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How does SPH Adaptive Size work in Ansys FreeFlow?

 

SPH Adaptive Size in Ansys FreeFlow works by refining and coarsening SPH elements in a controlled, hierarchical manner, improving accuracy only where needed while keeping overall computational cost low.

The method is based on discrete refinement levels. For each refinement level, the SPH element size is reduced by a factor of 2. This means that if the initial SPH element size is:

 

• Refinement level 1 produces elements of size s / 2
• Refinement level 2 produces elements of size s / 4
• Refinement level n produces elements of size s / 2ⁿ

 

When an element is refined, it is replaced by multiple smaller elements so that the total volume (in 3D) remains unchanged. For example, refining one element by one refinement level results in 8 new elements, each occupying one-eighth of the original volume. At refinement level 2, 64 elements are created, and so on. This guarantees consistent element spacing, conserved mass, and numerical stability.

 

Refinement is applied only in selected regions, using one of three approaches:

 

• Geometry boundary refinement, where elements are refined within a distance of selected wall boundaries. This distance is computed as the product of a Refinement Distance Factor times the initial SPH size. Both parameters, Refinement Distance Factor and SPH size, can be set by the user.
• Cubic Region of Interest (ROI), where all SPH elements inside a user‑defined cube are refined.
• Cylindrical Region of Interest (ROI), where elements inside a user‑defined cylinder are refined, with an optional hole ratio to exclude an inner region.

 

To prevent excessive growth in the number of SPH elements, Ansys FreeFlow software also applies automatic coarsening.

 

Coarsening is performed every 5 solver iterations and merges SPH elements in pairs rather than collapsing many elements at once. Elements are coarsened only if they are far enough from geometry boundaries, compatible in refinement history, sufficiently close to each other based on their smoothing length, and if SPH elements are outside ROIs defined for refinement by the user. This gradual coarsening strategy helps avoid instability and mass loss.

 

In summary, SPH Adaptive Size dynamically refines and coarsens SPH elements at every 20 and 5 solver iterations, respectively. It conserves SPH elements’ volume and spacing, and applies refinement only where needed, delivering higher accuracy in critical regions while maintaining efficient simulation performance.

 

Ansys FreeFlow™ smoothed-particle hydrodynamics (SPH) simulation software

Janaina Oliveira

Technical/Product Publications