Pressure field around a rectangular supersonic jet in screech

Abstract

Supersonic jet screech generated by a rectangular nozzle with a pair of side walls added is studied both experimentally and numerically. A detailed observation of screech phenomena contributes to an understanding of the mechanism of sound generation. The numerical simulation shows that computational aeroacoustics is effective for strongly coupled flow acoustic problems such as screech. Flow visualization by an acoustically triggered pulse laser schlieren system clearly shows the dynamic motion of the third shock cell in the flapping mode. A traveling shock is found to exist in the third shock cell, and the sound is generated at the moment when the traveling shock coalesces into the end shock of the third shock cell. The instantaneous pressure distribution on the side wall shows that a pressure wave convectively propagates along the shear layer in the near field of the jet, while it propagates acoustically in the vertical direction away from the jet. Numerical simulation shows similar aspects of screech to those obtained in the experiment : the screech frequency, the flowfield, the fluctuating pressure distribution, etc. The calculated acoustic intensity shows that the acoustic energy is radiated at the end of the compression portion of each shock cell. The source of the first shock cell seems to supply the acoustic energy to its own shear layer to sustain the oscillation. It is also shown that the dimple spots observed in the isoamplitude contours of the screech sound pressure are a result of the interaction between discrete sources located at the end of each shock cell.