Generation of Computational Mesh for CFPD Simulations - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 80

addition, a large number of grid generation software and open source codes exist
and the full listing is given in Appendix A. Typically a good meshing program will
be able to automatically fill the CAD geometry with triangles on surfaces (2D) and
tetrahedrals in a volume (3D). This process typically involves meshing the edges first,
followed by filling the enclosed face with elements connecting the edges (advancing
front method). Finally for a 3D geometry the interior volume is filled, and all the
elements are reconnected (Delauney method). In more recent times there has been
considerable development towards mesh containing polyhedral cells to fill the interior
domain. A clear potential benefit of applying polyhedral mesh is that it allows the
flexibility of an unstructured mesh to be applied to a complex geometry without the
overhead associated with a large tetrahedral mesh. The application of such cells is
still in its infancy. Nevertheless, polyhedral meshing has been shown, thus far, to
have tremendous advantages over tetrahedral meshing with regard to the attained
accuracy and efficiency of the numerical computations.
Unstructured meshes may also involve the use of hexahedral, pyramid, and wedge
cells in combination with tetrahedral cells, whereas a structured mesh is reliant on
hexahedral cells or the use of block-structured mesh only. A mesh with a combination
of different cell types is called a hybrid mesh and typically involves matching mesh
cells with the boundary surfaces and allocating cells of various element types in
other parts of the complex flow regions. Hexahedral elements are excellent near solid
boundaries (where flow field gradients are high) and afford the user a high degree
of control, but these elements are time consuming to generate. Prismatic elements
(usually triangles extruded into wedges) are useful for resolving near-wall gradients,
but suffer from the fact that they are difficult to cluster in the lateral direction due to the
underlying triangular structure. In almost all cases, tetrahedral elements are used to
fill the remaining volume. Pyramid elements are used to transition from hexahedral
elements to tetrahedral elements. Many codes try to automate the generation of
prismatic meshes by allowing the user to define the surface mesh and then marching
off the surface to create the 3D elements. While very useful and effective for smooth
shapes, the extrusion process can break down near regions of high curvature or sharp
discontinuities.
Hybrid grid methods are designed to take advantage of the positive aspects of both
structured and unstructured grids. Hybrid grids utilize some form of structured grid in
local regions while using unstructured grid in the bulk of the domain. This generally
leads to both accurate solutions and better convergence for the numerical solution
methods. However one disadvantage of hybrid methods is that they can be difficult
to use and require user expertise in laying out the various structured grid locations
and properties to get the best results. Hybrid methods are typically less robust than
unstructured methods. While the flow solver will use more resources than a structured
block code, it should be very similar to an unstructured code. Post processing the
flow field solution on a hybrid grid suffers from the same disadvantages as does an
unstructured grid. The time required for grid generation is usually measured in hours
or days.