Advances in Modeling Supersonic Jet Noise for Vertical Take-Off and Landing Aircraft
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Date
2021-04
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Abstract
The acoustic coupling of two closely separated, under-expanded Mach 1.27 jets impinging on a surface is studied using Large Eddy Simulations to develop a theoretical model capable of predicting "super-resonance". The impingement mechanisms that characterize the flow establish an acoustic feedback loop between the ground and the nozzle, generating intense acoustic tones, the theory of which is well established for single impinging jets (SIJ). However, in the case of dual impinging jets (DIJ), anomalous hydrodynamic and acoustic coupling mechanisms are present in the system that modulate the nominal SIJ acoustic feedback loop dominant in each jet. The hydrodynamic coupling mechanism includes the interaction of the fountain-flow generated between the jets with the convecting coherent structures in the shear layer; while acoustic coupling involves the acoustic waves generated by the impingement of one jet, propagated through the fountain-flow region, and received by the other jet. Although the resulting interference of the former mechanism is apparent, the efficacy and implications of an acoustic waves ability to cross the turbulent fountain-flow region is not well understood. The unique acoustic coupling mechanisms are presented in detail for a case study at an impinging height H/D=4 and jet separation distance S/D=4.3, where D is the nozzle exit diameter. Doak's momentum potential theory is applied to isolate the acoustic component of the LES flow field, providing characterization of the SIJ impinging tones, feedback paths, and modal behavior. The modal analysis isolated the SIJ feedback tones within each jet, as well as elucidated globally coupled impinging modes, which suggest the presence of an acoustically coupled, closed, DIJ feedback loop augmenting low-frequency SIJ harmonics. From these observations of the DIJ feedback mechanism, the theoretical SIJ acoustic feedback model of Powell is expanded to account for this additional acoustic coupling. Furthermore, a global "super-resonance" state is proposed when geometric and jet conditions facilitate the syncing of SIJ and coupled DIJ acoustic feedback loops, resulting in the successful prediction of overall sound pressure level trends for the DIJ system as a function of impingement height.
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Engineering: 1st Place (The Ohio State University Edward F. Hayes Graduate Research Forum)
Keywords
Aeroacoustic resonance, Impinging jets, Jet noise