Bugs in meshing

[OmkarJ]

There are some irritating bugs in the meshing that I would like addressed:

1. I use regions to have control on the mesh distribution. But often, change in region size does not have any bearing on mesh adn its no. of elements, and if you keep on changing the region size, you see a sudden change in mesh and element count changes massively.  e.g. For region size of 10 until 9.3, there is no change in the mesh, for region size of 9.2 the mesh element count nearly tripples. I have logged a case (08462140) on this earlier in which I was told I had discovered a bug.
2. Often, I change the region size on the go with right click on tree and not opening the region dialogue box, changing the size and the "spreading change." But for this to take effect, I have to "generate" the mesh. And whenver I do that, all the materials  that are assigned extruded mesh "forget" their  setting and switch to being tet mesh.  So if you had forgotten to reassign the setting each time you regenerate the mesh, you are in for a disappointment to see the sparse tet mesh spread over the extruded materials.
3. The volume growth factor apparently works for fully tet meshes but doesn't work when extruded meshes are present. Even if you first make a tet meh all along adn then try to extrude the surface, with volume factor enabled, it doesn't mesh. I haven't seen this limitation (volume growth factor not available in extrusion meshes) documented anywhere.
4. Sometimes, the extruded materials lose the settings just like that, without any reason after refinement, other settings being same! There  is no setting that hinders the extrusion mesh like uniform c/s, linear extrusion, 3D, uniform surfaces in the direction of extrusion, ends parallel etc.  There is a case (07837801) I have created long back that discusses this anomaly.
5. The "approximate element count" is so off the mark most of the times, that it is rather reasonable not to show it until it is withink at least 20% accuracy. For example, in the morning I was staring at the mesh dialogue that predicted the mesh count at 600k while the actual mesh was 4 million. This  is more than 600% error!

Additionally, I would really love to see the following suggestions that are probably the limitations and not the bugs considered:

1. Currently "uniform" appears in the mesh tree just as a setting, with no indication of what parts it is applied to. At the same time, the extruded mesh setting is not even mentioned. It would be a good addition if both the settigns are shown along with what parts they are assigned to.
2. Support for extruded part in all directions. Currently it is possible in only three principal directions.
3. Support for mesh adaptation in extrusion meshes. OR at least in tet meshes, in presence of extrusion meshes. Today, the adaptation is prohibited even if one part is extrusion. 
4. Multicore meshing in presence of extrusion parts. Moreover, making it a bit more efficient. This is a good feature.
5. Hex mesh with multiblock capability? I know this is a lot to ask, hence this is last 🙂

Sorry for the long list. But I just wrote everything that has bugged me over the year! Meshing is one of the most important aspect of CFD and occupies nearly half of the time of the CFD engineer. I really hope at least few suggestions mehtioned here are considered and implemented. 

Cheers

Omkar.

[hartogj]

Hi Omkar

Thanks for these detailed comments. Many of them are being reviewed in the context of some of your cases, others concern design changes.

If you could, it would be nice to get an understanding of where you use extruded mesh and some of the reasons why you prefer to use it? From your description it sounds like you use it fairly frequently.

Here is a quick review of the items you raised:

- The first issue is one we have logged and are looking into.

- I do not believe the second one has been raised before, but we will have a look.

- We will have a look at the behavior of the volume growth rate control with tet meshes.

- Material settings should not be lost when making mesh sizing adjustments. Do you have an example you can provide?

- The mesh size estimator uses the limited information available to make a fast estimate based on empirical data. Normally this estimate is pretty good when using basic meshing inputs (no regions, surface/gap refinement, etc) and working with typical geometry. But it's not perfect in all cases. We've passed this on to a developer so he is aware.

And for the enhancements:

• We have not gotten many requests for uniform meshing in the tree but can share that with our design team.
• For your request to extrude parts in all directions, do you mean directions not aligned with the X,Y,Z coordinate system? How often does this come up?
• We can look into adaptation in the presence of tet meshes. As you have seen, adaptation has been out for a few releases. We have just added support for adaptation with new material devices in version 2014 and are growing its capabilities.
• Multicore meshing is an interesting request. Currently how much time does it take to mesh your models with extrusions?
• Regarding your request for hex mesh with multiblock capability, what is it that you are ultimately looking to do?

Thanks,

Jon

[OmkarJ]

Bugs:

#2

JH: I do not believe the second one has been raised before, but we will have a look.

OJ: I haven't formally raised any request in cases or idea station, but Royce happened to mention this bug in one of his ideas, in the context of refinement setting being lost during mesh rebuild. I commented on that idea, adding that the same bug also affects the extrusion setting. It can be found here:

http://forums.autodesk.com/t5/Simulation-CFD-IdeaStation/Rebuild-Mesh-Definition/idc-p/4301158#M148

#3

JH: We will have a look at the behaviour of the volume growth rate control with tet meshes.

OJ: Just to clarify, the volume growth factor works well with unstructured tet meshes. But when extrusions are present elsewhere, the mesher refuses to have the volume growth factor enabled.

#4:

JH: Material settings should not be lost when making mesh sizing adjustments. Do you have an example you can provide?

OJ: Can you please advise as to how I can share the model securely?

Requirements:

#2: JH: For your request to extrude parts in all directions, do you mean directions not aligned with the X,Y,Z coordinate system? How often does this come up?

OJ: Yes, I meant the directions not aligned with principal axes (X/Y/Z). And admittedly, it is rarely needed, so it is not at the top in my bucket list.

#3

JH: We can look into adaptation in the presence of tet meshes. As you have seen, adaptation has been out for a few releases. We have just added support for adaptation with new material devices in version 2014 and are growing its capabilities

OJ: I appreciate the new additions, and I know it would be tricky to include the adaptation for the extrusion meshes, but it would be worthwhile to have the adaptation in at least the tet areas, when the extrusion are present elsewhere. We can always model the sections of gradients with tet mesh and let the adaptation take care of it, while the extruded portions are used mostly for the development of flow etc since the major gradients are in flow direction.

#4

JH: Multicore meshing is an interesting request. Currently how much time does it take to mesh your models with extrusions?

OJ: Since we need to include the piping layouts in our filtration equipment, the extrusion meshes are handy in reducing the mesh element count. But the rationale behind this request was not only making the multicore availble in presence of the extrusions but also making the multicore more efficient. A quick study on a 4.2 million element mesh revealed that improvement using multicore meshing (4 cores, no extrusions) over single core meshing (extrusions present) was just 10 seconds in 2:32 minutes (6%)!

#5

JH: Regarding your request for hex mesh with multiblock capability, what is it that you are ultimately looking to do?

OJ: The multiblocking helps in dividing the geometry without having to cut the geometry in the CAD stage, to aid in mapping the hex mesh. The advantages of the hex meshes are well documented, e.g. smaller running times as compared to the unstructured tet meshes, lesser computational and discretization errors etc. I have done few hex meshes in the past using ANSYS ICEM CFD, please have a look here: 

http://imageshack.us/a/img822/3425/o6ak.png

http://imageshack.us/a/img42/2549/29i4.png

(Please be patient with the image links, sometimes the server can be slow)

I understand that it is unreasonable to expect the capability of "mesh-only" software like ICEM in SimCFD. But then, few tools to help with the hex mesh can be really helpful.

Thanks for taking the notice of the comments patiently.

Omkar.