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[ICEM] Meshing Ground Effect and a Leading Edge Rotating Cylinder Correctly

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Old   March 9, 2017, 14:50
Default Meshing Ground Effect and a Leading Edge Rotating Cylinder Correctly
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Harry Clare-Paule
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Hi my thesis involves examining a NACA0015 with a Leading Edge Rotating Cylinder (LERC) in Ground Effect.

So far I have successfully meshed and achieved good results for: NACA0015 in a freestream, NACA0015 in Ground effect, NACA0015 LERC in freestream.

Now when I mesh the NACA0015 LERC in ground effect and import into fluent to solve I am unable to get the simulation to start as divergence is detected and some strange flow behaviour appears.

Attached are some photos of the meshes I have toyed around with.

Any suggestions or help would be greatly appreciated!
Attached Images
File Type: jpg Doesnt work 1.JPG (130.9 KB, 11 views)
File Type: jpg Doesnt work 3.JPG (146.7 KB, 15 views)
File Type: jpg Kind of works mesh.JPG (114.3 KB, 13 views)
File Type: jpg Kind of works mesh 2.JPG (122.0 KB, 13 views)
File Type: jpg Kind of works mesh 3.JPG (131.3 KB, 10 views)
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Old   March 10, 2017, 05:47
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Sebastian Engel
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Hello Harry,

you have some highly skewed cells in the throat area behind the cylinder.
I marked them in red in the following picture.
Fortunately, this can be fixed with a few extra splits, see the picture.
bettergrid.jpg

Always check your mesh quality. For example check Determinate and (Eriksson) Skewness. I believe there are mathematical proofs about numerical stability, but here are my rule of thumbs: I wouldn't start a simulation until the skewness is above 0.3. I recall some options of fluent to allow high-skewed-mesh simulations, but the solution quality is reduced.
The determinante in my experience also shouldn't be below 0.5. If possible above 0.7.

Another problem i see in your fifth picture is the extreme change in cell size. There are proofs that jumps in cell sizes shouldn't be above 1.2, relatively.
As you can see in your picture, in the throat area you have a tiny boundary-layer-cell right next to a (relatively) huge cell. This is a very unstable configuration.

With regards,
Sebastian
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Old   March 11, 2017, 07:56
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Harry Clare-Paule
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Hi Sebastian,

Thank you very much for your swift response and I appreciate your time,

I have begun re-meshing, introducing the new splits as you recommended. Unfortunately I am still having no luck, but I reckon it is because of mesh quality and the drastic changes in cell size which you pointed out. I am currently working on smoothing the mesh in order to improve skewness/determinant and hopefully I'll crack it soon.

I find the aspect of geometric symmetry an issue, I understand ICEM/FLUENT prefers a nice symmetric mesh and in my model I need to decrease the distance to ground to analyse the impact it has upon the aerofoil. Therefore the mesh above the aerofoil will be different to the mesh below, any tips on how tackle this problem?

Kind regards,
Harry
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