The incompressible Navier-Stokes equations are solved using a finite difference vorticity-stream function formulation. The vorticity transport equation ∂ω/∂t + (u·∇)ω = (1/Re)∇²ω is advanced in time, with the stream function ψ recovered via the Poisson equation ∇²ψ = −ω. Velocities follow from u = ∂ψ/∂y, v = −∂ψ/∂x. The Reynolds number Re = UL/ν controls the ratio of inertial to viscous forces.