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 $\mathrm{Re} = UD / \nu$. In the stable single-relaxation-time window ($\tau \ge 0.56$) the solver settles to a steady laminar wake, so the card shows what it computes: the stagnation point, the flow accelerating around the shoulders, the shear layers, and the downstream velocity deficit. Paint a body and the channel reroutes around it live.
WHAT TO TRY
- Draw obstacles into the flow: the half-way bounce-back at your shapes forces the stream around them and the dye streaklines bend to follow it. The channel reroutes live around whatever you paint.
- Read the two profiles below the field. The transverse cut shows the velocity deficit the body carves into the wake; the centreline cut stagnates to zero at the body and recovers downstream as the wake closes.
- Push the inflow U up and the viscosity tau down: the Reynolds number climbs, the wake lengthens, and the deficit deepens. Switch the field to vorticity to see the two shear layers peel off the shoulders.