Characteristics of the Shock Noise Component of Jet Noise

Abstract

The characteristics of the flow and the noise of shock-containing jets have been studied for nearly three decades. It is now established that broadband shock-associated noise is generated by the interaction of the downstream-convecting coherent structures of the jet flow with the shock cells in the jet plume. Past analyses of far-field data have been carried out with the total measured noise, which contains both the turbulent mixing noise and shock noise. In this study, these two components are first separated and extracted from the total spectra. Both convergent and convergent-divergent nozzles are considered. The decomposition is made possible by a recently developed scaling methodology for turbulent mixing noise, which provides excellent collapse of the mixing noise spectra from jets at all velocities but at a fixed temperature ratio. The characteristics of the shock component alone are investigated. A surprising effect of jet temperature on shock noise is established for the first time: the levels increase as the jet is first heated; however, the levels do not increase with further increase in jet temperature. The physical phenomenon responsible for this saturation of levels is not known at this time. The intensity for shock noise in the forward quadrant does not scale as the fourth power (shock exponent) of √|M 2 j -M 2 D | but spans a range from 2.9 to 6.17, depending on the radiation angle and the jet temperature ratio. It is not straightforward to collapse the shock spectra. It is also established for the first time that nonlinear propagation effects are manifested at lower radiation angles, in which the shock component is dominant. The physical phenomenon that triggers the onset of nonlinear propagation for the shock noise could not be identified. The characteristics of the correlation functions at the lower inlet angles for subsonic and supersonic jets are different, attesting to the different noise generation mechanisms.