Correlation of Large-Scale Structure Dynamics and Far-Field Radiated Noise in a Mach 0.9 Jet

Abstract

*† ‡ The main goal of the present work is to excite various instabilities of a high Reynolds number, highspeed jet, track the ensuing large-scale structures, and explore relations between the flow dynamics and the far-field sound. In this paper, results from a Mach 0.9 axisymmetric jet with a ReD of 0.76 x 10 6 are presented and discussed. The flow field is investigated using particle image velocimetry (PIV) and the far field is investigated using an array of 12 microphones at 30° to the downstream jet axis. The far-field pressure level is used to estimate the origin of sound waves in space and time within the jet. The jet is forced by eight localized arc filament plasma actuators equally spaced around the circumference of the nozzle, near the nozzle exit. The actuators operate over a wide frequency range (a few Hz to 200 kHz), and the phase between actuators can be controlled to excite various azimuthal modes (m = 0, 1, 2, 3, ±1, ±2 and ±4). Only the results for forcing around the jet column instability frequency (4kHz) with azimuthal modes m = 0, 1 and 2 will be presented here. When exciting the jet column instability at these lower azimuthal modes, flow visualization shows very robust structures and enhanced mixing. The PIV data confirm these results by exhibiting a larger decay in the jet’s centerline velocity and an increase in turbulent kinetic energy. The results show that the high amplitude noise radiated to 30° is originated shortly after the collapse of the potential core. In this region, phase-averaged PIV data shows the two sides of the mixing layer have interacted and the coherent part of the TKE along the jet centerline has grown, saturated, and is in the decay phase. The dynamics of the velocity fluctuations and TKE in the region of noise generation is highly dependent on the azimuthal mode excited.