Regarding nodal aspect ratio

[Anonymous]

Hi,

I have recently started using Simulation CFD in my master thesis and I'd like to start of by saying that I like your product.

However I have some general concerns regarding Nodal aspect ratio. Firstly, I have been looking for some rule of thumb values as to how large the NAR can be without having to much effect on the results and to not have convergence issues. The only value i have seen specified was from a Youtube video where it was stated that it should be lower than 10.000 (ten thousand).

Secondly, when I mesh my geometry (I use manual meshing since I need full control of the mesh due to restrictions from thesis supervisor) I always have a warning during surface meshing stating that "Mesh nodal aspect ratio exceeds threshold on surfaces ..." where all surfaces in the geometry are mentioned. This is followed by that the whole geometry is covered in orange dots (every element on the surface). However, I have been trying automatic meshing aswell, with and without mesh enhancement enabled, and i still have the whole geometry covered in orange dots.

I should say that initially when I import the geomtry from Inventor, I remove edges and surfaces with the geomtry tool included in Simulation CFD.

I'm thinking it has something to do with the CAD geometry. Does anyone have experience from this? I'm attaching a picture.

Regards,

Henrik

[Jon.Wilde]

Have you tried a manual mesh? Just to see if it works.

It is hard to see what might be causing the issue, as there are so many dots, I can't see the model 🙂

An NAR of 10,000 can be OK provided it is not near any area of interest. Ideally we need to be < 100.

Hope that helps,

Jon

[Anonymous]

Thanks for your answer,

I have tried both manual (with different volume sizes) and automatic mesh. For both cases with and without mesh enhancement/prism layers enabled. Still i get these orange dots. I attach a new figure where the orange dots are not displayed (same mesh).

I'm assembling four different parts in Inventor prior to importing it to Simulation CFD. I should also mention that the simulation works fine even though I have this warning. I'm just curious if I should mind this warning or not. Is it possible to modify the threshold value?

When i use Iso-surfaces to display the NAR, for a case where the geometry was filled with orange dots, my max value is 26. What about a lower threshold? Should NAR equal to 1 for an ideal element or 0? because most elements are << 1.

[LKania]

Excessive nodal aspect ratios are an indication that specified mesh sizes are not representative of the features of the geometry (i.e. things are poorly resolved) else there are problems with the underlying geometry itself. This is usually a localized phenomenon.You say your model is covered with orange dots; can you zip a sharefile (.cfz) with a meshing setup that shows this? The image you poste dof your mesh looks alright from what I can see.

[Anonymous]

I get the orange dots when I press generate mesh in the attached setup.

What the NAR values are for an ideal element? I'm a little concerned because I have most elements with a NAR value well below 1 (rather 0.001) in another setup.

[LKania]

Did you forget the attachment?

[Anonymous]

Cannot attach the file:

"The contents of the attachment doesn't match its file type."

[LKania]

Zip the file and try uploading.

[Anonymous]

Sorry 😜 Here it comes

[LKania · Accepted solution]

The nodal_aspect_ratio flag has unfortunately been mislabeled. A value greater than 0 enables the option and defines a threshold. If the local nodal aspect ratio exceeds the threshold, an orange dot will appear on the model. With a threshold of 1, you can be assured that an orange dot will appear at every mesh node. Try a value of 200 or so. 

The nodal aspect ratio is defined as the max-to-min ratio of all face/element edge lengths emanating from a mesh node (a measure of stencil distortion) so the minimum value you will ever see is 1 for a completely uniform stencil. Owing to length scale gradation in a mesh, you will typically see values greater than 1. 

If you examine the diagnostics panel in the meshing task, you might note that areas of the model are identified where model length scales are much less than mesh length scales; this situation gives rise to larger nodal aspect ratios. It may be necessary to reduce the minimum refinement length (defined in the edge diagnostics panel) and re-autosize to drive mesh length scales lower in those regions so that they are more representative of the model length scales. This will result in lower nodal aspect ratios. 

[Anonymous]

Thank you very much LKania, this was very helpful. I was sloppy and misread the information about this flag. I must have thought that the value 1 (as opposed to 0) would enable the solver to save NAR (which the stream function should do as I understand it). Again thank you!

With kind regards,

Henrik

[Royce_adsk]

To view the Nodal Aspect Ratio (NAR), you will need to oddly turn on the 'Stream Function' result quantity.  This is really an old place holder for the result from years ago. The stream function result is only used in 2D models.

I would suggest that you expore automatic mesh along side of your manaul meshing.  I don't see any reason why manual meshing would be required for this model.

Best regards,