Fareld ltering of subsonic jet noise: Mach and Temperature effects

Abstract

We present an analysis of the sound eld radiated by jets at different Mach numbers and temperature ratios. The methodology is similar to that developed by Koenig et al.1 in a previous study, where spatial and temporal structures of the sound eld of a Mach 0.9 cold jet are ltered and analysed by means of Proper Orthogonal Decomposition and wavelet transform. Using both the POD and wavelet-ltered signals the acoustic eld is decomposed into two components: a coherent structure (CS) component and a residuum (R). A source imaging procedure is then applied and the results compared with the CS component obtained by the two ltering operations. The main results include the following observations. (1) While the shallow-angle acoustic spectra of isothermal jets scale best with Helmholtz number, those of the heated jets scale better with Strouhal number. This difference suggests that non-compact effects do not play as important a role in the heated jet where downstream radiation is concerned, contrary to what is observed in isothermal jets; this in turn suggests a change in acoustic wavelength associated with the temperature of the jet, implying that source interference occurs in the core of the flows, an idea consistent with an axially-extended wavepacket source. (2) While in the isothermal jets no change in intermittency is observed as the Mach number is varied, when the jet is heated increasing the Mach number engenders a decrease in intermittency in the acoustic eld; we can conjecture from this that high Mach number hot jets comprise lower levels of source jitter (cf. Cavalieri et al.2). (3) The CS acoustic signatures identied by both wavelet and POD ltering present wavepacket superdirectivity. (4) The trends identied by the source imaging, as a function of Mach number for isothermal and heated jets, are consistent both with known changes in the mean flow structure and the results of linear stability theory. © 2011 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc.