Acoustic and Flow-Field Characteristics of Shock-cell Noise from Dual Flow Nozzles

Abstract

Experiments were conducted at the Boeing Low Speed Aeroacoustic Facility with a model-scale axisymmetric separate flow nozzle system to investigate the acoustic and flow field characteristics of shock containing plumes. Far field noise measurements with microphones covering a range of angles with respect to the inlet axis were made with the dual flow jet rig operated over a range of gas conditions. Data was acquired with the open free jet wind tunnel on (to simulate the flight effects) at a Mach number of 0.32. Detailed flow field measurements were made using a wake-rake to get a better understanding of the relationship between the shock-cell structures and the noise radiated. Shock-cell noise is a major contributor to the aft cabin interior noise of subsonic aircraft. Near field acoustic measurements were made with an inflow phased array of microphones to measure the shock-cell noise source distribution. The results presented here show a strong relationship between the amplitude of the shock-cell structure and the radiated noise. One of the surprising and interesting findings of this study is that the shockcell noise does not monotonically increase with power settings. There are points at which the shock-cell noise is lower than at adjacent power settings on either side. This variation in shock-cell noise with power settings is the result of a corresponding variation in the amplitude of the shock-cell structure. This is contrary to the jet mixing noise which increases monotonically with power settings.