Houdini 18.0 Finite Elements

Modeling meshes for use with FEM

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Guidelines

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

  • Use Solid Embed or use Remesh in combination with Solid Conform.

  • 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.

  • Inspect the quality of your tet mesh before you attempt to simulate

Manually creating simulation meshes

Simulation meshes for FEM may consist of tetrahedra, polygons or a mix of these two primitive types. 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 Solid Embed SOP to create a tetrahedral mesh from geometry.

    The Solid 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 Solid Embed offers, you can use Solid Conform SOP.

    However, you should not use Solid 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 Solid 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 Solid Conform.

  • For most types of simulations, you should use the Solid 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 Visualization SOP: First, enable "Create Quality Attribute" under the Attributes tab of your FEM object. Then, put an FEM Visualization SOP below your Dop Import node. Good quality tetrahedra and triangles are displayed in green, bad ones in red.

In addition to this, you want to avoid tetrahedra (triangles) that are very small in size, relative to the other tetrahedra (triangles) in the mesh.

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 look at slices of your tetrahedron mesh using the Clip geometry node.

  • Make sure you don’t have points very close together. This creates self-collision problems. You should either remove or fuse such points. (The Fuse node tends to create tetrahedrons in which the same point occurs multiple times as a vertex. The PolyDoctor node can help fix this problem.)

  • Larger objects (in world size) take longer to simulate.

  • If your simulation is very slow, try reducing the tetrahedron resolution. You can see the number of tetrahedrons in a model by pressing 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 target constraints instead.

Finite Elements

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