Stable Fluids — Semi-Lagrangian · iterative diffusion · pressure projection

Steady inflow from the left past a cylinder → Kármán vortex street. Drag to paint dye and inject velocity. Grid 256×256. Solver is unconditionally stable — push dt to the max or hit Stress dt and it will not blow up.

Status

STABLE

Mean |∇·u|

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Max |∇·u|

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Dye mass ∑ρ

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Mass drift

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Sim time / step

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FPS

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Timestep 0.120

Inflow speed 0.060

Viscosity 0.00002

Pressure iterations 30

Dye at inlet on

Density → color

low

high

Per step: add forces → diffuse (Gauss–Seidel) → project → advect (semi-Lagrangian backtrace) → project. Obstacle uses no-slip / no-flux (Neumann) boundaries; walls are free-slip; outlet is zero-gradient.