Aiaa-2000-2062 Linear Model of Mach Wave Suppression in a Dual-stream Jet

Reduction of Mach wave emission from a dual-stream jet is modeled via a linear temporal analysis in the frame of reference of the jet instability. The interface between the primary and secondary streams is simulated by a exible wavy wall with zero phase speed and arbitrary growth rate. The interface between the secondary and ambient streams is simulated by a vortex sheet. The secondary stream, which is adjacent to the wall, has variable Mach number and height. The semiin nite ambient stream is supersonic. The study focuses on the case of a subsonic secondary stream and compares it to the case of uniform supersonic ow over the wall. It is found that a subsonic layer over the wall creates two e ects: (a) the magnitude of the pressure disturbance on the wall is diminished and (b) the disturbance decays exponentially within the height of the layer. The pressure uctuation that is nally transmitted into the supersonic stream is very weak, leading to considerable noise suppression. The noise reduction increases with decreasing value of the secondary Mach number and with increasing height of the secondary ow. These trends compare favorably with experimental results in which neareld noise was measured in dual-stream jets with variable secondaryow Mach number and adjustable height of the secondary stream.