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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: Development of High Bandwidth Powered Resonance Tube Actuators with Feedback Control
Creator: Raman, G., Khanafseh, S., Cain, A. B., Kerschen, E.
Date: 2004-01-22
Publisher: Academic Press Ltd Elsevier Science Ltd
Description: A high bandwidth powered resonance tube (PRT) actuator potentially useful for noise and flow control applications was developed. High...
Show moreA high bandwidth powered resonance tube (PRT) actuator potentially useful for noise and flow control applications was developed. High bandwidth allows use of the same actuator at various locations on an aircraft and over a range of flight speeds. The actuator selected for bandwidth enhancement was the PRT actuator, which is an adaptation of the Hartmann whistle. The device is capable of producing high-frequency and high-amplitude pressure and velocity perturbations for active flow control applications. Our detailed experiments aimed at understanding the PRT phenomenon are complemented by an improved analytical model and direct numerical simulations. We provide a detailed characterization of the unsteady pressures in the nearfield of the actuator using phase averaged pressure measurements. The measurements revealed that propagating fluctuations at 9 kHz were biased towards the upstream direction (relative to the supply jet). A complementary computational study validated by our experiments was useful in simulating the details in the region between the supply jet and the resonance tube where it was difficult to gather experimental data. High bandwidth was obtained by varying the depth of the resonance tube that determines the frequency produced by the device. Our actuator could produce frequencies ranging from 1600 to 15,000 Hz at amplitudes as high as 160 dB near the source. The frequency variation with depth is predicted well by the quarter wavelength formula for deep tubes but the formula becomes increasingly inaccurate as the tube depth is decreased. An improved analytical model was developed, in which the compliance and mass of the fluid in the integration slot is incorporated into the prediction of resonance frequencies of the system. Finally a feedback controller that varied both the resonance tube depth and spacing to converge on a desired frequency was developed and demonstrated. We are optimistic that numerous potential applications exist for such high bandwidth high dynamic range actuators. (C) 2003 Published by Elsevier Science Ltd.
http://dx.doi.org/10.1016/S0022-460X(03)00212-8
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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: Jet-cavity Interaction Tones
Creator: Raman, G., Bencic, Tj, Envia, E.
Date: 2002-08
Publisher: American Inst Aeronaut Astronaut
Description: A fundamental study of resonant tones produced by jet-cavity interaction over a wide range of flow conditions covering both subsonic and...
Show moreA fundamental study of resonant tones produced by jet-cavity interaction over a wide range of flow conditions covering both subsonic and supersonic speeds is described. Two significant findings emerge. For the jet-cavity configurations investigated, a suitably defined reduced frequency parameter allows for a global classification of all jet-cavity tones into two main types. For the first type, the reduced frequency depends on the jet Mach number, whereas for the second type, the reduced frequency is independent of the jet Mach number. We propose simple correlations for the frequency of both types of tones. Based on earlier research, we had expected that the traditional classifications of cavity flows into the open, transitional, or closed variety would be insensitive to small changes in Mach number and would depend primarily on the cavity's LID ratio. However, use of the novel high-resolution photoluminescent pressure sensitive paint shows that these classifications are actually quite sensitive to the jet Mach number for jet-cavity interactions. However, these classifications provide no guidance for determining tone type, amplitude, or frequency.
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Title: Jet Thrust Vectoring Using a Miniature Fluidic Oscillator
Creator: Raman, G., Packiarajan, S., Papadopoulos, G.
Date: 2005-03
Publisher: Royal Aeronautical Soc
Description: This paper presents a new approach to vectoring jet thrust using a miniature fluidic actuator that provided spatially distributed mass...
Show moreThis paper presents a new approach to vectoring jet thrust using a miniature fluidic actuator that provided spatially distributed mass addition. The fluidic actuators used had no moving parts and produced oscillatory flow with a square wave form at frequencies up to 1(.)6kHz. A subsonic jet with an exit diameter of 3(.)81cm was controlled using single and dual fluidic actuators, each with an equivalent circular diameter of 1(.)06mm. The fluidic nozzle was operated at pressures between 20(.)68 and 165(.)47kPa. The objectives of the present work included documentation of the actuation characteristics of fluidic devices, assessment of the effectiveness of fluidic devices for jet thrust vectoring, and evaluation of mass flow requirements for vectoring under various conditions. Measurements were made in the flow field using a pitot probe for the vectored and unvectored cases. Some acoustic measurements were made using microphones in the near-field and for selected cases particle image velocimetry (PIV) measurements were made. Thrust vectoring was obtained in low speed jets by momentum effects with fluidic device mass flow rates of only 2 x 10(-4) kg/sec (0-6% of main jet mass flow per fluidic oscillator). Although a single fluidic device produced vectoring of the primary jet, the dual fluidic device configuration (with two fluidic devices on either side of the jet exit) produced mass flux enhancement of 28% with no vectoring. Our results indicate that fluidic actuators have the potential for use in thrust vectoring, flow mixing and industrial flow deflection applications.
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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: Development of Powered Resonance-tube Actuators for Aircraft Flow Control Applications
Creator: Raman, G., Mills, A., Kibens, V.
Date: 2004-12
Publisher: American Inst Aeronaut Astronaut
Description: The present paper addresses both active-flow-control actuator technology development and the demonstration of the effectiveness of actuators...
Show moreThe present paper addresses both active-flow-control actuator technology development and the demonstration of the effectiveness of actuators that could be easily integrated into practical aircraft applications. The actuator used is an adaptation of the Hartmann oscillator. Demonstration experiments that illustrate the effectiveness of this actuator include cavity tone suppression at transonic speeds and the reduction of jet-impingement tones. The actuator concept is based on a high-speed jet aimed at the mouth of a cylindrical tube closed at the other end. The result is a high-amplitude self-sustaining fluctuating field accompanied by an intense narrowband tone, all in the region between the supply jet and the resonance tube. Using unsteady pressure sensors and flow visualization, we explored the effect of varying actuator parameters such as the spacing between the power jet and the resonance tube, supply pressure, resonance-tube depth, diameter, shape, and lateral spacing. By varying the depth of the tube, the frequency could be varied from about 1.6 kHz to over 10 kHz and amplitudes as high as 156 dB (microphone location dependent) were obtained in the vicinity of actuation. To integrate this concept into practical aircraft applications, two generations of a more complex version of this device known as the powered resonance-tube bank (PRTB) were developed and demonstrated. Results indicate that by using high-frequency excitation at 5-kHz suppression levels in excess of 20 dB were consistently obtained over a range of operating conditions in both cavity and impingement flow situations. Based on our results, we have grounds to believe that a properly designed PRTB has significant advantages over conventional actuators such as acoustic, piezo, and oscillatory microstructures.
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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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