Page 298 - Computational Fluid Dynamics for Engineers

288 9. Grid Generation

9.7 Unstructured Grids

The unstructured grids of the previous section are typically quadrilaterals in two
dimensions. The generation of these grids involves complex iterative smoothing
techniques that attempt to align elements with the boundaries of physical do-
mains. This task can become very difficult if the geometry becomes very com-
plex. It is often necessary to decompose this complex domain into blocks with
simpler topologies and generate structured grids inside each block in turn, a
method called the multi-block approach. Unstructured grids can also be used
to provide a good representation of complex boundaries. Unstructured grids
are usually made of triangles in two dimensions. Because of the immense scope
of the field of unstructured mesh generation, this section will simply introduce
the more fundamental aspects. Figure 9.16 shows a typical unstructured grid
around a two-dimensional multi-element airfoil.
There are several algorithms for generating unstructured grids. The De-
launey triangulation method and the advancing front method are the most pop-
ular. The field is in rapid expansion, and there are schools of thought whether
the unstructured approach is better or worse than the structured approach to

Fig. 9.16. 2D Unstructured grid for Navier-Stokes computations of a multi-element airfoil
generated with the hybrid advancing front Delaunay method of Mavriplis [6].