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Fluent diverges when using inflation layers

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Old   August 9, 2013, 09:51
Question Fluent diverges when using inflation layers
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Bart A.
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I'm simulating a 2D wedge in a flow with the transient standard k-epsilon model. The Reynolds number is around 400000.
The geometry is shown below.

The mesh without inflation layers gives a solution with a CD of around 1.2. This is close to literature values of a 2D wedge in a flow.

However, when I add the inflation layers (and change nothing else in the mesh), the solution diverges after one timestep and I get a warning that the turbulent viscosity is limited to a viscosity ratio of 1e5 in several thousand cells.
In the end Fluent stops iterating with the message:

Code:
Divergence detected in AMG solver:y-momentum
The first inflation layer thickness is set such that y+ is around 30. Setting it such that y+ is 1 did not solve the divergence.

I've read up on the viscosity ratio warning in different posts here and in other internet resources, but the remedies (refining the mesh, under-relaxing the k and e) did not work for me.

I'm surprised that adding an inflation layer has such a big (negative) effect on my solution. Can someone explain what is happening here?
Should I just leave the inflation layers out all together (sounds like a bad idea)?

1. Geometry overview, flow comes from the left, normal to the boundary:


2. Geometry close-up:


The mesh without inflation layers looks as follows:

3. Mesh overview:


4. Mesh refined area:


5. Mesh around wedge:


And with inflation layers:

6. Mesh around wedge with inflation layers:


7. Mesh around wedge nose with inflation layers:
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Old   August 9, 2013, 10:10
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Alex
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Have a look at the cells at the sharp trailing edges of your geometry.
I guess they have very small angles.
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Old   August 9, 2013, 10:23
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Bart A.
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The trailing edge looks like this:



How do I prevent too sharp angles in the mesh?
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Old   August 9, 2013, 10:28
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An easy way could be to refine the mesh in this region (which you should do anyway because the mesh is rather coarse here).
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Old   August 9, 2013, 12:11
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Bart A.
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I've refined the mesh at the trailing edge by adding an edge sizing restriction on the edges of the wedge. It now looks like this:



Fluent does converge now. Thank you for your help.
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divergence, fluent 14, inflation layer

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