19.8.1 Yplus/Ystar Adaption Approach

The approach is to compute $y^+$ or $y^*$ for boundary cells on the specified viscous wall zones, define the minimum and maximum allowable $y^+$ or $y^*$, and mark and/or adapt the appropriate cells. Cells with $y^+$ or $y^*$ values below the minimum allowable threshold will be marked for coarsening and cells with $y^+$ or $y^*$ values above the maximum allowable threshold will be marked for refinement (unless coarsening or refinement has been disabled).

Figure  19.8.1 shows the mesh for a duct flow, where the top boundary is the wall and the bottom boundary is the symmetry plane. After an initial solution, it was determined that $y^+$ values of the cells on the wall boundary were too large, and $y^+$ adaption was used to refine them. The resulting mesh is shown in Figure  19.8.2. This figure shows that the height of the cells along the wall boundary has been reduced during the refinement process. However, the cell-size distribution on the wall after refinement is much less uniform than in the original mesh, which is an adverse effect of $y^+$ adaption.

See this section in the separate User's Guide for guidelines on recommended values of $y^+$ or $y^*$ for different near-wall treatments.

Figure 19.8.1: Duct Flow Mesh Before Adaption

Figure 19.8.2: Duct Flow Mesh after $y^+$ Adaption