2D lid-driven cavity flow solved by a lattice-Boltzmann method. Increase Re = ρUL/μ from laminar (streamlines) to turbulent (vortex shedding). Watch the recirculation zone develop.
Reynolds number Re = ρUL/μ is the ratio of inertial to viscous forces. Re≪1: Stokes flow (reversible, creeping). Re~100: steady recirculation. Re~400: unsteady. Re>2300: pipe flow transitions to turbulence. The lid-driven cavity is a canonical benchmark for fluid solvers.