Investigation of acoustic radiation from supersonic turbulent jets by double-pulse holographic interferometry

Abstract

A new experimental method was used to investigate optically the acoustical radiation field and the related fluctuating turbulent field from supersonic air jets. The method utilizes double exposure of a supersonic jet using a pulse ruby laser. The jet remains on during both exposures eliminating the no-flow exposure. The time separtion between the pulses can be varied, depending on the spectral component of the radiation and flow field to be observed. Optical records were obtained for supersonic jets with exit Mach number ranging from 1.0–3.0. It was found that high-frequency acoustic radiation mainly originates from the regions near the nozzle exit, 1–3 nozzle diameters, whereas the low-frequency noise is generated from regions of 8–10 diameters downstream. (The resulting three-dimensional optical data also reveal information on the relative intensity of the observed spectral component.) These findings are consistent with the acoustic data obtained by other investigators. The other experimental details on the acoustic radiation from supersonic jets and potential applications of this technique to other aeroacoustics problems are also discussed.