,

Before refining I get to about 10^-3 residual (standard for COMSOL) and after refining the value is about 10^-2.

Could it be that the boundary layer mesh is to fine and causing numerical diffusion?

Best Regards,

Rui
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Old   February 8, 2013, 06:28
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Filippo Maria Denaro
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Quote:
Originally Posted by RSilva View Post
Thank you for your reply Mohd123,

Before refining I get to about 10^-3 residual (standard for COMSOL) and after refining the value is about 10^-2.

Could it be that the boundary layer mesh is to fine and causing numerical diffusion?

Best Regards,

Rui
Hi,
for any consistent and stable discretization, in no way a grid refinement can cause an increasing in the numerical diffusion! Try check your iteration parameters
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boundary layer mesh, k-epsilon turbulence, low reynolds

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