gmsh not generating volume mesh

protontim

I have a complex part that I am trying to mesh in gmsh. It appears to mesh in gmsh, but when I look at the mesh it is a surface mesh not a volume mesh. I have volume mesh, quadratic elements, and fit midside nodes checked. I have a max element size of 1mm. Since I checked volume mesh that is what I expected I would get, but that does not seem to be the case. Any help would be appreciated.

Regards,
Tim

protontim

It looks like MECWAY/GMSH may think the STEP file is a surface file not a solid body. It is a solid body in Solidworks when I export it, but it looks like when it exports to step there may be a couple faces that don't get written properly - even though everything checks out in SW. Looks like I have more investigating to do. I will post my findings here when I get this working. Still open to suggestions.

prop_design

I think I saw something similar to this recently. There is a certain option you have to use with SolidWorks to get step files to work right. I don't recall exactly what that is though, possibly split periodic faces. I've never personally used SolidWorks, so I can't provide more help.

Update; Oops, what I was thinking of was actually one of your old posts. Took me forever to find it. I was searching other forums I follow, thinking it was there. http://mecway.com/forum/discussion/comment/5838/#Comment_5838

protontim

I have done a number of export and imports out of and into various CAD programs to verify the STEP is a solid. It still meshes only the surfaces though. Any suggestions as to how to proceed would be appreciated.

Victor

That can happen when it fails generating the volume mesh but after generating the surface mesh which it always does first.

With the surface mesh, can you then turn it into solids with Mesh tools -> Automesh 3D? That will also show you any gaps that would cause it to fail.

protontim

After removing some very small faces on the STEP model it appears to now mesh as a solid. The file I started with was a STEP file with a lot of complexity that I imported from one of our manufactures. It came in with a lot of errors. I cleaned up all the issues but apparently some of the very small face slivers were causing the import to bring it in as a surface model.

Is there a way to tell if the STEP file comes in as a surface model or a solid?

prop_design

one thing i do is check the volume in cad and again in mecway. making sure they are reasonably close. if there is a problem with the geometry, the cad program i use won't calculate volume. so i use that as a check of the geometry as well. i think in mecway, you have to mesh the part first, before it will calculate volume.

prop_design

it looks like netgen will at least show you if the imported geometry is a solid or not. use the iges/step topology doctor tool. it also shows some other things that might have let you spot the problems. you can download netgen from here; https://ngsolve.org/downloads

it has some old requirements for python 3.7 and visual studio redistributables. you need to install the requirements first. then install netgen.

protontim

@prop_design thanks for the tip on the topology doctor. I have seen others refer to that as well. Downloading it now. I will give it a whirl.

Do you happen to design aircraft props and rotors? We have a 9' diameter carbon composite rotor for a multi rotor autogyro that we designed that we could use some help with if that might be up your alley. You can see more at www.aergility.com

prop_design

cool aircraft. i don't do consulting anymore. i created a program called PROP_DESIGN. it was sort of a bucket list task for me. hopefully, i'm finished working on it now. spent the last 14 years making it better. it's not for your horizontal rotors though. the axial rotor you have would work though. you can analyze composite blades with mecway and the ccx solver. victor is working on a new shell element as well. so in the future, that capability may be even better. the issues i run into a lot are solid blades are really hard and sometimes impossible to mesh with netgen. i can't seem to get anything useful from gmsh. the other issue is lack of inertial loads. you can only apply centrifugal force and the aero loads (if it's a shell model). for aerobatic maneuvers, pitch roll, yaw, you are out of luck.

one very odd thing with netgen is the mesh settings change depending on where the model is. if you move or rotate the model, the mesh settings don't work the same. you can't really get the same mesh either, due to this issue. so you have to use mecway move rotate mesh, if you want the same mesh in different locations.

protontim

@prop_design, when I go to install the new netgen tools it tells me I need to uninstall my current installation. Not sure if this is refering to a previous installation I had installed in the past or the Mecway installation. Will I blow up the Mecway netgen tool if I do an unistall of netgen?

Turns out it is NGsolve that needs to be uninstalled - had an old copy of that installed. I will uninstall and hopefully that does the trick.

prop_design

no, it must be referring to a previous install of actual netgen. it won't affect mecway's netgen at all. i have a video of some getting started tips, that might help too.

https://youtube.com/watch?v=6amu7J26kqY

the part about starting off with admin privilege and changing the shortcut are helpful

prop_design

for your specific issue, in the topology doctor, i attached a picture of what to look for. on my model it says there is one solid. so on your problem geometry, i expect it to say you have some other entities in there and maybe not even a solid. the good thing here is it works before you even mesh anything.

protontim

WOW!!! I had split my complex part in half (with a 3D cutting surface) trying to get parts that would mesh and that I could reassemble with bonds later. After many hours of trying I got one half of the part to mesh, the other half refused no matter what I tried. I installed NGsolve (had to uninstall my original python because it was not the right rev) and ran the troublesome part in NGsolve. It meshed perfectly first try with no modifications to my original geometry!! I then tried the full blade, which never meshed and it meshed first try - no modifications to the geometry!!!

I hate to say it but it looks like I wasted numerous hours fighting to get the netgen in MECWAY to work, while the NGsolve meshed it immediately. Is there a big difference in the solving power of what NGsolve is using - there sure seems to be!

prop_design

yeah, that sounds like what i mention in the video. there seems to have been an undocumented change to netgen, that happened pretty recently. victor had already updated netgen for me, not that long ago. however, since then, they improved it more. it's also faster than the mecway versions. don't get too excited though, it still has many many problems. you just haven't run into them yet. i have encountered all sorts of things with netgen. oddly, sometimes mecway's versions work better. so it's very hit and miss.

protontim

Yes it is significantly faster - one nice feature is that you can specify the number of threads it uses. That should speed things up provided they get run on different cores.

prop_design

yeah that feature works well. only implemented for some operations, like volume optimization, right now.