Hex Meshing Bug

[makeclean]

Hi

Ive got a geometry that meshes fine with tets, and seems to mesh fine with sweep-target on hex, looking at the mesh in Trelis, however when I export that mesh to exodus, the resultant mesh is incorrect. Happy to share the inputs if that helps.

Thanks

Andy

[gvernon]

Hi @makeclean!

Are you able to share the inputs? If so I can take a look at this. It will be easiest for me if you can send me your Trelis file containing the geometry and a journal file used to make the mesh.

[makeclean]

Hi

Sorry its taken me so long to get back to you! The exodus file is large, could you please reach out with your email address and I’ll share a link to the exodus file.

What my investigations have shown (I think) is that the mesher meshes it correctly, but in writing it to an exodus file, it is somehow corrupted. The hex mesh is displayed correctly in Cubit/Trelis when meshed, but when I import that exodus file into Trelis or Visit/Paraview it appears corrupted as above.

You cant tell, but this is the mesh clipped after just being produced or reimported from the .cub file. So im confident that its either a bug in the exodus export. Ill try sending to another format, say openfoam, or abaqus and see if i can visualise it correctly that way.

[gvernon]

@makeclean - upload your file to transfer.coreform.com and then send the generated link to support@coreform.com

[makeclean]

Hi, when you wake up you folks should hopefully have it

[gvernon]

Hi @makeclean - so I’ve reviewed the file and have identified the issues.

Issues

1. The internal “twist” is also not compatible with the curve that partitions the exterior surface (highlighted):
   

   
   
   
   
   
   Essentially, this will cause elements to invert in order to respect both this straight curve and the tortuous curves inside the volume.

2. Volumes with a lot of “twist” often require a bit more assistance (in the form of webcut/partitioning) from the user to assist Cubit in laying down a valid mesh
   

   

Solutions

1. To remove this external curve we will regularize the volume: regularize volume <volume_id>
   

   

2. To webcut this volume we will create additional surfaces that are normal to the faces of the internal faces. We do this by:

   1. Creating vertices on the midpoints of the rectangle
      
      
      
      
   2. Then vertices on the circular curve that are “close to” these midpoint vertices.
      
      
      
      
   3. Then we create curves with the vertices on the circular curve that are opposite each other
      
      
      
      
   4. Then we sweep these curves, using one of the helical curves as a guide, to create the additional surfaces (sheet bodies)
      
      
      
      
      
   5. Then we webcut using these sheet bodies.
      
      
      
      
      

This will result in multiple volumes that more explicitly define the twist for the mesher.

You’ll need to update your boundary layers, but here’s a coarse mesh without the boundary layers, just to give you a feel for how the mesh looks:

Here’s a journal file that runs the commands, starting from the Trelis file you’ve sent:

## Open the file
reset
open "C:/Users/Owner/Downloads/swirly-hex.trelis"

## Delete mesh entities
delete mesh
delete boundary_layer all

## Block type was TETRA... change it to linear hexes
block 2 element type hex8

## Remove the extraneous, straight curve that will cause problems
compress ids
regularize vol 1
compress ids

## Create vertices on / near midpoints of the rectangular feature
create vertex on curve 3 4 1 2  midpoint 
create vertex on curve 13  close_to vertex 11 12 13 14  

## Create curves to be used for cutting the geometry
create curve vertex 15 17  
create curve vertex 16 18  

## Sweep the curves along helical path to form cutting surfaces
sweep curve 15  along curve 6  
sweep curve 16  along curve 6  

## Perform Webcuts
webcut Volume 1 tool Body 2 
delete Body 2 
webcut Volume 1 4 tool Body 3 
delete Body 3 

## Delete the free vertices we used for constructing the cut surfaces
delete vertex all
compress ids

## Imprint and merge volumes
imprint all
merge all

## Demo Mesh
vol all size 0.005
mesh vol all

Let me know if this was helpful!

Greg

[makeclean]

Thanks Greg! Super helpful, thanks for that. Out of curioisty, how does one view the mesh as in the last plot? A regular ‘slice’ does not give me that view.

[karl]

By default we do not draw the mesh internally. Turn on the cutting planes and to do a draw hex all from the command line.