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Dynamics of an impinging jet. Part 1. The feedback phenomenon

Published online by Cambridge University Press:  20 April 2006

Chih-Ming Ho  and
Nagy S. Nosseir
Chih-Ming Ho
Affiliation:
Department of Aerospace Engineering, University of Southern California, Los Angeles
Nagy S. Nosseir
Affiliation:
Department of Aerospace Engineering, University of Southern California, Los Angeles Present address: Department of Applied Science, New York University, 26–36 Stuyvesant St, New York, N. Y. 10003.
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Abstract

In a high-speed subsonic jet impinging on a flat plate, the surface pressure fluctuations have a broad spectrum due to the turbulent nature of the high-Reynolds-number jet. However, these pressure fluctuations dramatically change their pattern into almost periodic waves, if the plate is placed close to the nozzle (x0/d < 7·5). In the present study extensive measurements of the near-field pressure provide solid support for the hypothesis that a feedback mechanism is responsible for the sudden change observed in the pressure fluctuations at the onset of resonance. The feedback loop consists of two elements: the downstream-convected coherent structures and upstream-propagating pressure waves generated by the impingement of the coherent structures on the plate. The upstream-propagating waves and the coherent structures are phase-locked at the nozzle exit. The upstream-propagating waves excite the thin shear layer near the nozzle lip and produce periodic coherent structures. The period is determined by the convection speed of the coherent structures, the speed of the upstream-propagating waves as well as the distance between the nozzle and the plate. An instability process, herein referred to as the ‘collective interaction’, was found to be critical in closing the feedback loop near the nozzle lip.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 105 , April 1981 , pp. 119 - 142
Copyright
© 1981 Cambridge University Press

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