On the Aerodynamic Noise of a Rigid Flat Plate Moving at Zero Incidence

The aerodynamic noise resulting from the subsonic flow over a flat rigid plate at zero incidence has three origins. “Surface” noise due to fluctuating surface pressure is postulated to vanish by the author's image argument, except near the edges of the plate, where it is more appropriately called edge noise. Of dipole nature, its acoustic power depends on the velocity raised to between the fourth and fifth power, and consequently is to be expected to be of prime importance at low enough speeds. The contribution from fluctuating shear stresses is likely to be much smaller and so has been neglected. Quadrupole radiation takes place from from the turbulence of the boundary layer, producing layer noise and also from the turbulent wake, producing wake noise. Together, the latter two are suggested to have a spectrum with a single peak, bounded by slopes like f² and f^−7/4. Their noise power depends on nearly the eighth power of velocity, so is of increasing importance with speed. Analytical details rest on similarity concepts; the spectra in particular are subject to certain conditions. Also, the convection effects on the acoustical power and spectra are excluded on empirical grounds stemming from considerations of jet noise.