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-   -   [ICEM] Element Quality. (http://www.cfd-online.com/Forums/ansys-meshing/120125-element-quality.html)

marcoymarc July 1, 2013 07:38

Element Quality.
 
Hi guys,
i have a newbie dubt. Why element quality is so important for a sim to converge? Which numerical implications exist due to bad aspect ratio ? Thanks in advance

sadjad.s July 1, 2013 12:39

Answer
 
Hi mate,
I classify element quality in two ways:
1- Skewness; which is a criterion about angle of elements. Actually Fluent and almost all CFD commercial codes recommend to have cells with angles more than 18 degrees. With cell angles less than 18, there may be a possibility of production of error in domain which results in divergence.
2- Aspect ratio; ratio of largest side of element to smallest one. Some times having high aspect ratio is no problem, like boundary layers where aspect ratios more than 1000 may be allowed. But Fluent suggest to use "Double Precision" solver for cells with aspect ratio more than 100.

marcoymarc July 2, 2013 04:57

Yes ok, but what i'd like to know is how bad quality affects numerical stability..
Ty

marcoymarc July 2, 2013 13:10

How a bad aspect ratio is supposed to make a simulation diverge? Googled it all around, and found nothing "technical"; only a few words on good quality improving stability and accuracy, but not about the HOW.

sadjad.s July 2, 2013 14:12

Answer
 
It doesn't make solution diverge in all problems. Actually it depends.
First of all consider that in Control Volume approach, there can be only one value for variables (like velocity, pressure, temperature,...) in each cell.
Let me give an example; In boundary layer, we have no gradient along the wall. In that situation, high aspect ratio cells along the wall are no problem. They won't cause divergence neither.
But consider a high aspect ratio cell in high gradient region. While calculation, solver has to put only one value on that cell, while there may be gradient across that cell. This error (neglecting gradient) can cause divergence.

About skewness, you may have to refer to "Control Volume" and calculation of cell wall fluxes.

erayerisik May 19, 2014 06:58

mesh quality (orthogonal, skewness, element)
 
2 Attachment(s)
hi guys, I am new here and also sorry for my poor english :) I have checked site before write this here, however I could not find any solution.
I am working on analysing of an airfoil (naca2412). I have doubt with element quality. I had finished my mesh, then opened fluent, it says your orthogonal quality is under 0.01. What does it means? Also is anyone can give me some advice to improve my mesh quality? I attached pics.
Thank you all...
Attachment 30993

Attachment 30994


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