The aeroacoustics of a subsonic rectangular jet

Abstract

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. We present results for the near-field velocities and far-field acoustics of a rectangular jet. The results show a clear split in behaviour between roughly incompressible Mach numbers and higher Mach numbers. For the incompressible region we find that the peak frequency of uctuations in the near-field matches that in the far-field acoustics and that the relevant lengthscale for this behaviour is the momentum thickness of the exit boundary layer. The peak frequency being governed by StΘ = 8:5 × 10-3. Flow fluctuations in this region are visualised using high-speed Schlieren imaging. For higher Mach numbers the far-field acoustics is best described by a constant Helmholtz number behaviour peaking at kh = 0:1. From the far-field acoustics we can determine that there are no spanwise modes present in the jet and that there is strong evidence for the presence of wavepackets. The results also show a change in the directivity pattern with increasing Mach number; the low speed region exhibiting a slight figure-of-eight pattern becoming almost omnidirectional at higher Mach numbers.