2D Navier-Stokes (vorticity-stream formulation):
∂ω/∂t + (u·∇)ω = ν∇²ω
∇²ψ = −ω, u = ∂ψ/∂y, v = −∂ψ/∂x
ν = 1/Re. Two counter-rotating Gaussian vortices interact: they rotate around each other, then diffuse and reconnect as viscosity spreads vorticity.
Colormap: blue = negative ω, red = positive ω. Energy dissipation = ν·∫ω² dA.