Intense sound radiation by high-speed flow: Turbulence structure, gas properties, and near-field gas dynamics

Free-shear-flow turbulence with sufficiently fast advection speeds radiates Mach waves, with steepened and skewed pressure profiles. These form within about a mixing layer thickness and dominate the sound field. Their generation and propagation is investigated through comparison of numerical simulations of a temporally developing mixing layer with a series of model-flow simulations designed to isolate physical mechanisms. The first of these are numerical simulations of nonlinearly saturating instability waves, which despite being much simpler than corresponding turbulence reproduce key features of the sound. Motivated in part by this agreement, instability analysis is used to motivate the inclusion of artificial sources in turbulence simulations that are designed to induce specific alterations to the turbulence structures, leaving most of its broadband spectrum unchanged. Comparisons show how insensitive the radiation is to the particular structure. To assess how strongly the near-field sound is coupled to the turbulence, a high dilatational dissipation is imposed to suppress the waves. This significantly reduces radiated pressure intensity, but little changes the Reynolds stresses (<8%), which supports a source-plus-sound perspective. Given this, a low-dimensional nonlinear gas-dynamic mechanism is proposed for the generation and near-field propagation of the waves. The analysis uses a second-order wavy-wall asymptotic solution, and it reproduces the key observations: the sound-field structure, pressure skewness, and even the radiated pressure levels to within a factor of 2.