# Highly Skewed Cells

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 July 20, 2010, 10:22 Highly Skewed Cells #1 New Member   Chris Turner Join Date: Jun 2009 Location: Belfast Posts: 12 Rep Power: 16 Hi! I am running a simulation that has approximately 500,000 cells. It is a hybrid mesh of both hex and tet cells. The tet cells are being used as this is an optimisation problem where different geometries are being modelled and compared-the tet cells seemed to offer the most consistent cell size and count for different geometries compared to the hex options. In my model, I have approximately 0.01% highly skewed cells (above 0.97). My model still converges and seems to give reasonable results so I was wondering if these skewed cells matter? There are approximately 40 cells that are highly skewed in the entire model and none of them are clumped together, it seems to be one cell here and one cell there. Does anyone know of a way of getting rid of single highly skewed cells within fluent /Gambit? Thanks! Chris

 July 21, 2010, 01:03 #2 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,297 Rep Power: 40 In Gambit, if you are sure that the highly skewed cells don't come from your geometry (small angle, small edge, etc...) then you can try modifying slightly the cell's size (or Size Function's parameters). That's my workaround __________________ In memory of my friend Hervé: CFD engineer & freerider

 July 21, 2010, 04:38 #3 Member     Burak Join Date: May 2009 Posts: 90 Rep Power: 16 can we just change the mesh size of skewed cells, or must we remesh the whole domain?

 July 21, 2010, 05:07 #4 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,297 Rep Power: 40 you must remesh all the volume concerned. As another workaround, if the highly skewed cells are direct connected to a surface, you can try to correct the skewness of the cell by moving the node (belonging to the cell) on the surface (--> Face/Mesh/Move Face Nodes) __________________ In memory of my friend Hervé: CFD engineer & freerider

 July 21, 2010, 09:56 #5 Member     Burak Join Date: May 2009 Posts: 90 Rep Power: 16 my highly skewed cells are generally on(2D) or next to(3D) surface. So these recommendation will be very useful. Thanks Max

 July 21, 2010, 10:08 #6 New Member   Chris Turner Join Date: Jun 2009 Location: Belfast Posts: 12 Rep Power: 16 Thanks for the info Max. In your experience, if there are very little skewed cells (0.01%) and the CFD matches experimental data, is there any need to try and eliminate the skewed cells? I just think it may take a long time for not much improvement in result accuracy. Chris

July 22, 2010, 01:19
#7
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Maxime Perelli
Join Date: Mar 2009
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Posts: 3,297
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Quote:
 Originally Posted by chrisoturner Thanks for the info Max. In your experience, if there are very little skewed cells (0.01%) and the CFD matches experimental data, is there any need to try and eliminate the skewed cells? I just think it may take a long time for not much improvement in result accuracy. Chris
In my experience, I can say that highly skewed elements can affect the convergence with drops in the residuals (but not always).
But the question is: Are skewed cells in a specific region where you need to catch something?
At the other hand you reached results in agreement with experimental data, so....
I already gave up trying to fix some highly skewed cells, especially after one day of attempts, and I didn't encounter any convergence issue.
Nevertheless I try to keep max skewness below 0.9 (for unstructured grids)
__________________
In memory of my friend Hervé: CFD engineer & freerider

 July 22, 2010, 06:43 #8 New Member   Chris Turner Join Date: Jun 2009 Location: Belfast Posts: 12 Rep Power: 16 Thanks for the advice MAX! Once again, you have been very helpful! Chris