Mesh Generators

Modules
	chi_mesh.ExtruderMeshGenerator
	chi_mesh.FromFileMeshGenerator
	chi_mesh.MeshGenerator
	chi_mesh.OrthogonalMeshGenerator

Detailed Description

We split a MeshGenerator's execution into a phase that generates an unpartitioned mesh and a phase that then converts this mesh into partitioned chi_mesh::MeshContinuum (with both steps customizable). The phase that creates the MeshContinuum object can be hooked up to a partitioner that can also be designed to be pluggable.

Example A

nodes = {-1.0,-0.75,-0.5,-0.25,0.0,0.25,0.5,0.75,1.0}
meshgen1 = chi_mesh.OrthogonalMeshGenerator.Create({ node_sets = {nodes,nodes} })
chi_mesh.MeshGenerator.Execute(meshgen1)
 
chiMeshHandlerExportMeshToVTK("ZMeshTest")

In this example we created a set of nodes (monotonically increasing in value) for use with the chi_mesh.OrthogonalMeshGenerator. We supplied the same set twice meaning the generator will build a 2D mesh.

We did not specify a partitioner and therefore the generator will use the chi.PETScGraphPartitioner with type="parmetis" by default. Using 8 processes, we can see the mesh and it's partitioning below.

Example B

meshgen1 = chi_mesh.FromFileMeshGenerator.Create({ filename="TriangleMesh2x2.obj" })
chi_mesh.MeshGenerator.Execute(meshgen1)
 
chiMeshHandlerExportMeshToVTK("ZMeshTest")

In this example we created a mesh by reading it from a file, using the chi_mesh.FromFileMeshGenerator. The types of file-types we can support is ever growing. At the time of writing this we support the following formats:

• .obj Wavefront
• .msh gmesh,
• .e ExodusII,
• .vtu VTK Unstructured grid,
• .pvtu Pieced VTK Unstructured grid,
• .case Ensight Gold

Using 8 processes, we can see the mesh and it's partitioning below.

Example C

meshgen1 = chi_mesh.ExtruderMeshGenerator.Create
({
  inputs =
  {
    chi_mesh.FromFileMeshGenerator.Create({ filename=TriangleMesh2x2.obj" }),
  },
  layers = {{z=1.1, n=2},    -- First layer - 2 sub-layers
            {z=2.1, n=3}},   -- Second layer - 3 sub-layers
})
chi_mesh.MeshGenerator.Execute(meshgen1)
 
chiMeshHandlerExportMeshToVTK("ZMeshTest")

In this example we use an chi_mesh.ExtruderMeshGenerator with another mesh generator as an input to create an extruded mesh. Using 8 processes, we can see the mesh and it's partitioning below.

Using different Partitioners

ChiTech now has a set of Graph Partitioners (i.e. based off GraphPartitioner) that support different forms of partitioning, for example we have:

• chi.LinearGraphPartitioner, a very basic partitioner used for during the preparation of simulation meshes.
• chi.PETScGraphPartitioner, a flexible partitioner that can use all the partitioner options available in PETSc (defaults to using "parmetis").
• chi.KBAGraphPartitioner, the classical Neutron Transport KBA parallel partitioning with an overlaid orthogonal layout.

An example of changing the partitioning to PETSc's "average" option is shown below:

meshgen1 = chi_mesh.ExtruderMeshGenerator.Create
({
  inputs =
  {
    chi_mesh.FromFileMeshGenerator.Create
    ({
      filename="resources/TestMeshes/TriangleMesh2x2.obj"
    }),
  },
  layers = {{z=1.1, n=2}, {z=2.1, n=3}},
  partitioner = chi.PETScGraphPartitioner.Create({type="average"})
})
chi_mesh.MeshGenerator.Execute(meshgen1)
 
chiMeshHandlerExportMeshToVTK("ZMeshTest")

Another example using the KBAGraphPartitioner is shown below

meshgen1 = chi_mesh.ExtruderMeshGenerator.Create
({
  inputs =
  {
    chi_mesh.FromFileMeshGenerator.Create
    ({
      filename="resources/TestMeshes/TriangleMesh2x2.obj"
    }),
  },
  layers = {{z=1.1, n=2}, {z=2.1, n=3}},
  partitioner = chi.KBAGraphPartitioner.Create
  ({
    nx = 2, ny=2, nz=2,
    xcuts = {0.0}, ycuts = {0.0}, zcuts = {1.1}
  })
})
chi_mesh.MeshGenerator.Execute(meshgen1)
 
chiMeshHandlerExportMeshToVTK("ZMeshTest")