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Old   August 31, 2009, 21:05
Default mesh quality measurement
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Hello everybody,

As you know one of the parameters for measuring mesh quality is aspect ratio. I would be appreciated if any body can help me to know what is the optimum range for this parameter to have an appropriate quality for generated grid?
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Old   September 6, 2009, 17:23
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John Chawner
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Hello khosrow.

There are no universal guidelines for mesh cell aspect ratio. In fact, a pessimist might say that the only universal guideline for mesh quality is positive volume.

Acceptable (forget optimum) aspect ratio will depend on your solver. If you're using a commercial CFD code, contact their tech support engineers and I'm certain they'll be able to provide you with recommendations.

But acceptable aspect ratio also depends on cell type. For example, a highly stretch quad with an aspect ratio of 100 could very well be acceptable. A triangle with an aspect ratio of 100 could be horrible or good (depending on the included angles).

Acceptability also depends on what you're trying to compute. For example, you might be able to get away with poor grid quality if you're only interested in wall pressure. But if you're trying to compute heat transfer or boundary layer transition, the same grid may fail miserably.

Finally, it depends on where the cell is. A crappy cell out in the farfield can be tolerated while one in the boundary layer probably can't be.

Sorry this isn't a very good answer.
John Chawner / /
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Old   September 7, 2009, 02:22
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Ertan Karaismail
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I guess, fluent recommends a max aspect ratio of 5. However, sometimes it becomes very hard to satisfy this requirement. I generally try to keep it less than 15. It results in acceptable meshes for me. However, I totally agree with the previous reply. And as another point, that might be important, is that, different turbulence models (i.e. equations) have different mesh quality requirements. Especially, if you will use RSM, you should know that it is extremely sensitive to the mesh quality. It may not converge with the meshes acceptable for other model. There fore if you are gonna use RSM, you need to keep the mesh quality as high as you can, meaning that use of a low aspact ratio.
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Old   September 7, 2009, 11:09
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I quote this line from the GRUMMP UserGuide
"There must be a balance between resolution of the boundary and surface features and
complexity of the problem. In addition, for problems with isotropic physics, element aspect ratio must be
small to minimize linear system condition number and interpolation error"

but they do not define how small the aspect ratio must be. or how small is small.
This is a question of getting experience by doing, i.e. meshing
Good Luck
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