Broadband shock-associated noise from supersonic jets in flight

The stochastic model theory of broadband shock-associated noise from supersonic jets, developed in references [1] and [2], is extended to jets in flight. Forward flight affects broadband shock-associated noise generation through noise source modification. It also affects the intensity and characteristics of the noise radiated in a given direction through the mean flow convection effect. Noise source modification arises when the external flow associated with forward flight alters the mean velocity profile and shock cell structure of the jet. The mean flow convection effect arises because the radiated sound waves now have to propagate through a moving medium to reach the observation point. The present theory suggests that both the noise source modification effect and the mean flow convection effect would produce narrower broadband shock-associated noise spectral peaks with lower peak frequencies. Recently, Norum and Shearin [3,4] carried out a series of experiments to quantify the effects of forward flight on broadband shock-associated noise using an open wind tunnel. Extensive measured far field noise data are provided in reference [5]. In order to be able to compare the theoretical results with measurements, the theory is further extended to the case of a supersonic jet embedded in an open wind tunnel bounded by a vortex sheet. Detailed comparisons between the calculated results and the measurements of Norum and Shearin are reported here. Very good agreements are found over a broad range of jet Mach numbers and wind tunnel Mach numbers.