A simple model of shock cell noise generation and its reduction

Based on the data of near field surveys of the sound pressure from a choked jet, a simple model is proposed for the mechanism of the screech generation. A convected wave propagates downstream along the jet boundary and is modulated by its interaction with the shock-expansion waves of the jet. These interactions generate strongdipoleradiations. Using this model, an excellent reproduction of the essential features of the experimental results is obtained. By preventing the formation of shock waves inside the jet, the strong dipole radiation and hence the screech noise can be eliminated. Design data for perforated nozzles to achieve fuU expansion of the jet are provided. This avoids the mechanical complication of an adjustable convergent-divergent nozzle.