Observations on subsonic edge tones.

Abstract

Discrete tones, sometimes multiple, occur for turbulent jets with exit Mach numbers (M) between 0.14 and 1.0 at nozzle-to-edge distances of 3 ≤ h/d ≤ 55 (d=1 mm), with an 8° wedge. The minimum h/d for discrete tones to occur increases linearly with Mach number for 0.14 ≤ M ≤ 1.0. Broadband tones occur at h/d below this minimum distance. As in the classical edge tone, the frequency of the strongest tone increases proportionally with exit velocity at a constant h/d and is approximately inversely proportional to h/d(f∼h/d−1.1) at a constant exit velocity. Schlieren photographs reveal the asymmetric nature of the jet instabilities and the out-of-phase sound waves on either side of the jet. Up to three continuous stages occur, with frequency jumps between them. Ucon (convection velocity of the jet instability) varies from 0.22 Uexit to 0.45 Uexit (Uexit=fully expanded jet exit velocity) for 0.22 ≤ M ≤ 1.0 and 0.12 ≤Mcon≤0.45, Mcon=Ucon/ca (ca=sound speed). The sound pressure level (SPL) peaks within 1≤h/d≤5 of the minimum h/d. [Work supported by the Texas Advanced Research Program.]