While working with PrePoMax I ran into meshes with negative Jacobian elements. Since I couldn’t find a built-in fix in the GUI, I put together the workflow below using cgx as a workaround. Maybe this is useful to others running into the same issue, or maybe it’s something that could eventually be integrated into the GUI directly.
Thanks a lot for developing PrePoMax — it’s an amazing tool!
Protocol for eliminating negative elements
Generate the mesh in PrePoMax. This is where the negative elements were created.
Export the .inp file and open it in cgx.
In cgx, the set -NJBY is automatically created, containing all negative elements.
Repeatedly running mids -NJBY lin force unfortunately did not eliminate the negative elements in my case, so I took a different approach.
Running merg n all 0.01 merges all nodes that lie closer together than the specified tolerance (0.01 in this case). This removed all the negative elements.
Running send all abq generates a file all.msh containing the corrected mesh.
Manually rename the file extension from .msh to .inp. PrePoMax then recognizes the file and can import it.
Continue working with the new mesh in PrePoMax and start the job. With this corrected mesh, the calculation ran through successfully.
Did you try to don´t use the midsize nodes in the geometry? In my models this option very often cause non jaobian elements or other deformations on elements in curved faces.
The most common cause of negative jacobians in PrePoMax is adjustment in tie constraints or contact. Sometimes they are also generated when meshing problematic geometries.
PrePoMax creates a set for negative jacobians when submitting an analysis (or just the Check Model procedure), but you can also use the built-in mesh quality check tool to find them.
Yes, I used that option. Disabling “Mid-side nodes on geometry” would definitely be one approach. I think there are advantages to having the option enabled, since the elements naturally represent the geometry better that way. I was just looking for a process to get rid of the few problematic elements that might arise during mesh generation with “Mid-side nodes on geometry” enabled. Thanks for the help!
Midside nodes on geometry are disabled by default because, with Netgen and Gmsh, they far too often lead to negative jacobians. Of course, careful preparation of the geometry before importing it to PrePoMax may help reduce those issues. And if more accurate representation of curvatures is needed, they can be always enabled manually in a given case. I wouldn’t do it by default, though (there is a setting in Tools —> Settings to have it always enabled initially).
Maybe there is a good threshold/evaluation value at which projected nodes cause problems and can later be detected by PrePoMax and moved back to the midside position?
