SPH (smoothed particle hydrodynamics)
A method that represents a fluid as a cloud of particles, where pressure and density are computed by weighting in the neighbours within a radius — a field with no grid.
SPH represents a continuous medium with discrete particles carrying mass and velocity. The value of a quantity at a point — density, pressure — is found by summing a smoothing kernel over every particle within a radius. The method was described in 1977 by Gingold and Monaghan, and independently by Lucy, for astrophysics, and was later adopted for fluid simulation in film and games.
Because everything rests on neighbourhood, the bottleneck is finding which particles lie close — in practice a spatial hash that bins space into cells. SPH is Lagrangian (it follows the matter), unlike grid-based, Eulerian methods, and is related to particle-based position-based dynamics.