The separation of radiating and non-radiating near-field pressure fluctuations in supersonic jets

Abstract

Conventional analysis infers the jet noise source characteristics from either the turbulent fluctuations in the mixing layer, the near-field pressure fluctuations, or the far-field radiated sound. It is generally accepted that the first two techniques include both the radiating and non-radiating noise sources while the last senses only the radiated sound. Understanding of the characteristics of the non-radiating and radiating fluctuations helps to shed light on the noise source mechanisms. For this purpose, a quantitative analysis is introduced in this study to separate the radiating from the non-radiating near-field fluctuations. The analysis uses a high-fidelity unsteady jet flow and noise simulation for an internally mixed dual-stream nozzle. Based on the predicted far-field acoustic pressure on a virtual microphone array, a simple beamforming method is used to derive the equivalent wavepacket-like noise sources on a conical surface outside the jet mixing layer. These radiating noise sources are compared with the total near-field pressure fluctuations to examine their different characteristics. Wavepacket-like noise sources are filtered with the POD (Proper Orthogonal Decomposition) at discrete frequencies. Although the two equivalent noise sources show different amplitude distributions and phase velocities, they agree favorably with the Mach wave radiation theory as well as with the near-field turbulence. Both reproduce well the noise radiation pattern in terms of the phase, wavelength, radiation direction and noise levels when inserted into the wave equation.