In this document we demonstrate how to generate unstructured tetrahedral meshes for
oomph-lib, based on the output from Hang Si's open-source mesh generator
Tetgen . The mesh generation is performed in a two-stage process. First we use
Tetgen to generate the mesh "offline". Then we process the output files generated by
Tetgen to generate an
Tetgen home page contains a comprehensive User's Guide for the code and its many options, therefore we only present a brief overview of the code's most basic usage.
Tetgen creates the mesh based on the information about the mesh boundaries provided in an input file,
filename.poly, say. By default, three output files,
filename.1.face are created. They contain the information about the nodal positions, the element connectivity lists and the boundary faces respectively.
The input file for
Tetgen usually has the extension
poly and has the following format:
First line: [number of nodes] [dimension (must be 3)] [number of attributes] [number of boundary markers (0 or 1)]
One line: [number of facets] [boundary markers (0 or 1)]
One line: [number of holes]
Region attributes list
One line: [number of region]
b+1should be used to indicate that a node is located on the mesh boundary
bin the final
oomph-libdoes not use the "attributes" so their number should be set to zero.
Tetgen home page for further information.
To create the mesh from a given input file the command is
With these commands,
Tetgen will generate as few tetrahedra as possible. Finer meshes may be generated by imposing additional constraints via command line arguments. For instance
-an where n is the maximal volume wanted. (There is no space between
-aand the number specifying the volume!)
Again, we refer to the
Tetgen home page for a comprehensive listing of all available options.
To visualise the mesh, the program
tetview (distributed with
Tetgen ) can be used.
To illustrate the procedure, we demonstrate how to generate a mesh for the cube domain with a hole shown in the figure below. Note that the node numbers correspond to those in the
Tetgen input file. Boundary 1 is shown in blue; boundary 2 in magenta. In the corresponding
oomph-lib mesh, the boundaries are numbered from zero.
Here is a listing of the corresponding input file,
The output files that
Tetgen creates with the command
Here is a sketch of the resulting discretisation, as displayed by
oomph-lib provides a mesh,
TetgenMesh, that uses the output from
Tetgen to generate an unstructured
oomph-lib Mesh containing elements from the
TElement<3,NNODE_1D> family of the tetrahedral elements. The relevant interface is:
The driver code mesh_from_tetgen_poisson.cc demonstrates the use of this mesh for the solution of a 3D Poisson problem in the "cube domain with a hole", described in the previous section.
The code expects the names of
ele and *.
face files generated by
Tetgen as command line arguments and stores them in the namespace
The names of these files are then passed to the mesh constructor. Since the rest of the driver code is identical to that in the corresponding example with a structured mesh, we do not provide a detailed listing but simply show the plot of the computed results, together with a plot of the exact solution, for linear (four-noded) elements
...and quadratic (ten-noded) elements:
The driver code mesh_from_tetgen_navier_stokes.cc demonstrates the use of a
TetgenMesh for the solution of a 3D Navier-Stokes problem in the same geometry as in the previous example. We apply no-slip boundary conditions on the central hole and impose a unit vertical velocity. On the outer boundaries we pin the and velocity components and set them to zero, while leaving the axial velocity unspecified. The resulting flow field, shown in the figure below, corresponds to the flow that is generated when the rigid block in the centre of the domain rises vertically inside a rigid duct with "slippery walls". The duct is open at the ends , where parallel, axially-traction free outflow is imposed. The various slices show pressure contours and the in-plane velocity vectors.
We re-iterate that
Tetgen does not allow nodes to be located on multiple boundaries. It is therefore important to check the boundary numbers allocated by
Tetgen , e.g. by using the function
Mesh::output_boundaries(...). Boundary nodes should always be placed on the boundary with the most restrictive boundary conditions. If this is not possible, some post-processing of the mesh may be required.
TetgenMesh can only be used to generate four and ten-node tets (i.e. tets with tri-linear and tri-quadratic shape functions). It should be easy to generalise the "scaffold-mesh"-based mesh generation procedure to higher-order elements but this has not been done yet. Any volunteers?
Tetgen, , and create your own meshes.
A pdf version of this document is available.