Shock associated noise of inverted-profile coannular jets,part II: Condition for minimum noise

An experimental and theoretical investigation of shock-associated noise of inverted-profile coannular jets is described. For a fixed fan-stream Mach number, it is observed that the shock-associated noise often drops suddenly to a minimum as the reservoir pressure of the primary jet increases. When this happens, the almost periodic shock cell structure of the fan stream is found to nearly completely disappear. In this paper, an analytical model of this phenomenon is constructed and studied. It is theoretically established that this sudden change in the shock structure and hence the decrease in shock associated noise would occur when the primary jet flow is just slightly supersonic regardless of the Mach number and temperature of the fan stream. This minimum shock associated noise condition is confirmed in several series of experiments.