[snappyHexMesh] meshQualityDict: minVol for snappyHexMesh

kandelabr · June 20, 2017, 04:08

Hello,

I'm calculating the minVol in system/meshQualityDict with the following formula:

minVol = a^3 / 8^lvl / x

where
a = size of a blockMesh element
lvl = maximum refinement level
x = some arbitrary number

It seems that for values of x lower than 15-20 there are too many too small cells to produce a good mesh.

So my question is:
What should i choose for x, and why?

kandelabr · June 20, 2017, 12:47

Another question is, why is the minimum volume so important? Can I make a mesh with a too high resolution? The intuitive thinking would be that more elements mean more precise discretisation hence more exact results. Or is my thinking wrong?

Thanks!

kandelabr · July 1, 2017, 17:48

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wyldckat · July 2, 2017, 18:48

Quick answers:

Quote:

Originally Posted by kandelabr

Another question is, why is the minimum volume so important?

The simplest reason is that it can be critical for transient simulations (time-dependent), due to the Courant Number. Re-read the first tutorial on the OpenFOAM User Guide, where this is briefly explained.

Quote:

Originally Posted by kandelabr

Can I make a mesh with a too high resolution?

It's possible to have "too much of a good thing"...

Quote:

Originally Posted by kandelabr

The intuitive thinking would be that more elements mean more precise discretisation hence more exact results. Or is my thinking wrong?

... specifically in CFD, it's advisable to perform mesh convergence tests when ascertaining the accuracy. A mesh too refined can easily reveal the limitations of the models being used. A mesh refined in the wrong places can lead to problematic/non-physical results.

See this blog post for a few examples: OpenFOAM: Interesting cases of bad meshes and bad initial conditions

Also, look up (Google?) the difference between mesh requirements between DNS, LES and RANS (or RAS).

Quote:

For a moment, nothing happened. Then, after a second or so, nothing continued to happen.

-- Douglas Adams

Unfortunately not enough people are available at all times here at the forum, but with luck a few people are available a few times per week/weekend

kandelabr · July 3, 2017, 06:38

I know you're not obliged to answer anything and I appreciate any kind of help, I just made an elaborated 'bump'. IMO it's a good quote

Thanks (for now), let me do some googling...

June 20, 2017, 04:08	meshQualityDict: minVol for snappyHexMesh	#1
kandelabr Senior Member Nejc Join Date: Feb 2017 Location: Slovenia Posts: 196 Rep Power: 9	Hello, I'm calculating the minVol in system/meshQualityDict with the following formula: minVol = a^3 / 8^lvl / x where a = size of a blockMesh element lvl = maximum refinement level x = some arbitrary number It seems that for values of x lower than 15-20 there are too many too small cells to produce a good mesh. So my question is: What should i choose for x, and why?

July 3, 2017, 06:38		#5
kandelabr Senior Member Nejc Join Date: Feb 2017 Location: Slovenia Posts: 196 Rep Power: 9	I know you're not obliged to answer anything and I appreciate any kind of help, I just made an elaborated 'bump'. IMO it's a good quote Thanks (for now), let me do some googling... wyldckat likes this.

June 20, 2017, 12:47		#2
kandelabr Senior Member Nejc Join Date: Feb 2017 Location: Slovenia Posts: 196 Rep Power: 9	Another question is, why is the minimum volume so important? Can I make a mesh with a too high resolution? The intuitive thinking would be that more elements mean more precise discretisation hence more exact results. Or is my thinking wrong? Thanks!