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Title: Using Controlled Unsteady Fluid Mass Addition to Enhance Jet Mixing
Creator: Raman, G.
Date: 1997-04
Publisher: American Inst Aeronaut Astronaut
Description: A rectangular jet was excited by controlled unsteady fluid mass addition using two miniature fluidic jets placed on either side of its narrow...
Show moreA rectangular jet was excited by controlled unsteady fluid mass addition using two miniature fluidic jets placed on either side of its narrow dimension. The subharmonic of the primary's preferred jet column frequency [St(D-e) = fD(e)/U-J = 0.15] was forced in the antisymmetric mode because such forcing persists for longer downstream distances than the fundamental. Details of the phase-averaged flowfield, velocity gradient terms, velocity spectra, and the mean and fluctuating flowfields were documented. The fluidically excited mode grew and persisted in the flow beyond the potential core region. Unsteady fluid mass addition of 12% (4% momentum addition) per fluidic jet resulted in a 35% reduction of the potential core length and about a 60% increase in the normalized mass flux (percentages are with reference to the primary unforced jet). On the basis of the results, it appears that fluidic devices have the potential for use in shear flow control applications.
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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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Title: Shock-induced Flow Resonance in Supersonic Jets of Complex Geometry
Creator: Raman, G.
Date: 1999-03
Publisher: American Inst Physics
Description: Jets with complex shock-cell structures exist in numerous technological applications. This paper describes a fundamental study of shock...
Show moreJets with complex shock-cell structures exist in numerous technological applications. This paper describes a fundamental study of shock-induced flow resonance (commonly referred to as "jet screech") in supersonic jets with spanwise nonuniform shock-cell structures. Experiments that involve flow visualization and detailed mapping of the near field reveal unsteady aspects of shock-induced flow resonances, mode transitions, and directivity of the radiated noise. The following important results about the role of spanwise nonuniform shock-cells emerged: (1) It is possible to have two coexisting, independent feedback loops at nonharmonically related frequencies and different spanwise modes. (2) The same type of spanwise asymmetric mode was produced by two entirely different source configurations. (3) Nozzle geometry significantly altered the intensity and directivity of screech and broadband shock noise. The results presented here provide considerable insight into the fluid dynamics and acoustics of jets with spanwise oblique shock-cell structures and provide grounds for believing that shock-induced noise can be controlled by tailoring nozzle geometry. (C) 1999 American Institute of Physics. [S1070-6631(99)00103-8].
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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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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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Title: JET MIXING CONTROL USING EXCITATION FROM MINIATURE OSCILLATING JETS
Creator: Raman, G, Cornelius, D
Date: 1995-02
Publisher: AMER INST AERONAUT ASTRONAUT

Title: SUPERSONIC JET MIXING ENHANCEMENT USING IMPINGEMENT TONES FROM OBSTACLES OF VARIOUS GEOMETRIES
Creator: Raman, G, Rice, Ej
Date: 1995-03
Publisher: AMER INST AERONAUT ASTRONAUT
Description: This paper reports the results of an experiment that investigated the effect of impingement tones, generated by obstacles of various...
Show moreThis paper reports the results of an experiment that investigated the effect of impingement tones, generated by obstacles of various geometries, on the spreading of a supersonic jet flow. A rectangular supersonic jet was produced using a convergent-divergent nozzle that was operated near its design point (with shocks minimized). Immersing obstacles in the flow produced an intense impingement tone that then propagated upstream (as feedback) to the jet lip and excited the antisymmetric hydrodynamic mode in the jet, thus setting up a resonant self-sustaining loop. The violent flapping motion of the jet due to excitation of the antisymmetric mode, combined with the unsteady wakes of the obstacles, produced large changes in jet mixing. The experiment controlled the frequency and amplitude of the impingement tone excitation by varying the nozzle-to-obstacle distance and the obstacle immersion. Proper shaping of the obstacles made it possible to reduce the thrust penalty significantly.
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Title: AXISYMMETRICAL JET FORCED BY FUNDAMENTAL AND SUBHARMONIC TONES
Creator: Raman, G, Rice, Ej
Date: 1991-07
Publisher: AMER INST AERONAUT ASTRONAUT
Description: A circular jet was excited simultaneously by two harmonically related tones. The results of this excitation on jet behavior are reported for...
Show moreA circular jet was excited simultaneously by two harmonically related tones. The results of this excitation on jet behavior are reported for three pairs of Strouhal numbers [St(D) = f*D/U(j) = 0.2 and 0.4, 0.3 and 0.6, 0.4 and 0.8]. For each case, the initial phase difference between the two tones was varied in steps of 45 deg for one full cycle, and the amplitude of the fundamental and subharmonic tones was varied independently over the range of 0.1-7.0% of the jet exit velocity. Several results of this study agreed with other published findings, such as a critical amplitude or the fundamental being required for subharmonic augmentation and the initial phase difference being critical in determining whether the subharmonic is augmented or suppressed. In addition, the detailed documentation or several aspects of this phenomenon, measured in the same experimental facility in a controlled manner, brought out two important points that had eluded previous researchers. First, at high levels of the fundamental and subharmonic forcing amplitudes, the subharmonic augmentation is independent of the initial phase difference. Second, two-frequency excitation is indeed more effective than single-frequency excitation in jet mixing enhancement. Higher spreading rates seem to go along with higher subharmonic levels.
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Title: Cavity Resonance Suppression Using Miniature Fluidic Oscillators
Creator: Raman, G., Raghu, S.
Date: 2004-12
Publisher: American 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: 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: DISTURBANCE GROWTH TRIGGERED BY STEADY HEATING OF A JETS NOZZLE EXIT BOUNDARY-LAYER
Creator: Cornelius, D, Raman, G
Date: 1995-10
Publisher: AMER INST PHYSICS
Description: Experiments were conducted to investigate how heating only the nozzle exit boundary layer of an axisymmetric jet, with an unheated potential...
Show moreExperiments were conducted to investigate how heating only the nozzle exit boundary layer of an axisymmetric jet, with an unheated potential core, affects disturbance growth in the initial shear layer. The exit boundary layer had a minimum density ratio of 0.74, was laminar, and had a constant momentum thickness (theta) for all levels of nozzle heating used in this study. The fluctuating velocity (u') and temperature (t') in the exit boundary layer increased monotonically with increasing nozzle temperature. Low-amplitude acoustic excitation produced a more rapid growth of coherent velocity fluctuations for the heated case than for the unheated. (C) 1995 American Institute of Physics.
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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: Reversal in Spreading of a Tabbed Circular Jet Under Controlled Excitation
Creator: Zaman, Kbmq, Raman, G.
Date: 1997-12
Publisher: American Inst Physics
Description: Detailed flow field measurements have been carried out for a turbulent circular jet perturbed by tabs and artificial excitation. Two ''delta...
Show moreDetailed flow field measurements have been carried out for a turbulent circular jet perturbed by tabs and artificial excitation. Two ''delta tabs'' were placed at the nozzle exit at diametrically opposite locations. The excitation condition involved subharmonic resonance that manifested in a periodic vortex pairing in the near flow field. While the excitation and the tabs independently increased jet spreading, a combination of the two diminished the effect. The jet spreading was most pronounced with the tabs but was reduced when excitation was applied to the tabbed jet. The tabs generated streamwise vortex pairs that caused a lateral spreading of the jet in a direction perpendicular to the plane containing the tabs. The excitation, on the other hand, organized the azimuthal vorticity into coherent ring structures whose evolution and pairing also increased entrainment by the jet. In the tabbed case, the excitation produced coherent azimuthal structures that were distorted and asymmetric in shape. The self-induction of these structures produced an effect that opposed the tendency for the lateral spreading of the streamwise vortex pairs. The passage of the distorted vortices, and their pairing, also had a cancellation effect on the time-averaged streamwise vorticity field. These led to the reduction in jet spreading. [S1070-6631(97)03012-2].
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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: 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: INSTABILITY MODES EXCITED BY NATURAL SCREECH TONES IN A SUPERSONIC RECTANGULAR JET
Creator: Raman, G, Rice, Ej
Date: 1994-12
Publisher: AMER INST PHYSICS

Title: DEVELOPMENT OF PHASED TWIN FLIP-FLOP JETS
Creator: Raman, G, Rice, Ej
Date: 1994-07
Publisher: ASME-AMER SOC MECHANICAL ENG
Description: The flip-flop nozzle is a device that can produce an oscillating jet flow without any moving parts. There is now a renewed interest in such...
Show moreThe flip-flop nozzle is a device that can produce an oscillating jet flow without any moving parts. There is now a renewed interest in such nozzles due to their potential for use as excitation devices in practical applications. An experiment aimed at developing twin flip-flop jets that operate at prescribed frequencies and phase differences was performed. The phasing was achieved using two different nozzle interconnection schemes. In one configuration the two jets flapped in-phase and in another they flapped out-of-phase with respect to each other. In either configuration the frequencies of oscillation of both jets were equal. When one of the jets was run at a constant high velocity and the velocity of the second jet was increased gradually, the higher velocity jet determined the frequency of oscillation of both jets. The two flip-flop jet configurations described in this paper could be used to excite a primary jet flow in either an anti-symmetric (sinuous) or a symmetric (varicose) mode.
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