Jet-plate interaction tones relevant to over-the-wing engine mount concept

Abstract

It has been suggested that by mounting the jet engines of an aircraft over the wings, the wings would act as a noise shield. In 2015, Zaman et al. (J. Fluid Mech. vol. 779 (2015) 751–775) reported the results of their experiment intended to explore the feasibility of such an engine-wing configuration. Unexpectedly, they found that tones were emitted from their scaled laboratory experiment. They identified the tones as feedback tones. Many details and characteristic features of the tone phenomenon were measured and documented in their paper. However, it was uncertain as what the feedback mechanism/path was. The purpose of this paper is to propose that the feedback closure mechanism is the upstream propagating acoustic wave modes internal to the jet. Thus, the feedback loop is believed to consist of downstream propagating Kelvin-Helmholtz instability waves of the jet flow initiated at the nozzle exit by the excitation of the upstream propagating internal acoustic waves. As the instability waves grow in amplitude when propagating downstream, they cause the jet to oscillate. The oscillatory jet impinges on the trailing edge of the plate (representing a wing). The impact leads to the emission of tones and the generation of internal upstream propagating waves. These waves are supported by the jet flow. In this way, the feedback loop is closed. In this paper, results of computed tone frequencies, based on the above described feedback loop, are compared with measurements of Zaman et al. Good agreements are found. The effects of jet Mach number on tone frequencies are considered. Again, good agreements are obtained over a range of Mach numbers. Zaman et al. reported that no tones were observed when the jet Mach number exceeded 1.06. The present theoretical analysis shows that there is no upstream propagating acoustic wave mode when the jet Mach number is higher than 1.118 (fairly close to the experimental value). Therefore, at and above this Mach number the feedback loop is broken and hence there is no interaction tone. This offers a physical explanation of the observed high Mach number cut-off phenomenon.