Total particles: --
Step: 0
Mean density: --
About: The HPP lattice gas (1973) places particles on a square lattice moving in 4 cardinal directions. At each timestep: (1) Propagation — particles move to adjacent cells; (2) Collision — if exactly two particles meet head-on, they rotate 90°. Despite simple rules, HPP approximately satisfies Navier-Stokes equations (with anisotropy). FHP model on triangular lattice corrects this. Colors show local particle density; velocity arrows show mean flow direction.