Reynolds150

Drag C_d–

Lift C_l–

Strouhal–

Step0

Vorticity

Inspector Hover over the flow Move the pointer across the field to read local values.

Drag to stir and inject dye. Drag the body or the probe to move them. Hold Shift to stir over them.

Fluid Lab

A real-time 2D incompressible Navier–Stokes solver (stable fluids with MacCormack advection and pressure projection). Stir it, put things in the way, and watch vortices form.

Playback speed 1.0×

Presets

Flow

Reynolds number (viscosity) 150 Flow speed U 0.60 Vorticity confinement 0.10 Buoyancy (gravity × expansion) 0.0 Dye dissipation 0.15 %

ν = U·L / Re. The grid has its own numerical viscosity, so very high Re values saturate.

Obstacle & tools

Body size L 12.5 % Body angle 0°

Mouse tool

Brush radius 5

Display

Velocity vectors Tracer particles Inlet smoke streaks Wake probe

Colour gain 1.0×

Grid resolution

Wake probe drag the probe on the field to move it

probe v / Ulift C_l

Strouhal St = f·L/U: –
Shedding period: –
Drag C_d (mean): –
Lift C_l (rms): –
Viscosity ν: –
Frame time: –

What you are seeing

Stable fluids. Each step adds forces, advects velocity, dye and temperature with a second-order MacCormack scheme, diffuses with the viscosity, then projects the velocity onto a divergence-free field by solving a Poisson equation for pressure (red-black Gauss–Seidel).
Kármán street. Above Re ≈ 50 the wake behind a cylinder becomes unstable and sheds alternating vortices at St = fL/U ≈ 0.2. Lift oscillates at the shedding frequency, drag at twice that.
Buoyancy. Hot fluid is lighter and rises (smoke plume). Heating from below and cooling from above makes Rayleigh–Bénard convection cells. A velocity shear layer rolls up into Kelvin–Helmholtz billows.

Forces use the surface pressure plus a crude wall-shear term on the staircase boundary and are normalised by ½ρU²L per unit span; treat them as estimates, not wind-tunnel numbers.