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75,100 Views 5 Replies Last post: Sep 28, 2010 7:14 PM by jitender.kumar RSS
anravi Lurker 2 posts since
Jan 18, 2010
Currently Being Moderated

Aug 26, 2010 1:06 AM

Grid aspect ratio

How to reduce grid aspect ratio (<60)?. Most of the compact model download from Flotherm Pack aslo has aspect ratio more than 200.

john_parry Contributor 14 posts since
May 8, 2009
Currently Being Moderated
Sep 8, 2010 5:49 AM in response to: anravi
Re: Grid aspect ratio

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

dirk_niemeier Active 45 posts since
Oct 20, 2008
Currently Being Moderated
Sep 8, 2010 6:23 AM in response to: john_parry
Re: Grid 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!

 

Cheers,

Dirk

john_parry Contributor 14 posts since
May 8, 2009
Currently Being Moderated
Sep 8, 2010 6:38 AM in response to: dirk_niemeier
Re: Grid aspect ratio

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.

jitender.kumar Lurker 3 posts since
Sep 28, 2010
Currently Being Moderated
Sep 28, 2010 7:14 PM in response to: anravi
Re: Grid aspect ratio

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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