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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: 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: INITIAL TURBULENCE EFFECT ON JET EVOLUTION WITH AND WITHOUT TONAL EXCITATION
Creator: Raman, G, Zaman, Kbmq, Rice, Ej
Date: 1989-07
Publisher: AMER INST PHYSICS

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: Nozzle-Orientation Effects and Nonlinear Interactions Between Twin Jets of Complex Geometry
Creator: Joshi, R, Panickar, P, Srinivasan, K, Raman, G
Date: 2006-04
Publisher: American Inst Aeronaut Astronaut
Description: In this paper we focus on understanding the behavior of twin nozzles of complex geometry in various yaw orientations. To the best of our...
Show moreIn this paper we focus on understanding the behavior of twin nozzles of complex geometry in various yaw orientations. To the best of our knowledge there are no published studies addressing the effects of nozzle orientation on the coupling of twin jets of complex exit geometry. We study the behavior of 1) uniform-exit rectangular nozzles, 2) single-beveled nozzles in a codirected configuration, and 3) single-beveled nozzles in a contradirected configuration. Experiments were carried out at fully expanded Mach numbers ranging from 1.28 to 1.72. Bevel angles of 10 and 30 deg were considered, and microphones located at the nozzle exit plane quantified the coupling using both linear and nonlinear spectral-analysis methods. Nonlinear characteristics were quantified using the nonlinear interaction density metric with a cross-bicoherence cut-off threshold of 0.4. The following interesting results emerged from this study: 1) When nozzles having uniform rectangular exits are yawed, the sound-pressure levels in the internozzle region reduce as the yaw angles are increased, and, at a very high yaw angle, the symmetric coupling regime that existed at the high fully expanded Mach number range (without yaw) is replaced by an antisymmetric coupling regime in the same range. 2) Geometrically similar exits from uniform-exit rectangular nozzles and beveled nozzles in the contradirected configuration showed similar characteristics when studied using linear techniques. However, they revealed information that was hitherto unknown when studied using nonlinear spectral-analysis techniques. It is believed that the results presented in this paper will provide benchmark data to those simulating/designing complex-geometry nozzle systems.
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Title: FLIP-FLOP JET NOZZLE EXTENDED TO SUPERSONIC FLOWS
Creator: Raman, G, Hailye, M, Rice, Ej
Date: 1993-06
Publisher: AMER INST AERONAUT ASTRONAUT
Description: An experiment studying a fluidically oscillated rectangular jet flow was conducted. The Mach number was varied over a range from low subsonic...
Show moreAn experiment studying a fluidically oscillated rectangular jet flow was conducted. The Mach number was varied over a range from low subsonic to supersonic. Unsteady velocity and pressure measurements were made using hot wires, piezoresistive pressure transducers, and pitot probes. In addition, smoke flow visualization using high-speed photography was used to document the oscillation of the jet. For the subsonic flip-flop jet, it was found that the apparent time-mean widening of the jet was not accompanied by an increase in the mass flux. Fluidically oscillated jets up to a Mach number of about 0.5 have been reported before, but to our knowledge there is no information on fluidically oscillated supersonic jets. It was found that it is possible to extend the operation of these devices to supersonic flows. The streamwise velocity perturbation levels produced by this device were much higher than the perturbation levels that could be produced using conventional excitation sources such as acoustic drivers. In view of this ability to produce high amplitudes, the potential for using a small-scale fluidically oscillated jet as an unsteady excitation source for the control of shear flows in full-scale practical applications seems promising.
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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: 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: MODE SPECTRA OF NATURAL DISTURBANCES IN A CIRCULAR JET AND THE EFFECT OF ACOUSTIC FORCING
Creator: Raman, G, Rice, Ej, Reshotko, E
Date: 1994-10
Publisher: SPRINGER VERLAG
Description: A modal spectrum technique was used to study coherent instability modes (both axisymmetric and azimuthal) triggered by naturally occurring...
Show moreA modal spectrum technique was used to study coherent instability modes (both axisymmetric and azimuthal) triggered by naturally occurring disturbances in a circular jet. This technique was applied to a high Reynolds number (400,000) jet for both untripped (transitional) and tripped (turbulent) nozzle exit boundary layers, with both cases having a core turbulence level of 0.15%. The region up to the end of the potential core was dominated by the axisymmetric mode, with the azimuthal modes dominating further downstream. The growth of the azimuthal modes was observed closer to the nozzle exit for the jet with a transitional boundary layer. Whether for locally parallel flow or slowly diverging flow, even at low levels of acoustic forcing, the inviscid linear theory is seen to be inadequate for predicting the amplitude of the forced mode. In contrast, the energy integral approach reasonably predicts the evolution of the forced mode.
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Title: EVALUATION OF FLIP-FLOP JET NOZZLES FOR USE AS PRACTICAL EXCITATION DEVICES
Creator: Raman, G, Rice, Ej, Cornelius, Dm
Date: 1994-09
Publisher: ASME-AMER SOC MECHANICAL ENG
Description: This paper describes the flowfield characteristics of the flip-flop jet nozzle and the potential for using this nozzle as a practical...
Show moreThis paper describes the flowfield characteristics of the flip-flop jet nozzle and the potential for using this nozzle as a practical excitation device. It appears from the existing body of published information that there is a lack of data on the parameters affecting the operation of such nozzles and on the mechanism of operation of these nozzles. An attempt is made in the present work to study the important parameters affecting the operation and performance of a flip-flop jet nozzle. Measurements were carried out to systematically assess the effect of varying the nozzle pressure ratio (NPR) as well as the length and volume of the feedback tube on the frequency of oscillation of this device. Flow visualization was used to obtain a better understanding of the jet flowfield and of the processes occurring within the feedback tube. The frequency of oscillation of the flip-flop jet depended significantly on the feedback tube length and volume as well as on the nozzle pressure ratio. In contrast, the coherent velocity perturbation levels did not depend on the above-mentioned parameters. The data presented in this paper would be useful for modeling such flip-flop excitation devices that are potentially useful for controlling practical shear flows.
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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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