Mach Number Effects on Jet Noise Sources and Radiation to Shallow Angles

A LL jets have noise generation due to turbulent mixing; such mixing leads to the production of Reynolds stresses, the source term within the Lighthill acoustic analogy [1]. In addition to this source, a jet that possesses coherent, large-scale structures that travel with velocities exceeding the local speed of sound will emit Mach wave radiation, a highly efficient noise production mechanism. The amplitude of jet noise scales with the jet area and the exit velocity to an angle-dependent power n. There is disagreement, however, in the value of n. Based on dimensional analysis, Lighthill [1] foundn to be 8. In an extensive examination of jet noise scaling, Viswanathan [2] observed that n 9:8 for the measurement location (30 deg) and ambient to total temperature ratio of unity examined in this work. Venkatesh et al. [3] presented an extensive set of subsonic jet noise measurements that show the region of maximum noise generation coincides with the collapse of the uniform, irrotational jet core. This technical note examines the impact of Mach number on the mechanism of jet noise generation, the scaling of jet noise, and the region of maximum noise generation. It does this through examination of the time and frequency domain properties of the shallow angle jet noise from Mach 0.9, and ideally expanded Mach 1.3 and 2.0 jets. A recent set of experiments showed that a Mach 1.3 jet possesses large amplitude acoustic peaks within the time-domain data and these peaks could be used to relate the generation of far field radiated sound to the evolution of the mixing layer [4–7]. These acoustic peaks are examined in this technical note to characterize the noise generation mechanism and source region of the three jets. For all Mach numbers studied, the large amplitude acoustic peaks originate from an ellipsoid-shaped region that is centered past the end of the potential core and the noise source region narrows in the cross-stream with increasing Mach numbers. Unlike at Mach 0.9 and 1.3, the Mach 2.0 acoustic peaks have temporal asymmetry, which is related to the change in noise generationmechanism from turbulencemixing to Mach wave emission.