Hexes or tets? take the best of both worlds!

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Hybrid mesh generated by MG-Hybrid out of a quad dominant surface mesh

Mesh generated by MG-Hybrid, with hexahedra/prisms induced by the corresponding input boundary elements preserved, conformal transition, and hexahedra in the core

Ever wanted to fill automatically a closed surface composed of quadrilaterals with hexahedra only? Bet you have… Bad news is that it is so difficult, nobody really can. In fact there are even cases where you mathematically can’t at all.
One robust way of generating hex meshes automatically is to build both surface and volume at the same time -this is what MG-Hexa does- , but there’s no way you can make a conformal assembly with another existing mesh. Even keeping the geometry is a challenge (just think of trying to fit a square in a sharp angle…)
On the other hand, filling closed triangular surfaces with tets while preserving exactly the input surface is extremely robust, thanks to tools like MG-Tetra. This means that the geometry is always preserved, and that you can mesh several parts independently. But you only get tets, even if your solver would prefer hexahedra.

So what are the options?
Option 1 : change your solver, so that it can be always satisfied with tets… or,
Option 2 : take the best of both worlds: put hexes wherever you can, while preserving the input surface mesh!

Hybrid mesh generated by MG-Hybrid out of a triangular surface mesh

Mesh generated by MG-Hybrid, with prismatic elements induced by triangular input surface, conformal transition, and hexahedra in the core

This is where MG-Hybrid, a new to-be product of the MeshGems suite, comes into action.
MG-Hybrid, for which a preview is released as part of MeshGems 1.3, is a hybrid  volume mesher. MG-Hybrid conforms exactly to the input quad or mixed tri/quad surface, and fills the corresponding volume in a conformal way with hexahedra wherever possible, or tets, or a collection of other simple elements. Actually, MG-Hybrid is built on top of the tet mesher (MG-Tetra) and the hex mesher (MG-Hexa) with other complementary meshing capabilities, like boundary layers generation or conformal patterns between element types. The input mesh must be a closed and manifold surface mesh, which will be preserved throughout the meshing process. All you have to do is: specify where you want the layers near the surface, how many of them and what height they have, and if you also want hexes in the core. MG-Hybrid is planned for product release in MeshGems 2.0, so have fun in the meantime with this preview!

Main features

  • Hexcore meshing (with transition element for conformity)
  • Automatic Boundary Layer/extrusion meshing
  • Support of multi-normals around convex ridges and corners
  • Automatic control and processing of collision
  • Mixed elements : tetrahedra, pyramids, prisms, hexahedra
  • Extends the applicability of MG-Tetra and MG-Hexa to mixed surfaces (conformal)

Examples

Given a  surface containing quadrilaterals and triangles, MG-Hybrid offers several possibilities:

  • MG-Hybrid can generate layered elements which are orthogonal to the boundary: quads are grown into hexahedra while triangles are grown into prisms. At the same time, the core of the mesh is built with hexahedra. The space between hexahedral zones will be glued conformally using various element types (tetrahedra, pyramids, prisms).
  • MG-Hybrid can generate layered elements which are orthogonal to the boundary: quads are grown into hexahedra while triangles are grown into prisms. A conformal transition layer is then built using various element types (tetrahedra, pyramids, prisms), and the remaining volume is filled with tets.
  • MG-Hybrid can grow quadrilaterals into pyramid volume elements, and the remaining internal volume will then be automatically filled with tetrahedra.

Boundary layers
Specific enhancements are implemented to address boundary layers: multi-normals around convex ridges and corners; full control of location of boundary layers; control of heights and number of layers.

Boundary layer mesh generated by MG-Hybrid

Boundary layer mesh generated by MG-Hybrid, with multi-normal processing