CFD mesh quality: understanding accuracy and convergence

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A simple demonstration of how a poor mesh from a cell geometry perspective (right) results in lower discretization error than one with “perfect” cells (left).

An excellent article on CFD meshing by John Chawner, from blog.pointwise.com:

“We know embarrassingly little about how the mesh affects the CFD solution,” said Prof. Carl Ollivier-Gooch of the University of British Columbia.

That statement is counter to what we all know to be true in practice, that a good mesh helps the computational fluid dynamics (CFD) solver converge to the correct answer while minimizing the computer resources expended. Stated differently, most every decent solver will yield an accurate answer with a good mesh, but it takes the most robust of solvers to get an answer on a bad mesh.

The crux of the issue is what precisely is meant by “a good mesh.” Syracuse University’s Prof. John Dannenhoffer points out that we are much better at identifying a bad mesh than we are at judging a good one. Distinguishing good from bad is clouded by the fact that badness is a black-white determination of whether the mesh will run or not. (Badness often only means whether there are any negative volume cells.) On the other hand, goodness is all shades of gray – there are good meshes and there are better meshes. <more>

If you’re a CFD person, I think you’ll enjoy this article.  Here are a couple of summary points from the article that are fun and interesting:

“One researcher was able to show a complete lack of correlation between mesh quality and solution accuracy.”

“Use as many grid points as possible… In many cases, resolution trumps quality.”

 

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