MG-Hexa:

A 100% Hex Fully-Automatic Mesher with Boundary Layers (*)

It is based on the octree method, so as to combine full automation and reliability. Its method is quite simple: the mesher starts by inserting the input geometry (provided as a set of triangles) into the object’s bounding box, and refines it recursively according to a set of criteria. This octree mesh is made conformal thanks to the insertion of connecting patterns called conforming patterns. As for all full-hex mesh generation methods, the octree method has its good sides (automation, extreme reliability for all complex geometries), and its bad sides (fine geometry accuracy control, size of meshes produced), but at least it provides a viable approach to many industrial needs.

Since the goal is to obtain well shaped hexahedra in all sorts of complex geometries, MeshGems-Hexa inserts a layer of hexes around the boundary to ensure a good quality of hexahedra near the body. Generating highly anisotropic hexahedra near the boundary, as is required to obtain an accurate simulation of boundary layer effects in CFD, can actually take advantage of this: The layers of  hexas which were built during the meshing process can be used to generate these boundary layer meshes, provided they can be extended much further apart from the geometry, and sliced in order to generate the expected layers.

Generating boundary layer meshes as part of a 100% hex mesh is a capacity that MeshGems-Hexa is able to propose. The user simply specifies the number of layers required, the height of the first layer and a geometrical growth rate as well as the region where the boundary layers have to be built. Out of these information, the size of the last layer as well as the total size of layers will be computed and checked against the size of the smallest boundary hex element, in order to adjust the boundary layer size to the geometry . Furthermore, most of the time, the boundary layer size is dictated by physical considerations and is unrelated to the size of the remaining volume hexes. This may lead to strong size discrepancies between the outer layer elements and the neighbouring volume element belonging to the remaining “isotropic” mesh. To bridge the gap and provide a smooth transition between the two parts of the mesh (the anisotropic part of the boundary layers, and the isotropic part corresponding to the remaining volume), some additional layers, called blending layers, are inserted in between. Finally, an imprinting capability is also available, to make sure boundary layers are imprinted on faces which do not have a boundary layer to grow themselves (like symmetry planes, periodic faces, inlets or outlets, etc.).

(*) : THE “BOUNDARY LAYERS” FEATURE OF MG-HEXA IS A STANDARD OPTION OF THIS COMPONENT, BASED ON THE METHODOLOGY AND ALGORITHM OF MG-HEXA. THE EXTRUSION CAPABILITY AVAILABLE IN MG-HYBRID FOR THE “BOUNDARY LAYERS” GENERATION USES A TOTALLY DIFFERENT APPROACH.