I am relatively new to cubit software and I am looking to build and mesh 3D muscle models with multiple parts ie muscle belly, proximal and distal tendon-aponeuroses.
Whilst I am now familiar with cubit boolean functions (subtract etc) and have successfully meshed a few muscle models - these models tend to have very large number of element (500,000-800,000). I think this is largely due to the complexity of the attachment from tendon to muscle. To ease the computational time for the FEA process I am hoping to reduce this number closer to 50,000 - 100,000.
Ideally I want the distal tendon-aponeurosis to interact directly across the surface of the muscle tissue, whilst the proximal tendon-aponeurosis is partly intra-muscular (like a key in a hole) and partly extra-muscular or surface level to the muscle tissue. You can see this in the file attached.
So my question is: what is the best way to simply align the parts/volumes as tightly as possible to each other to best allow these structures to inter-connect and be meshed as a continuum?
One of the core principles for creating a conforming mesh is the concept of imprint and merge. Imprinting takes the curves from one body and imprints them into the adjacent body. Merging then make surfaces, curves, and vertices that are shared between bodies and makes it so that Cubit only sees one of the entities.
In your problem, if I open the model and issue the commands
open "~/Downloads/BFLH_version1.cub5"
imprint all
merge all
vol all scheme tetmesh
mesh vol all
Cubit returns and indicates that it did the following after the merge:
...Merging all features in the model
...Merging all Surfaces in the model
Consolidated 3 pairs of surfaces
...Merging all Curves in the model
Consolidated 0 curves
...Merging all Vertices in the model
Consolidated 0 pairs of vertices
Three surfaces were merged and there were no unmerged curves or vertices left after merging the surfaces. That is exactly what we want. The surfaces merged completely.
Now if I mesh the model, I get about 800K tetrahedra. Notice that in the image the mesh is very dense in areas of high curvature or near small features. This is frequently the desired result.
We can decrease the mesh density by turning off Geometry Sizing.
This drops the number of elements to around ~167K without modifying the default size.
You should be able to get the desired number of elements by increasing the size parameter. Using
volume all size 70
gives 105K tetrahedra.
I did notice that I get a warning from the GUI when I tetmesh the volumes. I think that may be a false positive. Are these the surfaces that should be merged? Note that the top surface is folded over.
This is super helpful. I had been using the imprint and merge function previously - but I had been using the subtract boolean function prior to imprint and merge and this certainly appears to have added more overlapping surfaces etc.
The surfaces that have merged appear correct on your version. I was also hoping you may provide some advice to help align the geometry of these surfaces prior to meshing. For example the tendon-aponeurosis and muscle tissue are in contact along the entire surface with no gaps (please see screenshot that illustrates said gap).
Also when I perform your instructions I get fewer elements than you note ~15240 (with a far less smooth geometry still intact) - I have attached this model so you can see what I mean.
delete mesh
volume all scheme tetmesh
volume all size 70
As far as morphing the geometry to remove the gaps, that could be difficult. What are the constraints? Do you have to maintain the volume or general shape of the tendons?
Yes, ideally I need to maintain the general volume and shape of the tendons - but due to MRI error there is often small gaps due to these very thin tendon structures.
I can modify these using the CAD software used to build the solid model parts - but this can be quite difficult and time consuming.
You are probably best served making the geometry changes in the CAD system.
If you are doing many models of this type, you could look at creating python code that would move nodes onto the surface. Maintaining volume would require maintaining the volume of the attached tetrahedra. That gets pretty tricky, but doable given enough effort. After that you need some sort of tied contact to enforce attachment. This would probably not give you a conformal mesh.
The idea of a tied contact might be an FEA trick to solve the problem. You could create rigid beams from the tendon to the muscle in the gap areas to tie the two together without modifying the geometry. Given your geometry, you would likely have to create the beams manually.
