Marking or refining the grid based on volume magnitude is often used to remove large cells or to globally refine the mesh. The procedure is to mark for refinement any cell with a volume greater than the specified threshold value.
Marking or refining the grid based on the change in cell volume is used to improve the smoothness of the grid. The procedure is to mark for refinement any cell that has a volume change greater than the specified threshold value. The volume change is computed by looping over the faces and comparing the ratio of the cell neighbors to the face.
For example, in Figure 26.8.1 the ratio of V1/V2 and the ratio of V2/V1 is compared to the threshold value. If V2/V1 is greater than the threshold, then C2 is marked for refinement.