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Jet resonance in truncated ideally contoured nozzles
Published online by Cambridge University Press: 27 May 2021
Abstract
Unsteady side loads observed in supersonic nozzles operating in over-expanded regimes are most often associated with intrinsic unsteadiness of the shock system and separation line, featuring random motions with mainly broadband low-frequency contributions. A tonal flow behaviour, rather associated with energy peaks of fluctuating wall pressure in the middle frequency range, is also found to emerge for particular operating conditions in a truncated ideally contoured nozzle. The corresponding flow field is here investigated to understand its origin and show how it modifies side-load properties. The temporal and spatial organization of wall pressure and jet velocity field are first experimentally characterized based on synchronized acquisition of both wall pressure along rings of pressure probes located within the nozzle, and high-rate time-resolved particle image velocimetry velocity fields measured in a plane section crossing the jet downstream of the nozzle exit. The external jet aerodynamics and internal wall pressure field are first shown to be clearly linked, but only at this frequency peak for which a significant coherence emerges between first azimuthal mode of fluctuating wall pressure and first azimuthal mode of fluctuating external velocity field. A delayed detached eddy simulation is carried out and validated against experimental results in order to reproduce this tonal flow dynamics. The analysis of simulation data shows that the tonal flow behaviour of first azimuthal mode is indeed more globally felt within the whole flow structure where both upstream and downstream propagating waves are shown to coexist, even far downstream of the nozzle exit. The analysis shows that both waves possess support in the jet core and have a non-negligible pressure signature in the separated region. The spectral proper orthogonal decomposition of fluctuating pressure field at this tonal frequency reveals that the nature and intensity of lateral pressure forces is directed by the resonance related to the upstream- and downstream-propagating coherent structures, which imposes the shock waves network to respond and modulate the pressure levels on the nozzle internal surface.
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- © The Author(s), 2021. Published by Cambridge University Press
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