Learning Forum

[viliusv]

Hi,

I am struggling with creating a mesh for my project. I am working on structural analysis of a wing that consists of two hollow-square shaped cross-section spars and a number of airfoil-shaped ribs (skin also, but for now I just aim to analyse the previous two). The spars are protruding through the ribs and so I have shared topology option enabled over entire object. Something to note is that spars also have a hole through-the-thickness at one free end to attach them to a fixed plate in fuselage.

I want to 1) create a preferably hex mesh for spars since theyre basically a basic hollow square beams 2) for ribs, make some other mesh like multizone 3) adjust the region around bolt holes with inflation. Problem is that since they are sharing topology, I cant use stuff like multizone or sweep since it gives error. i tried working with named selection but that does not solve the issue. In short, I want to ask how to create different meshes on spars and ribs while maintaining shared topology for smooth global mesh and good quality of mesh?

Thank you.

[peteroznewman]

The spars and ribs should be surface geometry, not solid geometry. Use the SpaceClaim Midsurface tool to create surface geometry from the solid geometry. Some cleanup may be required since the hole in the rib will be larger than the midsurface on the spar, but SpaceClaim has ways to extend edges to close gaps. Then you can use the Share button and meshing will be easy in Mechanical.

[viliusv]

Why is that so? After all, I will later update the model by including skin to which I will apply a real pressure distribution obtained from cfd simulation.

I see your point of using surface geometry given the small thickness dimensions but I do not believe it is appropriate for my configuration. I'd much rather like to ask if I can create mesh individually by suppresing bodies that are not being meshed in the moment and then press share topology to ensure that the different meshes get automatically connected together?

[peteroznewman]

The reason to turn the spars and ribs into surface geometry is to make them easy to mesh. Deformation results will be comparable to a hex mesh.  Stress results from shell elements will have a lot fewer stress singularities than the hex element mesh which will make evaluating stress simpler.  The skin over the ribs should also be surface geometry and the real pressure distribution can be applied to those surfaces.

I don't understand why you think a surface model would not be appropriate for the spars and ribs. Perhaps you can insert a few images to show an example where only a solid body would work. I can understand if you have detailed fittings to connect thin structures like the skin and the ribs together or a detailed fitting where the spars connect to a solid fitting in the fuselage. There is no problem having a mixed solid/surface model to get both of those.

To have shared topology between some geometry such as spars and ribs, put them in one component below the top level assembly. Open that component and use the Share button.  Then from the top level assembly, add another component and move the solid bodies for fittings into that component. Those will not share topology with the spars and ribs. Once that is read into Mechanical, you can use various methods such as a Beam Connection to simulate a bolt connecting the edge of a hole in the spar surface to the face in a solid representing the threaded hole in a fitting. 

[viliusv]

I actually implemented the shell elements and to test it I modelled just the spar fixed at one end and having a point load at the other. the deformation i got at the free end was much underestimated compared to analytic solution (PL^3/3EI). However, when I did the same with solid elements, I got the same deformation and stress as in analytic solution despite horrible mesh. The mesh generated for shell model had high element quality, so its unclear to me why is it modelled incorrectly.

I attach a picture of my mesh with solid elements. the ribs appear to have high quality mesh while spar who is arguably simpler geometry is severely distorted. maybe you have some advice on how to

resolve that?

[viliusv]

As a follow up question, do you know if when I mesh spars, ribs and skin independently, will I be able to just press share topology and the meshes at the intersections will join? i.e. how could i mesh individual elements separately while ensuring mesh is uniform at intersection in the end? Thank you.