Reducing Jet Noise in Over-Expanded Nozzle Flow

摘要

A flow resonance accompanied by emission of acoustic tones takes place in an over-expanded convergent-divergent nozzle when we operate it at comparatively low jet Mach numbers.This phenomenon, as distinguished from conventional screech tones, is referred to as "transonic tones".Unlike screech tones, the peak resonant frequency for transonic tones increases with jet Mach number, and the peak sound pressure lever exceeds 110 dB.In this work, we investigate basic characteristics of transonic resonance and tones using a circular convergent-divergent nozzle in an anechoic room.Effects of a boundary layer trip on suppression of transonic resonance and tones are also evaluated.Exhausted jet flow is visualized by the Schlieren method using a high-speed video camera.Results of acoustic measurements show that several predominant peaks, corresponding to transonic tones, and a stage change are observed.For reduction of the transonic tones, we mounted a boundary layer trip inside the nozzle, and it effectively eliminates these tones and suppress unsteadiness of the flow inside the nozzle.