The von Kármán Vortex Street
Theodore von Kármán described one of fluid mechanics' most beautiful phenomena: when a steady flow encounters a bluff body, it does not remain steady. Instead, vortices detach alternately from each side of the body in a remarkably regular pattern — the von Kármán vortex street. Each departing vortex creates a lateral impulse, generating an alternating lift force perpendicular to the wind direction. This alternating force is the root cause of vortex-induced vibrations in chimneys, towers, cables, and bridge decks.
The Strouhal Number
The frequency of vortex shedding is governed by the Strouhal number: f_s = St × V / D. For circular cylinders, St ≈ 0.2 across a remarkable range of Reynolds numbers (300 to 300,000). This universality makes Strouhal number prediction reliable: given the wind speed and body diameter, the shedding frequency is predictable. In this simulation, adjusting wind speed or diameter changes the shedding frequency, and you can see the vortices form and shed at the predicted rate.
The Lock-In Phenomenon
The most dangerous condition occurs when the shedding frequency approaches a structure's natural frequency — the lock-in zone. Instead of the shedding frequency increasing linearly with wind speed, the structural motion takes control: the oscillating structure synchronizes the vortex shedding to its own natural frequency over a range of wind speeds. Vibration amplitudes grow dramatically, limited only by structural damping. The Tacoma Narrows Bridge collapse, while driven by flutter rather than vortex shedding, popularized awareness of wind-structure resonance. True vortex lock-in has caused failures of cooling tower stacks, marine risers, and suspension bridge hangers.
Engineering Countermeasures
Preventing vortex-induced vibration requires either separating the shedding frequency from the natural frequency or disrupting the vortex coherence. Helical strakes — spiral fins wrapped around the body — break up the spanwise correlation of vortex shedding, reducing the net alternating force by 90%. Tuned mass dampers add damping at the critical frequency. Aerodynamic fairings streamline the cross-section, preventing regular shedding entirely. For bridge decks, vortex shedding is often the controlling dynamic load case for fatigue of hangers and deck elements.