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Aeroacoustic Features of Coupled Twin Jets with Spanwise Oblique Shock-Cells
Title
Aeroacoustic Features of Coupled Twin Jets with Spanwise Oblique Shock-Cells
Creator
Panickar, P., Srinivasan, K., Raman, G.
Date
2004-11-22
Publisher
Academic Press Ltd Elsevier Science Ltd
Description
This paper experimentally investigates the aeroacoustics of coupled twin jets of complex geometry. The study was motivated by the fact that...
Show moreThis paper experimentally investigates the aeroacoustics of coupled twin jets of complex geometry. The study was motivated by the fact that twin jet configurations that are commonly used in aircraft propulsion systems can undergo unpredictable resonant coupling resulting in structural damage. Further, nozzles with spanwise oblique exits are increasingly being considered for their aerodynamic and acoustic advantages, as well as stealth benefits. Although several studies have examined aspects of twin jet coupling, very little data is available on the coupling of jets from nozzles of complex geometry. Our study focuses on twin convergent nozzles with an aspect ratio of 7 with spanwise oblique exits operated over the fully expanded Mach number range from 1.3 to 1.6. The inter-nozzle spacing (s/h) was varied from 7.4 to 13.5. However, the focus remained on the lower spacing that is more representative of aircraft applications. Several interesting results have emerged from this study: (1) Coupling of twin nozzles with a beveled exit was observed only when the beveled edges faced each other and the nozzles formed a 'V' shape in the inter-nozzle region. Specifically, if the two beveled edges were oriented away from each other to form an arrowhead (W) shape no coupling was observed. (2) Despite the presence of spanwise antisymmetric, spanwise symmetric and spanwise oblique modes for the single nozzles, only the first two modes were evident in the coupling. (3) The symmetric coupling produced unsteady pressures in the inter-nozzle region that were up to 7.5 dB higher than the antisymmetrically coupled case. (4) Dynamic tests conducted by moving the nozzles apart while they were operating or by continuously changing the stagnation pressure at fixed inter-nozzle spacing revealed that coupling modes could co-exist at non-harmonically related frequencies. These dynamic tests reproduced the static test data. (5) The frequency of both coupling modes agrees with the higher order waveguide modes based on Tam's theory. (6) Differences in broadband shock noise between the 'V' and 'A' configurations were also documented. Our results provide an understanding of complex twin jet coupling and will serve as benchmark data for validating computational models. (C) 2003 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jsv.2003.10.011
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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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