Andy Farnell, Designing Sound (Ch 35: Air & Wind) · Strouhal Aeolian Vortices · Helmholtz Cavities
1. Strouhal Vortex Shedding (Aeolian Tones): When air streams past cylindrical obstacles (wires, edges, branches) of diameter d at velocity v, alternating vortices create acoustic pressure oscillations at f = St × (v / d). Smooth cylinders exhibit a Strouhal number St ≈ 0.20.
2. Broadband Shearing & Turbulence: Pink noise filtered through dynamic high-Q and low-Q bands simulates boundary-layer drag, obstacle friction, and low-frequency gust wandering.
3. Cavity Whistling: Air shearing across hollow architectural apertures excites resonant Helmholtz and pipe modes (the classic howling chasm).