# Boundary Layer Low Re Turbulent Flow

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 February 5, 2013, 09:52 Boundary Layer Low Re Turbulent Flow #1 Member   Join Date: Apr 2011 Posts: 56 Rep Power: 15 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

 February 7, 2013, 00:36 #2 New Member   Mohammed Join Date: Feb 2013 Posts: 12 Rep Power: 13 Maybe you have reached convergence if stall at high residual value. At what residual value it stalls before refining and after refining?

February 7, 2013, 07:30
#3
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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

Quote:
 Originally Posted by Mohd123 Maybe you have reached convergence if stall at high residual value. At what residual value it stalls before refining and after refining?

February 8, 2013, 05:06
#4
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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.

Quote:
 Originally Posted by RSilva 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

February 8, 2013, 05:28
#5
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Filippo Maria Denaro
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Quote:
 Originally Posted by RSilva 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

 Tags boundary layer mesh, k-epsilon turbulence, low reynolds