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Title: Resonant interaction of a linear array of supersonic rectangular jets: An experimental study
Creator: Raman, G, Taghavi, R
Date: 1996-11-10
Publisher: CAMBRIDGE UNIV PRESS

Title: Flow characteristics of a rectangular multielement supersonic mixer-ejector
Creator: Taghavi, R, Raman, G
Date: 1996-10
Publisher: AMER INST AERONAUT ASTRONAUT

Title: Resonant interaction of a linear array of supersonic rectangular jets: An experimental study
Creator: Raman, G, Taghavi, R
Date: 1996-11-10
Publisher: CAMBRIDGE UNIV PRESS

Title: ENHANCED MIXING OF MULTIPLE SUPERSONIC RECTANGULAR JETS BY SYNCHRONIZED SCREECH
Creator: Taghavi, R, Raman, G
Date: 1994-12
Publisher: AMER INST AERONAUT ASTRONAUT

Title: Flow characteristics of a rectangular multielement supersonic mixer-ejector
Creator: Taghavi, R, Raman, G
Date: 1996-04
Publisher: SPRINGER VERLAG
Description: This study describes a technique that combines the benefits of focusing schlieren and phase conditioning. Focusing schlieren blurs and drops...
Show moreThis study describes a technique that combines the benefits of focusing schlieren and phase conditioning. Focusing schlieren blurs and drops contrast of non-critical features whereas phase conditioning emphasizes periodic flow features, and their combination produces unique results. The supersonic jets that we studied produced an intense tone referred to as screech. The measured screech tone signal was used as input to the phase conditioning circuit that adjusted the strobing light source to the vertical synchronization pulse of a CCD camera. The sharp video images obtained by this technique could either be frozen or continuously swept through one period of screech to acquire a slow motion video record of the jet unsteadiness. Two cases were visualized in this study: first, an underexpanded jet from a convergent rectangular nozzle at various fully expanded Mach numbers. Second, a supersonic jet emerging from a convergent-divergent rectangular nozzle at a design Mach number of 1.4, artificially excited by impingement tones. The results of this study illustrate the usefulness of this system in visualizing oscillatory flows.
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Title: Aeroacoustic Characteristics of a Rectangular Multi-element Supersonic Jet Mixer-ejector Nozzle
Creator: Taghavi, R., Raman, G.
Date: 1997-10-23
Publisher: Academic Press Ltd
Description: This paper provides a unique, detailed evaluation of the acoustics and aerodynamics of a rectangular multi-element supersonic jet mixer...
Show moreThis paper provides a unique, detailed evaluation of the acoustics and aerodynamics of a rectangular multi-element supersonic jet mixer-ejector noise suppressor. The performance of such mixer-ejectors is important in aircraft engine applications for noise suppression and thrust augmentation. In contrast to most prior experimental studies on ejectors that reported either aerodynamic of acoustic data, the present work documents both types of data. Information on the mixing, pumping, ejector wall pressure distribution, thrust augmentation and noise suppression characteristics of four simple, multi-element, jet mixer-ejector configurations is presented. The four configurations included the effect of ejector area ratio (AR = ejector cross-sectional area/total primary nozzle area) and the effect of non-parallel ejector walls. The configuration that produced the best noise suppression characteristics has also been studied in detail. The present results show that ejector configurations that produced the maximum pumping (secondary (induced) flow normalized by the primary flow) also exhibited the lowest wall pressures in the inlet region, and the maximum thrust augmentation. When cases having the same total mass flow were compared, one found that noise suppression trends corresponded with those for pumping (per unit secondary area). Surprisingly, the mixing (quantified by the peak Mach number, and flow uniformity) at the ejector exit exhibited no relationship to the noise suppression at moderate primary jet fully expanded M-j (the Mach number that would have been attained under isentropic expansion). However, the noise suppression dependence on the mixing was apparent at M-j = 1.6. The above observations are justified by noting that the mixing at the ejector exit is not a strong factor in determining the radiated noise when noise produced internal to the ejector dominates the noise field outside the ejector. (C) 1997 Academic Press Limited.
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Title: Coupling of Twin Rectangular Supersonic Jets
Creator: Raman, G., Taghavi, R.
Date: 1998-01-10
Publisher: Cambridge Univ Press
Description: Twin jet plumes on aircraft can couple, producing dynamic pressures significant enough to cause structural fatigue. For closely spaced jets...
Show moreTwin jet plumes on aircraft can couple, producing dynamic pressures significant enough to cause structural fatigue. For closely spaced jets with a moderate aspect ratio (e.g. 5), previous work has established that two coupling modes (antisymmetric and symmetric) are kinematically permissible. However, the dynamics of twin-jet coupling have remained unexplored. In this paper a more fundamental assessment of the steady and unsteady aspects of twin-jet coupling is attempted. While we document and discuss the nozzle spacings and Mach numbers over which phase-locked coupling occurs, our concentration is much more on answering the following questions: (a) What mechanism causes the jets to couple in one mode or the other? (b) Why do the jets switch from one mode to another? (c) Are the two modes mutually exclusive or do they overlap at the transition point? Our results reveal, among many things, the following. (i) For very closely spaced twin jets in the side-by-side configuration phased feedback based on source to nozzle exit distance of adjacent jets does not fully explain the coupling modes. However, the 'null' phase regions surrounding the jets where the phase of an acoustic wavefront (arriving from downstream) does not vary appears to correlate well with the existence of the symmetric mode. When the 'null' regions of adjacent jets do not overlap antisymmetric coupling occurs and when they do overlap the jets couple symmetrically. We provide a simple correlation using a parameter (a) that can be used as a simple test to determine the mode of coupling. (ii) The switch from the antisymmetric to the symmetric mode of coupling appears to occur because of an abrupt shift in the effective screech source from the third to the fourth shock, which in turn causes the 'null' phase region surrounding the jets to grow abruptly and overlap. (iii) The two modes are mutually exclusive. Our results provide considerable insight into the twin-jet coupling problem and offer hope for designing twin-jet configurations that minimize damage to aircraft components.
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Title: Resonant interaction of a linear array of supersonic rectangular jets: An experimental study
Creator: Raman, G, Taghavi, R
Date: 1996-02-25
Publisher: CAMBRIDGE UNIV PRESS
Description: This paper examines a supersonic multi-jet interaction problem that we believe is likely to be important for mixing enhancement and noise...
Show moreThis paper examines a supersonic multi-jet interaction problem that we believe is likely to be important for mixing enhancement and noise reduction in supersonic mixer-ejector nozzles. We demonstrate that it is possible to synchronize the screech instability of four rectangular jets by precisely adjusting the inter-jet spacing. Our experimental data agree with a theory that assumes that the phase-locking of adjacent jets occurs through a coupling at the jet lip. Although synchronization does not change the frequency of the screech tone, its amplitude is augmented. The synchronized multi-jets exhibit higher spreading than the unsynchronized jets, with the single jet spreading the least. We compare the near-field noise of the four jets with synchronized screech to the noise of the sum of four jets operated individually. Our noise measurements reveal that the more rapid mixing of the synchronized multi-jets causes the peak jet noise source to move upstream and to radiate noise at larger angles to the flow direction. Based on our results, we have grounds to believe that screech synchronization is advantageous for noise reduction internal to a mixer-ejector nozzle, since the noise can now be suppressed by a shorter acoustically lined ejector.
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Title: Mixer-ejector Wall Pressure and Temperature Measurements Based on Photoluminescence
Creator: Taghavi, R.r., Raman, G., Bencic, Tj
Date: 2002-04
Publisher: American Inst Aeronaut Astronaut
Description: Ejector side-wall pressure distribution is a key indicator of supersonic jet-mixer-ejector performance. When documenting pressure patterns on...
Show moreEjector side-wall pressure distribution is a key indicator of supersonic jet-mixer-ejector performance. When documenting pressure patterns on an ejector wall using pressure-sensitive paint (PSP), one has to consider temperature variations caused by the supersonic jet flow within the ejector because these can cause significant local errors in the PSP results. If the temperature sensitivity of PSP is not corrected for in complex internal supersonic flows, large localized errors could contaminate the results. In the present work, temperature-sensitive paint maps the temperature distribution on the ejector wall and corrects PSP results point-by-point for temperature sensitivity. The experiments were conducted on multijet supersonic mixer-ejector configurations with straight, convergent (6-deg), and divergent (6-deg) side walls. A comparison of corrected and uncorrected PSP readings shows that at M-j = 1.55, the error with respect to true data from static pressure ports can be reduced from 4.98 to 2.84% for the case of a simple ejector with parallel walls. For the complex 6-deg convergent ejector at M-j = 1.39, the error reduces by almost an order of magnitude (from 20.83 to 2.66%). Our results indicate that the use of this correction technique can significantly reduce PSP errors in complex internal supersonic flow situations.
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Title: Pressure Sensitive Paint Demonstrates Relationship Between Ejector Wall Pressure and Aerodynamic Performance
Creator: Taghavi, R., Raman, G., Bencic, T.
Date: 1999-05
Publisher: Springer Verlag
Description: This paper provides an example of the application of Pressure Sensitive Paint (PSP) to complex internal suspersonic flows and demonstrates the...
Show moreThis paper provides an example of the application of Pressure Sensitive Paint (PSP) to complex internal suspersonic flows and demonstrates the relationship between ejector wall pressure and aerodynamic performance. Details of such jet mixer-ejector nozzles are relevant to jet noise reduction programs. Several ejector configurations with straight, convergent, and divergent side walls were used in our experiments. The side-wall that was painted with PSP was also instrumented with an array of 156 pressure taps connected to Electronically Scanned Pressure (ESP) modules, enabling simultaneous measurement of "true" reference pressures. The PSP results agreed very well with the "true" reference pressures and also provided a detailed map of the complicated pressure patterns that could not be detected using the pressure taps. Finally, we also demonstrated the direct relationship between ejector side-wall pressure distribution and ejector performance characteristics such as exit mean flow uniformity, pumping, and thrust augmentation.
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