Fluid Painter: Lattice Boltzmann Sandbox

A D2Q9 BGK lattice-Boltzmann channel flow on a 192x96 grid: steady inflow on the left, zero-gradient outflow on the right, and half-way bounce-back at user-drawn obstacles. The relaxation time $\tau$ sets the kinematic viscosity $\nu = (\tau - 1/2)/3$ and hence the obstacle Reynolds number $\mathrm{Re} = UD / \nu$, so lowering $\tau$ drives the wake from a steady recirculation toward unsteady vortex shedding.