Mesh Issues

Highly Skewed Cells at Sliding or Rotating Interfaces or Small Gaps

In circumstances where the cell orthogonality or connectivity is compromised, the solver can experience instability with Bounded Central Differencing (for LES) or Hybrid-Bounded Central Differencing (for DES). Under these circumstances, second-order upwind spatial discretization is recommended; it is more robust than Bounded Central Differencing. However, expect the flow structures to suffer smearing from the upwind contribution of the second-order scheme. The main effect is the reduction of the spectral magnitudes at higher frequencies, and, possibly, the redistribution of energy to lower frequencies.

Refinement Interfaces on a Trimmed Mesh

There is some evidence that (acoustic) pressure reflections or spurious (acoustic) convective disturbances can occur at mesh coarsening interfaces for trimmed meshes. As a result, the local flow fluctuating structures swamp the acoustic pressure effects. The likely cause is local orthogonality errors that reduce the spatial accuracy toward first-order accuracy. Using 20 cells per acoustic wavelength is recommended. But, where the local discretization due to refinement is reduced to first order, use 40 cells per acoustic wavelength.

Another solution to the issue is to use an equivalent polyhedral mesh with gradual coarsening/refinement. The polyhedral mesh eliminates reflections, but a speed penalty factor of around 1.5 exists for an equivalent mesh size—directly proportional to the increased number of faces per cell.