Explore Karman vortex shedding and lock-in phenomenon in fluid-structure interaction
When fluid flows past a bluff body at sufficient Reynolds number (Re > 47 for a cylinder), alternating vortices shed from each side, forming a Karman vortex street downstream. The shedding frequency follows f_s = St * U / D, where St is the Strouhal number (~0.2 for cylinders). Vortex shedding exerts a periodic transverse force on the structure.
When the vortex shedding frequency f_s approaches the structure's natural frequency f_n (typically U_r = 4-8), the two frequencies synchronize. Vibration amplitude grows dramatically as the structure and flow reinforce each other. This positive feedback is a leading cause of fatigue failure in bridges, chimneys, and marine risers.
Tacoma Narrows is a famous warning case about wind-excited bridge instability, but this page is only a simplified cross-flow oscillator analogy, not a torsional bridge reconstruction. The lesson it does capture is that very low structural damping can make wind-driven oscillations far more dangerous.
Ocean currents flowing past cylindrical risers and mooring lines cause VIV, leading to fatigue damage. The oil and gas industry uses helical strakes to disrupt vortex formation. Reduced velocity U_r is the key design parameter for VIV assessment.
Watch the flow field: alternating red/blue vortices form the Karman street. Increase velocity gradually: at U_r ~ 5, vortices synchronize with the structure and amplitude surges — this is lock-in. The amplitude response curve shows the characteristic bell-shaped peak.
1) Slowly increase U from 0.1 to 2.0 and watch the amplitude peak near U_r = 5. 2) Compare lock-in vs high damping preset: damping suppresses vibrations. 3) Switch shape: square has different St and CL. 4) Use the low-damping bridge preset to see how weak damping amplifies response.