Generation of Computational Mesh for CFPD Simulations - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 87

Fig. 4.16 A triangular cell
having an angle of β between
the surfaces normal to the
triangular parts of the faces
connected to two adjacent
triangles

In many grid generation packages, the problem can be overcome by a grid smoothing
algorithm to improve the element warp angles . Whenever possible, the use of tetra-
hedral elements should be avoided in wall boundary layers. Prismatic or hexahedral
cells are preferred because of their regular shape.
Since there is no restriction on the use of the type of cells in an unstructured grid, a
hybrid mesh can be used which combines different element types, such as triangular
and quadrilateral in two dimensions or tetrahedral, hexahedra, prisms and pyramids
in three dimensions. This allows maximum flexibility in matching appropriate cells
with boundary surfaces and allocating cells of various element types in other parts
of the complex flow regions. For the example of a 90 ◦ bend geometry, grid quality
can be enhanced through the placement of quadrilateral or hexahedral elements in
resolving the viscous boundary layers near the walls whilst triangular or tetrahedral
elements are generated for the rest of the flow domain as was shown in Fig. 4.9 . This
normally leads to both accurate solutions and better convergence for the numerical
solution methods.
Finally, special grid design features such as the H-grid, O-grid or C-grid in-
troduced earlier need careful consideration to locate block interfaces because this
significantly improves the overall quality of a block-structured mesh. The presence
of arbitrary mesh coupling, non-matching cell faces, or extended changes of element
types at block interfaces should always be avoided in critical regions of high flow
gradients or high shear. Wherever possible, finer and more regular mesh in these
critical regions should be employed. This also applies to regions that exhibit signif-
icant changes in the geometry or where suggested by error estimates. In all cases,
it is recommended that the CFD user check the assumptions made when setting up
the grid with regards to the critical regions of high flow gradients; and if necessary,
proceeds to rearrange the grid nodal points.

4.4.2 Local Refinement and Solution Adaptation

In order to capture critical flow regions an adequate mesh resolution is needed. Such
regions may include flows around obstacles that cause flow separation, attachment
and recirculation, near-wall boundaries, interface shear regions, and converging and
diverging regions. These regions typically exhibit sharp flow property gradients (e.g.