Houdini 19.0 Finite elements

Creating simulation meshes for use with FEM

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Guidelines

  • Start with a polygonal mesh that’s airtight and has no self-intersections.

  • To create a simulation tet mesh, use Tet Embed or use Remesh in combination with Tet Conform.

  • Use the FEM Validate SOP to inspect the quality of the mesh before attempting to simulate it.

  • Always model your objects to real size. Make sure you set the correct Unit Length in the Hip File Options before you start creating your DOP network.

Manually creating simulation meshes

Simulation meshes for FEM may consist of tetrahedra, polygons, polylines, isolated points, or a mix of all these. Tetrahedra provide your objects with local rigidity and local volume preservation, creating the impression of a solid object. Surface triangles may be fused on top of the tetrahedral mesh to create extra strength on the surface. This may be used to create dynamic wrinkling effects, among other things.

The Solid Object shelf tool will automatically add nodes to the geometry network of your objects to convert them to tetrahedrons before importing them into the simulation network.

However, if you want to convert geometry to tetrahedrons yourself for more control, use the following guidelines for good results.

  • Use the Tet Embed SOP to create a tetrahedral mesh from geometry.

    The Tet Embed SOP is usually the most effective option for finite element simulation. It may create a tetrahedral mesh that is slightly larger than the input surface representation. You can control the resolution of the tet mesh with the “Sizing” parameters on the node. The original rendering mesh may be embedded into this larger tet mesh using the Embedded Geometry option on the FEM Solid Object and FEM Hybrid Object.

  • If you want finer control over the mesh than Tet Embed offers, you can use Tet Conform SOP.

    However, you should not use Tet Conform directly on an existing model that’s intended for rendering. Models that are used for rendering typically contain too many polygons to make a fast simulation possible. Also, the shapes of the polygons in a rendering geometry are a poor choice for finite element simulation, leading to poor simulation speeds or poor simulation results. Before Tet Conform, should can use tools like VDB, Remesh and PolyReduce to prepare the mesh for tetrahedralization.

    When a rendering model is all that’s available, you should first apply a Remesh SOP before the Tet Conform.

    You want will want to avoid creating tetrahedra (triangles) that are very small in size, relative to the other tetrahedra (triangles) in the mesh.

  • For most types of simulations, you should use the Tet Embed SOP to create a tet mesh.

Quality Inspection and Improvement

To get good results with FEM, it is essential that you use a good quality simulation mesh; the more each tetrahedron (triangle) resembles an equilateral tetrahedron (triangle), the better. You can inspect the quality of your simulation mesh using the FEM Validate SOP: Simply attach an FEM Validate SOP below the SOP Node that has the geometry that you want to simulate. Using the Type parameter of the FEM Validate SOP you can inspect the mesh quality and detect inversions.

If you find that your mesh has bad tetrahedra and bad triangles, there are a number of ways to improve the mesh. It usually helps to convert for input polygonal mesh to VDB and then back to polygons through Remesh or PolyReduce. Results may improve drastically if the Equalize Lengths parameter on PolyReduce is set to 1.

Tips and notes

  • Don’t worry too much about getting a high-resolution tet-mesh. That is what embedding is for.

  • Avoid tetrahedrons with very small edges and small internal angles. The more regular (equal length sides) each tetrahedron, the better. If the polygonal mesh that you start with has very irregular triangles, use a Remesh node before the tetrahedralizing.

  • You can use the FEM Validate SOP to look at slices of your tetrahedron mesh, by enabling View Slice.

  • If your simulation is very slow, try reducing the tetrahedron resolution. You can see the number of tetrahedrons in a model by pressing MMB on a node in the geometry network.

  • Be careful if you SOP Solvers with finite element simulations. Unlike other solvers (for example, the Grain Solver), you should not directly modify the position (P) and velocity attributes (v) in a finite element simulation. If you want to influence a finite element simulation, use the various FEM Constraint types instead.

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