Grid aspect ratio

Document created by kgokavar on Jun 7, 2011
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How to reduce grid aspect ratio (<60)?. Most of the compact model download from Flotherm Pack aslo has aspect ratio more than 200.


Just curious, why do you want to reduce the aspect ratio?


Because (in the past?) new users were told in training that aspect ratios <20 are good, 20-200 are bad and everything >200 is probably killing the convergence.


The worst aspect ratio I ever had was around 10000 and I solved the case without any problems. I would definitely not worry about an aspect ratio of 60 (peanuts ). Even >200 might be ok.


What can be done?

- create geometry with a reasonable tolerance, avoid very small gaps where possible.

- set an appropriate minimum grid size - some keypointed grid lines will be removed. Can be dangerous!

- add more grid. Use localized grids.


If in doubt, raise an SR!





You are probably right. In general purpose CFD where using a distorted hex mesh you would need to have a much lower aspect ratio than 60. Using Cartesian based meshes allows much higher aspect ratios to be used in both solid and fluid regions. I have solved a case with an aspect ratio of 1000:1 in FloTHERM with no numerical artifacts in the pressure and flow field.


Unless there is a specific issue in this case (which is why I asked the reason for the concern) I would not worry at all.


Thanks  Dirk and John for reply and inputs.

I fully agree with Dirk,  the instructions during the training period (in the past and even now) will get embedded in the users memory as a rule of thumb and will be difficult to erase.  And such discussions  helps to come out of such mindset.

Like John I have also come across case solved with aspect ratio >200 and not convinced with results because of above mindset.


It will be good if more experts share their views.


Many times grid aspect ratio is less than 20 but still solution not converge. it is low level stable. So I agree with above comments. Grid aspect ratio may go more than 200.


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