Decoupled Boundary Handling in SPH

Abstract: Particle-based boundary representations are frequently used in Smoothed Particle Hydrodynamics (SPH) due to their simple integration into fluid solvers. Commonly, incompressible fluid solvers estimate the current density and corresponding forces in case the current density exceeds the rest density to push fluid particles apart. Close to the boundary, the calculation of the fluid particles' density involves both, neighboring fluid and neighboring boundary particles, yielding an overestimation of density, and, subsequently, wrong pressure forces and wrong velocities leading to the disturbed fluid particles' behavior in the vicinity of the boundary. In this paper, we present a detailed explanation of this disturbed fluid particle behavior, which is mainly due to the combined or coupled handling of the fluid-fluid particle and the fluid-boundary particle interaction. We propose the decoupled handling of both interaction types, leading to two densities for a given fluid particle, i.e., fluid-induced density and boundary-induced density. In our approach, we alternately apply the corresponding fluid-induced and boundary-induced forces during pressure estimation. This separation avoids force overestimation and reduces unintended fluid dynamics near the boundary, as well as a consistent fluid-boundary distance across different fluid amounts and different particle-based boundary handling methods. We compare our method with two regular state-of-the-art methods in different experiments and show how our method handles detailed boundary shapes.