Active Noise Control of Supersonic Impinging Jet Using Pulsed Microjets

J. Choi* A. M. Annaswamyt n/l~~ssach~u.srtts Inutitute of Technology, Cambridge, M A , 02139

0. Egungwu * F. S. Alvii Floridu A & M Unzversity and Florida State University, Tallahassee, FL

Supersonic im~~inging jets produce a highly unsteady flowfield leading to a noisy en- vironment with high dynamic pressure loads on nearby surfaces. In prior work, it was demonstrated that microjet injecti on along the circumference of the main jet nozzle di- rectly into the shear layer of the main jet disrupts the feedback loop inherent in high-speed impinging jet flows, thereby significantly reducing the adverse effects produced. The mi- crojet action was due t o steady blowing whose flow rate was either uniform along the circumference or varied using the eigenmode of the flow. While these methods yielded some successes, ulliform reduction of all operating conditions was not t o obtained. In this paper, pulsing microjet injection is chosen as a new control scheme to retain a uniform suppression of impinging jet noise. Through pulsing injection, a fairly good amount of noise reduction w ls achieved using 42% of the mass flow rate that led to the same level of noise reduction using steady microjet injection. As the duty cycle was increased, it was observed that the pulsed injection completely destroyed the distinct impinging tone for almost all heights.

Nomenclature

Nozzle diameter of rnai~ljet. 111

Diameter of lift plcte, rn Distance from lift Illate t o ground plane, m Mass flow rate, kg, s Density of air, kg/] n3 Mean velocity of nr icrojet, ni/s Cross sectional are.t of microjet, m2 Amplitude of veloc ty f luct~~at ion, m/s Number of holes in the lift plate Number of rnicroje ,s

I. Introduction

The interaction betwfwi the ground and the high speed jet emanating from a STOVL . i r c r d t nozzle generates discrete, high-amplitude acoustic tones. Such discrete tones cause high levels of noise and acoustic fatigue of nearby s t ruc tu~es and are also the cause of lift loss arid ground erosion. Suppression of these tones requires destabilizing the feedback loop,' which can be realized by suitably disturbing the shear layer near the nozzle exit. Two me1 hods of unsettling the shear layer, which include using coaxial flow2 and counter flow,3 have showed a moderate reduction in noise level. A more flexible way of affecting the shear layer that

*Graduate Research Assist ant +Senior Research Scientist. Member AIAA t ~ s s o c i a t e Professor. Ser~ictr Merrher AIAA

A~riericarl Institute of Aeroriautics and Astronautics

43rd AIAA Aerospace Sciences Meeting and Exhibit10 - 13 January 2005, Reno, Nevada

AIAA 2005-798

Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

has been attenlpted iri K , f . [ ' I is to introduce microjets along the periphery of the nozzle exi:. Due t o their small size, these rnicrojet i CiLIl be optinially distributed along the circumference and can also ,e switched on arid off on-derrialitl.

While it has been shown in the previous work' tha t an open-loop coritrol strategy tha.t employs the microjets is effective in :.uppressing the irripirigerrient tones, the amount of suppression is deperiderit t o a. large extent on the opelatirig cor~ditions. Since in practice, the flow conditioris are expected to change drastically, a more attra1:tive control strategy is one that employs feedback and has the ability t o control the irnpingemerit tones rver a large range of desired operating conditions. Recently, a closed-loop control method was introduced" I o suppress the tones under various operating conditions based on 'proper orthogonal decomposition' and produced additional noise reduction compared t o open-loop control s t r~kegy. But the performance is still sensitive t o changes in operating conditions such as the distance between nozzle exit and the impinging region. Hence, we rieed t o devise new methods t o obtain consistent noise recluction under various jet operating conditions. In this paper, we present the result obtained by using pulsed niicrojets in the impingement tone problerrl. Our expectation was that for a given mass flow rate, a pulsed injection can generate larger morr~enturn than steady continuous microjet injection, and hence can perhaps make a stronger impact on the 1l8,ise reduction mechanism.

Flow control techniql~es using pulsed forcing actuator have recently been used. In reference5 Wiltse and Glezer introduced all open-loop control strategy via high frequency forcing in the inertial subrange of a free shear layer on a 1, ,w speed flow. They found that broadband velocity fluctuations were reduced a t low frequency but increaied a t high frequencies. StanekG7 and Sinha8 adopted the high frequency forcing technology into control 01 the cavity flows and RamanQas recently reported the result applied on control of impinging tone. More recently, Kastner et a1.l" could reduce a resonant peak using HTFA (Hartmann Tube Fluidic Acutator), a ver:. high speed actuator for control the impinging jet noise. This work was mainly confined to reducing flow induced noise in the sense of optimal control that the effort was focu5ed on creating satisfactory noise reduction under small mass flow rate of actuator which required t o increa.se the pulsing speed to ensure a sufficieilt niomentu~n. It should be noted that high speed pulsing cannot always guarantee the best noise reduction l~ecause it can occasionally increases the broadband noise. Hence, in this paper, we present a slightly differer t pulsing strategy.

11. Experimental Details

A. Experimental Fac ility and Test Configuration

The experiments were ci rried out a t the STOVL supersonic jet facility of the Fluid Mechanics Research Laboratory (FMRL) located a t the Florida State University. A schematic of the test geometry with a single impinging jet is shown i i Figure ' . This facility is used primarily to study jet-induced phenomenon on STOVL aircraft hovering in arid out of the ground effect. Further facility details can be found in Krothapalli et al.ll

G Ground Plate

Figure 1. Test geometry

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The nieasurenients were conductcrl using an axisyrnmctric, convergent-divergent (C-D) nozzle with i\

design Mach number of I .5. The throat and exit diameters (d , cle) of the nozzle are 2.54c1n and 2.75cni (see Figure I & 2 ) . The clivel,gent part of the nozzle is a straight-walled conic section with a 3' divergence angle from the throat t o the nozzle exit. Although tests were conducted over a range of Nozzle Pressure Ratios (NPR, where NPR = stigrlation pressure/ambie~~t p ress~~re) , the results discussed in the present paper are limited t o NPR = 3.7 tliat corresponds t o an ideally expanded Mach 1.5 jet, A circular plate of diameter D (25.4 c~n- l0d) was flush mounted with the nozzle exit and, henceforth referred t o as the 'lift plate', represents a generic airclaft planform. A l m x In1 x 25 111111 aluminum plate serves as the grc~und plane and is rnounted directly under the nozzle 011 a hydraulic lift.

Seconddry plenum Prinvv Plenum chambers a

@-@- Control valvcr

Figure 2. (a) Lift plate/microjet layout, (b)Microjet feed assembly

Active flow control w;ts implemented using sixteen microjets, flush mounted circumferentia.lly around the main jet as shown in f i g r e :?a. The jets were fabricated using 400 p m diameter stainless tubes and are oriented a t approximatel 30° with respect t o the rmin jet axis. The supply for the microjets was provided by cornpressed nitrogen ( ylinders through a main and four secondary plenum chambers. In this manner, the supply pressures to each bank of rnicrojets could be independently controlled. The microjets were operated over a range of NPR = 5 to 7 where the combined mass flow rate from all the microjets was less than 0.5% of the primary jet mass l u x .

B. Pressure Measurc!ment

Near-field noise was mea: ured using B&I< TM microphones placed approximately 25 cm away from the jet. The distribution of unste ldy loads on the lift plate was measured by six high frequency response miniature KuliteTM pressure transliucers, placed symmetrically around the nozzle periphery plate, a t r/d =1.3 from the nozzle centerlirie (figure. 2 ) . The transducer outputs were conditioned and simultaneously san~pled using National Instrunierits dig tal data acquisition cards and LnbViewTM software. Standard statistical analysis techniques were used to ,)btain the spectral content a r d the Overall Sound Pressure Level (OASPL) from these riiessurenients.

C . Control of Supersonic Impinging Tone Using Modulated Microjets

For a given mass flow rate I ~ L = pAUO, the force inclucecl by steady microjet irijection is given by the rate of ~ n o m e ~ i t u m change in t in~e . Using the same mass flow rate, unsteady injection can exert more force on the shear layer of the flow thim steady injection, in an average sense. Equation ( I ) described belclw shows that ,

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if thc unsteady How is represented in sinusoidal form, the a.dditiona1 force increase is realized by pA (R2 /2 ) :

B e c a ~ ~ s e the increased forcing strength is expected t o cause stronger impact on the noise reduct!.on mechanism, a snital)le pulsing strate:y is sought for the new actuation scheme. Two different niethods for introducing pulsing are explored. One method is t o use a high speed valve that modulates the flow from upstream. The other method is t o use ;L rotating cap actuator that generates p~llsing directly a t the micro,jet exit. These actuation schemes are discussed in the following section in detail.

1. Modulated Microjet - High Speed Valve

The easiest way t o gene] ate pulsing is using a high speed valve. The traditional feedback control paradigm requires tlie control inpu: to be modulated a t the natural frequericies of the system (for example, see Rowley et al.'". This, in turn, laandates that the external actuator have the necessary bandwidth fsx operating a t tlie natural frequencies. In the problem under consideration, the edge tones associated with the flow-field are typically of a few kilohertz. Given the current valve technology, modulating the microjet:j a t the system frequencies is nearly impossible. It follows that the first approach, as presented above, is reduced t o pulsing microjet injection a t the frequency of sub harmonics of the resonance frequencies. The specification of the relevant high speed vahe is listed in table I . Figure .i shows the schematic diagram of -the high speed valve assembly. Highly pressurized flow is passing through a filter, supplied t o the inlet of the block and drained out of the conne :ting tube. The high speed valve inside the assembly block modulates the exit flow. Unsteady pressure trans~iucer by K u l i t e T M , located at the end of the connecting tube measures the total pressure fluctuation.

High Speed Valve

pressure Unsteady Pitot tube

Figure 3. Schematic diagram of high speed valve assembly

To verify the correct function of the valve, a n experiment was conducted under the condition of 50 % duty cycle and a t 165 ps supply pressure. The result shown in figure ; indicates noticeable flow rriodulation issuing out of the connecting tube a t the 150 Hz valve speed. The next step was t o size down the diameter of the comecting tube s.1 as to match the microjet size at the other end. Because the length and diameter of the How channel are major factors in determining the flow resistance, several experiments were conducted t o mi~iiniize tlie resistal ce, an example of which is given here. An adaptor which decreaes the channel diameter frorn 0.50" t o 1).25" was plugged a t the end of the connecting tube. Because the cross sectio~lal

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Table 1. Specification of high speed valve

I Vendor I CleariAirPower I

area of the tuhe l~ecomes narrow dowristream direction, flow speed through the tube increase:;. However the upper limit of flow speed is bounded by the sonic limit, hence the resulting modulation of the flow speed is almost col~ipletely decayed as seen in figure <. Since the overall performance of this pulsing strategy was not

Valve

(SP-051)

Driver

(SD-1)

sntisf;tctorv. it was not pursued any further.

150 IIz 165 psi , High speed valve

Speed

Q (@loopsi) Precision

Supply Pressure

Response Time

Mics.

Figure 4. Flow modulation measurement without adaptor

D. Modulated Microjet - Rotating Cap

< 200Hz

115SLPbI

~ t 2 5 p s e c

< 300 psi

Because fiow modulation applied a t the upstream end results in steady injection, direct niodulation from the exit wils tried, where thc pulsing device was collocated at the microjet exit. This device was chosen t o be a rotating cap, designed 50 as t o interrupt the microjet streaks. In particular, if a rotating c . 2 ~ is placed a t the exit as shown in figu-e (j, the tooth inside the cap, while rotating, periodically hlocks the microjet hole. By corltinl~ing the rotation, a pulsing action of microjets is achieved.

'I'lie inner and outer a c e of bearing (Kaydon, KAA15AGO) are tightly fitted t o the small lift plate and rotating cap respective11 A motor mounted on the lift plate drives the rotating cap connected by a belt. Fig11rc) .. shows the desig.1 of the rotating cap and its installation in the experin~ental setup. The design of the l i f t plat,<, is slightly (.hanged t o install the rotating cap actuator. As seen in figure y , the lift plate is composed of two parts, a small lift plate and a big lift plate. The small lift plate assembled with the rotating cap is fixed ;tt the center of the big lift plate. Finally, the lift plate is supported by three arms attached a t the end of' the holder grabbing the nozzle side firrnly. Figure !I shows the ir~stallatiori of lift plate into the nozzlt..

111 f i p ~ ~ r ~ 1 1 3 , unsteady pressure data measured by Kulite shows a well modulated flow velocity with respect to time. The FFT plot also indicates the exact pulsing frequency of the ~nicrojet;. In addition t o provitling a direct method of pulsing the microjet flow, the rotating cap technique is also amenable t o achirviug p;tranietric changes in the pulsed flow. Several properties including the amplitude, frequency, arid duty cycle o f the pulsing can be changed by varying the design parameters of the pulsing cap. For example,

Opening

3 msec

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Closirig

2 msec

l . 6 n l l 2 V f D C )

Adaptor

150 Hz 165 psi

Figure 5. Flow modulation measurement with adaptor

Figure 6. Conceptual diagram for rotating cap actuator

et Microiets

Small Lift Plate .

Kulite

Motot

Figure 7. Rotating cap design

Pulley

Gof 1 1

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Figure 8. Assembled feature of lift plate and nozzle

Figure 9. Experimental setup installed with lift plate

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the h t y cycle of this pusing defined as the ratio of valve opening time t o pulsirig period is (decided by the size of tooth in the cap a i shown in figure 1 i , while both the duty cycle arid the niagnitude of fluctuation is determined hy the suppl!: pressure delivered to rnicrojet chamber. Pulsing speed is solely controlled by the rotation speed of the cap.

Steady flow

Figure 10. Performance of collocated rotating cap actuator; Total pressure data in time series (a) Case of steadily injecting microjt t (b) Case of pulsing microjet at the speed of 304.74Hz (c) FFT plot of pulsing microjet

42% duty cycle

Hole 74% duty cycle

F'igure 11. Modification of duty cycle by changing hole size

A sinlilar design t o the rotating cap approach discussed above has been reported in R.eference13 for suppressing jet noise. A comparison between our design and that of reference13 is briefly illustrated in figure 12. The major distinction between the two is the distance from the rnicrojet injection point t o the shear hyer of ~ n a i n jet. 11. reference13 (shown in figure ]'-a), this distance is 2.25 mm, five times as large as the microjet diameter. 011r design, shown in figure 12-b, collocates the actuator with the nozzk exit, thereby allowing the microjet flov t o have an inclination angle with respect t o the flow direction. In contrast, in reference,'"he nlicrojet njection is forced t o remain normal t o the main flow. The lack of success reported in r e f e r e n c e l h a y be dul: t o this distance, since the penetration depth of inicrojet injection into the shear l a y ~ r is known t o play a ( ritical role in the noise suppressing mechanisnl.

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Main Jet Main Jet

Figure 12 (a) Rotating cap design in reference,13 (b) Collocated rotating cap

111. Results

A. Noise Reduction by Synchronous Microjet Pulsing

The results obtained usir g the collocated rotating cap for impingement control are shown in Figure 1 ;<. The noise level is measured b] the microphone located a t 25 cm away from the nozzle axis because it was observed that the Kl~lite transduct r mounted on the lift plate does not capture the correct signal due t o the vibration of the motor. In figure :;, steady microjet injection case was also compared to check the pulsing efficacy under the sarne experimt ntal condition. The supply pressure t o the microjets was set t o 115psi during the operation. Under the mine supply pressure, the peak value of flow n~ocldation clue to pulsing corresponds to the flow exit speed during steady injection, hence the duty cycle of pulsing detern~ines t h ~ relative mass flow rate clue to pulsing as cornpared to the steady injection case. The speed of pulsing is determined by the rrlotor speed, size of ,-otating cap and number of teeth in the cap. Here the resultant pulsing speed by tlie rotatil~g cap was set to 121.09 Hz, which is a moderate speed arid does not lead t o a broadband noise increase. Figure {:$-(a) illustrates that pulsing of microjets using the collocated rotating cap can generate an acceptable level of noise reduction. Another attractive feature of this actuator is that the noise reduction shown in figure ]:{-(a) mas achieved using 42% of the mass flow rate of the steady microjet injection case. As the duty cycle was in( reased, it was observed that the pulsed injection completely destroyed the distinct impinging tone for almos all distance conditions, and is shown in figures I.L(b). For example, the frequency content in figure 1;:-(b) shows tha t the impingement tone has been eliminated at height h/d = 3.5 because of the pulsed injection. F.foreover, the required mass flow rate is less than the steady injection case (74%). These results are encour tging because this technique has resolved the long-cherished problem of "uneven reduction for different he:glit."

B. Noise Reduction by Microjet Pulsing with Phase Difference

It has been observed in Krothapalli et al." that the flow characteristics evolve from a helical mode t o a syrlilnetric mode and ret lrn t o a helical mode as the nozzle to ground distance becomes larger. Figure 1 i shows the phase difference between the sigrials measured by Kulites mounted on the lift plate. A small deviation in tlie pliase d~fference between the signals implies that the flow characteristics are syrnrnetric. From figures ! I ( a ) and ( ))! we note that there is a strong correlation between the amount of noise reduction and tlie How mode. For exarriple, a drastic noise reduction occurs a t the height of h/d = 4.0 where the

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3 3.5 4 4.5 5 5.5 6

hld

No Control Steady Pulsing

Figure 13. Experimental result using synchronous pulsing scheme (a) Case of 42% duty cycle (b) Case of 74% duty cycle

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uncontrolled flow is dorni~~atetl by ;I syrrilnetric mode. The frequency content corresponding to the height (in Figure i i (b ) ) showed a listirict difttwrice too. At the height where a helical mode prevails, the frequency content is distinguished l ~ y several harrnor~ic frequencies, while the symmetric mode is promirient a t the instant where the frequency content is coritaminated by other frequencies. The uneven pa.ttern in noise reduction throughout all heights is clearly correlated t o the dominant flow characteristics.

-20 hid = 3 0 i 2 4

,Id = 4 o j

F6 - 2 4 6

2 4 6 Kulite pos tion

3 35 4 4 5 5 55 6

hld

I-+- M mnlrd S y m ~ootnl(ltBprla)l

Figure 14. (a) Phase difierence in Overall Sound Pressure Level using 300 microjet injection, NPR = 3.7, (b) Frequency content of pressure fluctuation (measured by kulite I), (c) Overall Sound P r e s s ~ ~ r e Level using symmetric microjet inject ion

From the above ot)se.vation. it follows that the dominant flow characteristics are expected t o play a crucial role in noise red ~c t ion niechanism. Hence, a control strategy that employs phase difference to suitably influence the dorniriant flow modes is potentially more effective in reducing noise. If the number of holes in the rotating cap IS the same as that of microjets, all the microjets can be pulsed synchronously. To achieve phase difference, the number of holes in the cap is kept different from that of microjets. A simple example shown in figure I , illustrates a set of 3 microjets and 4 holes in the rotating cap block and open

these rnicrojet steaks pe ,iodically. This configuration produces pulsing with 120" phase difference. The phase difference between microjets adjacent t o each other is given by:

where N,, is number of holes in the lift plate and N,, is riuniber of microjets. In the i~ripinging jet problem, 18 holes were made in tht, rotating cap while 16 microjets were installed in the lift plate. Following equation ( 2 ) , the phase difference letween an adjacent pair of microjets is 45". Two experinients were conducted by changing the rotating d i r~c t ion of the cap to check the effect of phase leading and lagging on noise reduction. In addition, to compare its efficacy to the synchro~iously pulsed injection case, Overall Sound .Pressure Level plot of synchronous pulsil~g with 42 '% duty cycle was also included. For fair comparison, the supply pressure delivered t o the microjet charnhcr was set t o 115 psi, and pulsing speed was also set t o 121.09 Hz which is same as the synchronc us case. The corresponding duty cycle of the phase difference scheme was 50%'.

l lo f 1 ;

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The result is shown in figure I ().It is clear that the effect of pulsing with 45" phase difference doesn't show noticeable inlprovetnent c wer the syrichronous scheme. We can attribute this result to the phase difference, 45", which may not he tile appropriate value for realizing rriaximum noise reduction for a g:.ven operating condition. The optimal v,tlue of this parameter will be investigated more detail in the future.

Small hft Plate Rotatmg Cap

M~crojet Hole

Hole 1 open Hole 2 open Hole 3 open

Figure 15. Concept of pulsing with phase difference (a) Configuration of lift plate and rotating cap (b) Microjet pulsing in the consecutive stages

IV. Conclusion

In this paper, several control strategies were presented t o suppress supersonic impinging jet noise. The unsteadiness of the microjet flow can the increase forcing strength and make distinct impact on the shear layer of the main jet. He ice, rrlodulation of microjets was suggested as a new control actuation scheme. At first, modulation of the f ow from the upstream end was suggested t o generate a pulsed microjet injection. However, the flow nlodu1;~tion using this scheme is bounded by sonic choking condition, resulting in the flow t o behave like a steady illjection. The flow modulation scheme using high speed valve was discarded. An alternative method, dire( t modulation of the flow from the nozzle exit, was suggested using rotating cap actuator. The saw tooth jtructure at the inner circle of rotating cap periodically blocks and opens microjet streaks, which generates l~ulsing injection. This strategy generated high speed pulsing a t the speed of several hundred Hz without any difficulty.

Two experiments wele conducted using the rotating cap actuator: (i) synchronous pulsing, and (ii) pulsing with phase differc nce. Synchronous pulsing a t the frequency of 121.09 Hz resulted in noise reduction as steady injection while using only 42% mass flow rate of the steady case. But as in the case of steady injection, the efficacy of >ynchronous pulsing a t low duty cycle is dependent on the nozzle-to-ground height. However, as the duty cyf:le was increased, it was observed that the pulsed injection completely destroyed the distinct impinging tone for almost all distance conditions. Thus, we have resolved the long prohleni of "uneven noise reduction :or different height condition" as shown in figures i ;$-(b), through high duty cycle pulsing strategy.

Pulsing with phase dilference can be achieved through a specially designed rotating cap. Pulsing with 45' phase difference has shown a sinlilar response as the synchronous pulsing case. The optirnal phase difference that results in an effecti\+? noise reduction is currently being investigated.

Acknowledgments

This work was s u p p ~ r t e d by a grant from AFOSR, monitored by Dr. J. Schmisseur. We would like t o thank the staff of FhIRL, for their irivaluable help in conclucting these tests. We are grateful t o Mr. Choutapalli for his help In conducting the tests, and Robert Avant for his advice in designing the experinie~ital setup. In addition, Boh11.v DePriest's prompt support make it possible to implement several crucial tests.

12of ' 1 -- - . . - .

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On * Off

CCW: Counter Clockwise Rotation

142 1 3 3 5 4 4 5 5 5 5 6

Wd

+No Control 11 5 p s ~ t 115 ps~ (synchromffi) -- - i

(4

CW: Clockwise Rotation

Figure 16. Implementatlon of pulsing with phase difference (a) Counter clockwise rotation (b) Clockwise rotation

1 3 o f ! :

Allmican Institute of Aeronautics and Astronautics

References

'hlvi. F S., Elavaraan R., Shih, C., Garg, G., and Krothapalli, K.. "Control of Supersonic Impinging Jet Flow Using Rlicrojets." 2000-2236. AIAA paper, 2000.

'Sheplak, M and Spina. E. F. , 'LControl of high-speed impinging-jet resonance." AIAA .Jozimal, Vol. 32, No. 8, 1994, pp. 1583- 1588.

"Shill, C. , Alvi, F. S., a l d Washington, D., "Effects of CounterHow on the Aeroacoustic Properties of a Supersonic Jet," J o ~ m ~ n l of Azrcraft, Vol. 36, \To. 2, 1999, pp. 451-457.

'Lou. H., Alvi, F. S., Shlh, C., Choi, J., and Annaswarny, A. MI., '.Active Control of Supersonic Impinging Jets: Flowfield Properties and Closed-loop Strategies," 2002-2728, AIAA paper, 2002.

5Wiltse, . I . M. and Glewr, A., "Direct Excitation of Small-Scale Motions in Free Shear Flows," Physics of Fluids, Vol. 10, No. 8 , 1998, pp. 2026-2036.

'Starrek, M. J., Sinha, V., Seiner, .J. M., Pearce, B., and Jones, RI. T . , "High Frequency Flow Control-Suppression of Aero-Optlcs 111 Tactical Directed Energy Beam Propagation & The Birthe of a New Model," 2002-2272, AIA.4 paper, 2002.

7Stanek, hl. J . , Raman G., Ross, J. A., Odedra, J., Peto, J . , Alvi, F . S . , and Kibens, V., "High Frequency Acoustic Suppression - T h e Role of h h s s Flow, T h e Notion of Superposition, And The Role of Inviscid Instability - A New Model (Part 11);' 2002-2404, AIAA paper. 2002.

'Sinha. N., Arunajatesan, S., and Seiner, J. M., "Computational and Experimental Investigations of Cavity Attenuation Using High Frequency Contrc~l," 2002-2403, AIAA paper, 2002.

'Raman. G. and Kiben:;, V., "Active Flow Control Using Integrated Powered Resonance Tube Actua~ors ," 2001-3024, AIA.4 paper, 2001.

"'Kastner, .J. and Samimv, M., "Effects of Forcing Frequency on T h e Control of an Inipingirlg High-speed Jet," 2003-0006, AIAA paper, 2003.

"t<rothapalli, A., R a j a k ~ p e r a n , E., Alvi, F . S., and Lourenco, I,., '.Flow field and Noise Characteristics of a Supersonic Impinging Jet," .Journal of Fluid Mechanzcs, Vol. 392, 1999.

"Rowley, C. W. , Williar is, D. R., Colonius, T . , Murray, R. M., MacMartin, D. G. , and Fabris, D., .'Mo.clel-based control of cavitv oscillatiorls. I1 - Sys' em identification and analysis," 2002-0972, AIAA paper, 2002.

"31bralrirn, M. K., Kuniinura, R. , and Nakamura, Y., "Mixing Enhancement of Compressible Jets by Using Unsteady RIicrojet,~ as Actuators," A I A A Journal, Vol. 40, No. 4, 2002, pp. 681--688.

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