# Abuout turbulent viscosity

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July 19, 2012, 02:46
Abuout turbulent viscosity
#1
New Member

Zhe Lu
Join Date: Apr 2012
Posts: 12
Rep Power: 7
Hi everyone:
I'm calculating a case with an airfoil flying in ground effect. The height, h/c, is 0.01, and Re is about 2e7. Everything looks alright, except that the turbulent viscosity is over the default limit of 1e5 in some cells(red ones in the picture below).
And there are no similar phenomena in other fields like turbulent kinetic energy, velocity, etc. So, anybody has any suggestions? Thanks in advance!
Attached Images
 turb vis rat.jpg (47.1 KB, 19 views)

 July 19, 2012, 02:52 #2 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,650 Rep Power: 26 I have some questions about your setup. The tiny little spot at the bottom center of the image is the airfoil, right? What are the boundary conditions for your simulation and where are they applied? I am especially curious about the boundary condition for the "floor" and the inlet. Could you show the mesh you used in the same view as the contour plot of the turbulent viscosity ratio is taken? Last edited by flotus1; July 19, 2012 at 05:58.

July 19, 2012, 07:21
#3
New Member

Zhe Lu
Join Date: Apr 2012
Posts: 12
Rep Power: 7
The little spot is the airfoil. Other than the airfoil, there are 4 boundaries: inlet, outlet, "floor" and "ceiling". I set the floor as a moving wall with the same velocity as the flow and set inlet as velocity inlet. The outlet is pressure outlet and the ceiling is symmetric. Plus, the turbulence model is realizable k-e with enhanced wall treatment. The picture I uploaded might be a little bit misleading, actually there is some distance between the inlet and the red cells, so here is a new one.
Attached Images
 mesh.jpg (95.7 KB, 15 views) turb vis rati.jpg (86.0 KB, 13 views)

 July 19, 2012, 07:22 #4 New Member   Zhe Lu Join Date: Apr 2012 Posts: 12 Rep Power: 7 This thread doesn't make any sense, but I don't know how to delete it...

 July 19, 2012, 07:41 #5 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,650 Rep Power: 26 The boundary conditions are ok. It seems like the viscosity ratio is high in cells with a high aspect ratio. Am I right assuming that the cells near the inlet have the same height as the cells in the boundary layer of the airfoil because of the block structure of the grid? I know this will result in a very high cell count, but try to obtain better aspect ratios in the cells near the inlet and the outlet with smaller cells in the streamwise direction.

July 20, 2012, 04:17
#6
New Member

Zhe Lu
Join Date: Apr 2012
Posts: 12
Rep Power: 7
Quote:
 Originally Posted by flotus1 The boundary conditions are ok. It seems like the viscosity ratio is high in cells with a high aspect ratio. Am I right assuming that the cells near the inlet have the same height as the cells in the boundary layer of the airfoil because of the block structure of the grid? I know this will result in a very high cell count, but try to obtain better aspect ratios in the cells near the inlet and the outlet with smaller cells in the streamwise direction.
You're right about the high aspect ratio cells. I was trying to obtain fine enough mesh near the airfoil and the ground, and the aspect ratio of the cells near the inlet reaches 1e6... But the red cells are not those with the highest aspect ratio. Anyway, I will follow your suggestion and have a try, thank you!

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