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Criteria for the Existence of Helical Instabilities in Subsonic Inpinging Jets
Title
Criteria for the Existence of Helical Instabilities in Subsonic Inpinging Jets
Creator
Panickar, Praveen, Raman, Ganesh
Date
2007-10
Publisher
American Inst Physics
Description
Understanding instability modes in impinging jets is important for the design of advanced vertical takeoff and landing aircraft. Our initial...
Show moreUnderstanding instability modes in impinging jets is important for the design of advanced vertical takeoff and landing aircraft. Our initial experimental observations of helical modes in subsonic impinging jets were contrary to expectations based on published stability theory results. This paper focuses on the curious phenomenon of the occurrence of these, hitherto unexpected, helical modes in subsonic impinging jets and provides, both experimental and analytical proof for their existence. Past results, based on linear stability analysis, that call for an absence of helical modes in cold, subsonic impinging jets have been re-examined to reconcile new experimental data. Based on this re-examination, a new threshold Mach number has been proposed, above which helical modes are permissible. The revised theory put forth in this paper indicates that the threshold Mach number for the existence of the helical mode depends on the jet temperature and that this threshold decreases as the jet temperature increases. This threshold Mach number has been experimentally verified for unheated jets. Additionally, the experimental results reveal that the threshold Mach number also depends on the stand-off distance. Finally, it has been shown that the experimentally obtained mean Strouhal numbers for the helical mode show excellent agreement with the Strouhal number of the least dispersive wave of the same mode calculated at various Mach numbers. The results presented here offer hope for better understanding of impinging jet instability modes. (C) 2007 American Institute of Physics.
http://dx.doi.org/10.1063/1.2798804
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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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