Presets
Playback
Flow and fluid
Pick a fluid, then change speed and diameter: Re = U D / nu.
Layers
Model checks
The science
Strouhal number and the Karman street
Above Re = UD/ν of about 47 the steady wake behind a circular cylinder becomes unstable and sheds vortices alternately from the two sides at a frequency f = St U / D. In the laminar range St = 0.198 (1 - 19.7/Re) (Roshko's fit); above Re of about 190 the wake turns three-dimensional and St settles near 0.2. The same mechanism makes wires hum (aeolian tones) and can drive chimneys and bridge cables into resonance.
Point-vortex model
Each shed vortex is a Lamb-Oseen vortex with circulation Γ (about 1.8 U D) and a finite core, so the velocity it induces at distance r is (Γ/2πr)(1 - e-r²/rc²). The vortices move with the free stream plus the velocity induced by all the others. For an ideal two-row street with row spacing h and vortex spacing a the street travels at U - (Γ/2a) tanh(πh/a), which is why vortices drift downstream slower than the stream (about 0.85 U).
Honest simplifications: the cylinder only appears as potential flow for the tracers; vortices are released at the prescribed Strouhal frequency rather than computed from the Navier-Stokes equations; viscous decay and the three-dimensional breakdown above Re = 190 are not modelled (a small jitter is added to hint at irregularity); no drag or lift forces are computed.
What to try
- Steady wake (Re 30) preset: no vortices are shed, the flow stays symmetric.
- Water rod preset: a clean street with St about 0.17 appears.
- Turbulent wake preset: shedding becomes irregular and St approaches 0.2.
- Raise the vortex strength: the street slows relative to the stream, as Karman predicted.