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Non-linear Acoustic Waves in Resonators Screech
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ScreechUnder certain conditions, shock –containing jets produce an intense tone referred to as screech. The screech tone is produced through four key mechanisms. First, is instability wave growth in a shock containing supersonic jet, then the instabilities interact with the shocks causing sound to traverse the regularly spaced shock wave system of the jet. Next, interference causes a powerful emission in the upstream direction, and finally the propagating sound waves interact with the nozzle lip to produce and maintain a resonant feedback loop. To study and understand screech we look at the phenomena associated with the mechanisms responsible for screech. Those phenomena include the screech instability mode models (instability wave growth), the shock-cell models and unsteady shock motions (instability shock- interactions), source location and details of sound propagation in the near field (acoustic feedback), and scattering by nozzle lip and coupling of acoustics with hydrodynamic instabilities (receptivity process).
Half of a century has passed since A. Powell discovered screech, and it still plays a critical role in the design of advanced aircraft because it can cause sonic fatigue failure. Such failures have been observed before on the British Aircraft Corporation’s VC-10 and on the F-15 and B-1B of the United States Air Force. It is well known that the twin-jet plumes on aircraft can couple, producing very high dynamic pressures in the inter-nozzle region, which in turn can cause sonic fatigue of external nozzle flaps. Other problems to look at with screech are the effects of the geometry of the jet nozzle and the effects of screech in enhancing jet mixing |
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