What is SPH Adaptive size coarsening, and how does it work in Ansys FreeFlow?

Coarsening is the process of reducing SPH resolution when refined elements are no longer needed.

Ansys FreeFlow applies automatic coarsening to maintain efficiency.

• Coarsening is performed every 5 solver iterations (fixed behavior)
• Instead of merging many elements at once, the solver merges SPH elements two by two
• This gradual process avoids numerical instabilities and mass loss

Two SPH elements are coarsened only if all three conditions below are satisfied:

1. Distance from geometry
   The elements must be far enough from any geometry boundary:
   • Distance > Refinement Distance Factor × Initial SPH Size
     This prevents instability near walls.
2. Refinement level compatibility
   Each SPH element tracks its refinement history.
   • Coarsening occurs only if the combined refinement count is below the maximum allowed by the refinement level.
3. Distance between elements
   The two elements must be close enough to each other:
   • Their distance must be smaller than the average smoothing length
   • The smoothing length is computed using the Kernel Distance Factor and the SPH element size.