Tonal dynamics and sound in subsonic turbulent jets

Abstract

Acoustic waves trapped in the potential core of subsonic turbulent jets have recently been observed and explained by Towne et al. We show that these waves also radiate outside the jet, primarily into the upstream arc. We provide an experimental identification of the Mach-number dependence of the phenomenon, which indicates that the modes are active even when evanescent, probably due to turbulent forcing. Finally, we show that for Mach numbers lower than about 0.8, the strong tonal dynamics and sound radiation (up to 170dB) that occur when a sharp edge is placed close to the jet are related to a resonance mechanism involving convective hydrodynamic wavepackets and a ‘slow’, upstreampropagating, trapped acoustic mode. A Helmholtz scaling of the resonance at higher Mach number suggests involvement of the ‘fast’ trapped modes in the range 0.8 ≤ M ≤ 1.