Alternating vortex shedding behind a cylinder · Strouhal number St = fD/U ≈ 0.2
The Kármán vortex street forms when a fluid flows past a blunt body at moderate Reynolds number (Re ~ 40–1000). Vortices are shed alternately from opposite sides, forming two staggered rows. Stable configuration: h/a = (1/π)arcosh(√2) ≈ 0.281. Strouhal number St = fD/U ≈ 0.2 for Re ~ 10³–10⁵.