Aircraft flight effects on high frequency sound emerging from a constant area jet pipe flow

Abstract

This paper examines the effect of flight on the sound radiated by a high frequency source embedded in a constant area jet pipe in the presence of flow. Ray acoustics theory and classical results for sound transmission at an interface of relative motion are used. The diffraction of sound at the nozzle lips, the inhomogeneity and irregularity of the interface and the possibility of instability waves being triggered by the incident sound are neglected. Some of the waves characterized by wave-fronts pointing upstream are shown to be convected downstream by the flow and to illuminate the forward arc after refraction at the jet interface. The amount of energy emitted by the source, which is trapped inside the flow, depends only on internal jet pipe conditions. However, the portion of the forward arc which is illuminated by this energy, is a function of flight speed. The radiation into the ambient atmosphere at rest of a basically omnidirectional source peaks at the edge of the downstream zone of silence and falls off rapidly when the observation angle is increased. The flight to static comparison reveals an interesting forward arc amplification due to flight but this occurs in a range of angles where the radiation is basically rather feeble.