For each of the following situations, determine whether it is better to use a regular Cartesian, irregular structured, or unstructured grid. Provide arguments supporting your answer. If a grid with varying step or cell size is justified, explain how the size must vary and why.

a) Flow in a gap between two concentric spheres rotating with different velocities around a common axis (the spherical Couette flow).

b) Flow of air past a car moving in a tunnel (see Figure 2.5).

c) Flow of ocean water around the supporting structure of an off-shore oil drilling platform.