Kármán vortex street simulation

Above a critical Reynolds number, the wake behind a cylinder becomes unstable and vortices peel off alternately from each side, forming a regular street. It is why wires sing in the wind and why tall chimneys have spiral strakes.

Interactive, free and runs in your browser. Part of the Fluid Dynamics simulations.

What you will see

Vortices shed from the top and bottom of a cylinder in turn and drift downstream in two staggered rows, red for anticlockwise and blue for clockwise. Thousands of dye particles are carried by the flow, revealing the swirling street.

Try this

  1. Raise the Reynolds number and watch the vortices get stronger.
  2. Count how often a vortex sheds from the top.
  3. Follow one vortex downstream and watch it move slower than the flow.

Common misconception

The vortices do not shed from both sides at once. They alternate, and that alternating sideways force is what makes cables and chimneys vibrate.

Exam-style question

Wind at 0.5 m/s flows past a 2.0 cm diameter pole. With a Strouhal number of 0.20, how often are vortices shed?

f = St × U / D = 0.20 × 0.5 / 0.020 = 5 Hz.

Key equations

  • Re = ρUD / μ
  • St = fD / U ≈ 0.2

Curriculum

Enrichment: fluid mechanics.

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