Generation of low frequency sound in turbulent jets

An analysis of the sound radiated by three turbulent, high speed jets is conducted using Lighthill's acoustic analogy [Proc. Royal Soc. London A, Vol. 211, 1952]. Computed by large-eddy simulation the three jets are at different operating points: A Mach 0.9 cold jet, a Mach 2.0 cold jet, and a Mach 1.0 heated jet. The last two jets have the same jet velocity and differ only by temperature. All three jets correspond to conditions originally investigated by Tanna [J. Sound Vib., Vol. 50, No. 3, 1977]. Comparisons between the jet LES predictions and experimental data show reasonable agreement: The simulations demonstrate all of the expected trends with changes in jet velocity and/or heated but have slightly higher, by roughly 0.01 U_j, fluctuation levels. In the far-field the LES sound fields are low-pass filtered versions of their experimental counterparts, having similar OASPL predictions but with limited high frequency sound in their spectra. From these jets the comparison between the Lighthill-predicted sound and the directly computed sound is excellent for all jets and for the two angles (30° and 90°, measured from the downstream jet axis) considered. The momentum (ρu_iu _j) and so-called entropy (p - P_∞ - a _∞²(ρ - ρ_∞)) contributions are examined in both the far-field acoustic spectra and in their near-field source distribution. It is found that significant phase cancellation exists in the source and that, in the far-field, additional cancellation occurs between the momentum and entropy components. Connections are made between the present results and the quieting of high-speed jets with heating.