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Cessation of Screech in Underexpanded Jets
Title
Cessation of Screech in Underexpanded Jets
Creator
Raman, G.
Date
1997-04-10
Publisher
Cambridge Univ Press
Description
In significantly underexpanded jets, screech inherently ceases to exist. This paper studies screech cessation in a supersonic rectangular jet...
Show moreIn significantly underexpanded jets, screech inherently ceases to exist. This paper studies screech cessation in a supersonic rectangular jet and provides an explanation for its occurrence. Experimental data are presented for fully expanded Mach numbers, M-j, ranging from 1.1 to 1.9. Screech becomes unsteady beyond M-j = 1.65 and ceases to exist beyond M-j = 1.75. The reason for this cessation has remained a mystery, and this paper examines three suspects: (i) the theory of a frequency mismatch between screech tones and the band of the most-amplified jet instability waves, (ii) the notion that Mach disk formation disrupts the shock-cell structure and weakens the screech-producing shocks, and (iii) the idea that acoustic feedback and receptivity diminish at high levels of underexpansion. A thorough interrogation of experimental data shows that (i) is not the main cause of screech cessation here, (ii) plays an insignificant role, and (iii) appears to have been largely responsible for screech cessation. Cessation occurs because feedback to the jet lip is diminished due to excessive expansion of the jet boundary. Further, since the jet lip now reflects and scatters low intensity sound, the end result is poor receptivity at the initial shear layer. This theory is substantiated by the re-activation of screech when the nozzle lip thickness is made larger than the expanded jet boundary. Finally, increasing lip thickness is seen to produce a systematic shift (to higher M-j) of the onset of cessation. The results of this study are of direct relevance to the sonic fatigue problem in aircraft structures, because understanding screech helps prevent such damage.
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Supersonic Jet Screech: Half-century from Powell to the Present
Title
Supersonic Jet Screech: Half-century from Powell to the Present
Creator
Raman, G.
Date
1995-08-19
Publisher
Academic Press Ltd
Description
Under certain conditions, shock-containing jets produce an intense tone referred to as screech. Screech was discovered about half a century...
Show moreUnder certain conditions, shock-containing jets produce an intense tone referred to as screech. Screech was discovered about half a century ago by Alan Powell in England. Here I recount developments in supersonic jet screech - from Powell's first observation in 1951 to now. During this period more than 200 papers have been published - many offering only incremental advances. This paper provides a concise screech resource including a historical perspective, a summary of recent developments and a critical assessment of the state of the art. Topics include modulation of instability waves by shocks, shock-cell models and screech frequency prediction models, unsteady shock motions and clues about their role in shock noise generation. also. detailed nearfield measurements and computer simulation methods now available are discussed. However, despite the advances, screech amplitude prediction remains an elusive but increasingly important goal not only due to concerns about sonic fatigue failure of aircraft structures but because knowledge gained by the study of screech can be applied to a variety of resonant flow situations, including jet impingement, cavity resonance, and closed-loop active flow control. (C) 1999 Academic Press.
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Advances in Understanding Supersonic Jet Screech: Review and Perspective
Title
Advances in Understanding Supersonic Jet Screech: Review and Perspective
Creator
Raman, G.
Date
1998-02
Publisher
Pergamon - Elsevier Science Ltd
Description
Screech tones are produced by imperfectly expanded jets under certain conditions. This paper provides an overview of developments in the field...
Show moreScreech tones are produced by imperfectly expanded jets under certain conditions. This paper provides an overview of developments in the field of supersonic jet screech. The overview includes a historical background, summary of recent developments, and a critical assessment of our current understanding of screech. The subject material has been categorized as described in the table of contents given below. (C) 1998 Elsevier Science Ltd.
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