Role of Coherent Structures in Supersonic Impinging Jet                    Noise and its Control                      Raja...
important to understand the details of the flow field of these jets with and without a particular control scheme for thede...
(10 m3) and is used to drive the facility. The measurements were made at both design and off-design conditions ofMach 1.5 ...
trigger for the synchronized camera and laser strobe. The laser frequency of 15 Hz requires the pulsation frequencyto be a...
figure includes the data obtained at two values of h/d, 4 and 4.5 with and without control. As stated earlier, h/d = 4 ist...
a) h/d = 4                                                 b) h/d = 4.5                  Figure 7. Mean velocity distribut...
a) ϕ = 0º                                          b) ϕ = 60º     c)   ϕ = 120º                                     d) ϕ =...
C. Phase locked velocity field    In order to better understand the flow physics associated with impinging jets with and w...
where the subscripts 1 and 2 correspond to streamwise and cross-stream directions respectively. Identification ofvortical ...
those roll up and become once again coherent. The outcome of the flowfield results with control agrees with those ofacoust...
[11] Kweon, Y. –H., Miyazato, Y., Aoki, T., Kim, H. –D. and Setoguchi, T., “Control of supersonic jet noise using a wire d...
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Role of Coherent Structures in Supersonic Jet Noise and Its Control

  1. 1. Role of Coherent Structures in Supersonic Impinging Jet Noise and its Control Rajan Kumar*1, L. Venkatakrishnan$2, Alex Wiley*3 and Farrukh S. Alvi*4 * Florida Center for Advanced Aero-Propulsion (FCAAP), Florida State University, Tallahassee, FL – 32310 $ National Aerospace Laboratories, Council of Scientific and Industrial Research, Bangalore, INDIA - 560017 This paper describes the results of a study examining the flow and acoustic characteristics of a Mach 1.5 ideally expanded supersonic jet impinging on a flat surface and its control using microjets. Emphasis is placed on two conditions of nozzle to plate distances (h/d), of which one corresponds to where the microjet based active flow control is very effective in reducing flow unsteadiness and nearfield acoustics and other with minimal effectiveness. Measurements include unsteady pressures using high response pressure transducers, nearfield acoustics using microphone and particle image velocimetry (PIV). The nearfield noise and unsteady pressure spectra at both h/d show discrete high amplitude impinging tones, which in one case (h/d = 4) get significantly reduced with control but in the other (h/d = 4.5) remain unaffected. The PIV measurements, both time-averaged and phase- averaged were used to understand the basic characteristics of impinging jet flowfield and the role of coherent vortical structures in the noise generation and suppression. The results show that the flowfield corresponding to the case of least control effectiveness comprise well defined, coherent and symmetrical vortical structures and will require higher levels of microjet pressure supply for noise suppression. I. IntroductionS UPERSONIC impinging jets have received considerable attention in the past because of theirimportance in a wide range of applications from theShort/Vertical Take-off Landing (S/VTOL) aircraft toturbine blade cooling. The flow field of the supersonicimpinging jets is known to be highly unsteady especiallyin an S/VTOL aircraft configuration. This can haveadverse effects such as high noise levels, unsteadyacoustic loads and sonic fatigue on the aircraft andsurrounding structures, ground erosion, and ingestion ofhot gases into the engine nacelle and a lift loss of theaircraft during hover. On a carrier deck, the aircraftexhaust impinges on the deflector plate and produceshigh noise levels and make the deck environment highlynoisy and cause a serious health concern to thepersonnel working on the deck. Although a substantialamount of research has been carried out in the past onsupersonic impinging jets and its control using variouspassive and active control methods, and theireffectiveness has been detailed in the literature1-10, butthe problem is still far from being resolved due to thecomplex flow field associated with these jets. It is very Figure 1. A schematic of feedback loop for1 impinging jet Research Scientist, Department of Mechanical Engineering, Senior Member AIAA.2 Scientist, Experimental Aerodynamics Division, Senior Member AIAA.3 Research Assistant, Department of Mechanical Engineering, Student Member AIAA.4 Professor, Department of Mechanical Engineering, Associate Fellow AIAA. 1 American Institute of Aeronautics and Astronautics
  2. 2. important to understand the details of the flow field of these jets with and without a particular control scheme for thedevelopment of a robust control system. The impinging jet flow field is a highly resonant flow field governed by a well known feedback loop1-12 (Fig. 1).The concept of the feedback loop and itsunderstanding has its roots in the pioneeringresearch of Powell, who explains the feedbackloop associated with edge tones generated byhigh speed jets. A number of the generalfeatures of the feedback loop associated withimpinging tones are similar to that elucidatedby Powell4 for edge tones. In a similar manner,as noted by Tam and Ahuja5 and detailed byKrothapalli et al.6, the feedback loop in theimpinging jet is initiated as instability waves inthe shear layer of the jet at the nozzle lip. Theseinstability waves grow in size into large-scalevortical structures as the jet travels downstream.In the case of impinging jets, the ground planeacts as a physical obstruction similar to the“edges” in edge tones. Upon impingement,these vortices generate large pressurefluctuations, which in turn travel upstream inthe ambient flow in the form of acoustic waves. Figure 2. Effectiveness of microjet control, NPR = 3.7Upon reaching the nozzle exit, these acousticwaves excite the shear layer and complete the feedback loop. Most of the control systems studied so far includinghigh momentum microjet control have tried to suppress this feedback loop and seem to work efficiently over alimited range of geometrical and flow conditions. In particular, effectiveness of microjet control in reducing noiseshows a strong dependence on the nozzle-to-plate distance, nozzle pressure ratio and temperature ratio of theimpinging jet. Figure 2 (data taken from Ref. 11) shows the variation of effectiveness of microjet control (in termsof reduction in overall sound pressure levels, ∆OASPL) with nozzle-to-plate distance. The results clearly show thatat certain values of h/d, microjet control is very effective in reducing OASPL (e.g., at h/d = 3.5, ∆OASPL = ~12 dBfor lift plate sensor), whereas that is not the case at other values of h/d (e.g., at h/d = 4.5, ∆OASPL = ~3 dB). Theseresults hence pose a number of questions such as why the control technique which is so effective at some testcondition is not so effective at other test condition. Do the flow features of jet change drastically with a small changein nozzle-to-plate distance? Does the strength of the feedback loop or evolution of large scale structures vary withh/d? It is very important to answer these questions to design a robust and highly effective control technique tomitigate high noise levels associated with supersonic impinging jets. In this study we have taken a closer look atthese results and have made an attempt to answer some of these questions. In addition we have made phase lockedparticle image velocimetry measurements at two values of h/d, one corresponding to the case where microjet controlis very effective and the other where it is not so. II. Experimental SetupA. Test facility The experiments were carried out at the STOVLsupersonic jet facility of the Advanced Aero-PropulsionLaboratory (AAPL) located at the Florida State University.This facility is mainly used to study jet-induced phenomenonon STOVL aircraft during hover. It is capable of runningsingle and multiple jets at design or off-design conditions upto M = 2.2. In order to simulate different aircraft to groundplane distances, the ground plate is mounted on a hydrauliclift and can be moved up and down. A high pressurecompressed air (~160 bars) is stored in large storage tanks 2 Figure 3. A photograph of the STOVL facility American Institute of Aeronautics and Astronautics
  3. 3. (10 m3) and is used to drive the facility. The measurements were made at both design and off-design conditions ofMach 1.5 jet issuing from a converging-diverging axisymmetric nozzle. The design Mach number of the nozzle was1.5 and was operated at Nozzle Pressure ratio (NPR, where NPR = stagnation pressure/ambient pressure) of 3.7,corresponding to ideally expanded jet condition. The test Reynolds number based on exit velocity and nozzlediameter of the jet was 7 x 105. For these experiments, the stagnation temperature of the jet was kept constant at300K, corresponding to a temperature ratio, TR =1.0 (where, TR = stagnation temperature / ambient temperature).The throat and exit diameters (d, de) of the nozzle are 2.54 cm and 2.75 cm respectively. The diverging section of thenozzle is a straight-walled with 3° divergence angle from the throat to the nozzle exit. A circular plate of diameter25.4 cm (= 10d) was flush mounted with the nozzle exit. This plate, henceforth referred as lift plate, represents ageneric aircraft planform and has a central hole, equal to the nozzle exit diameter, through which the jet is issued. Atotal of sixteen microjets were flush mounted circumferentially on the lift plate around the main jet to implement theactive flow control. The jets are issued using 400 µm diameter stainless steel tubes mounted at an inclination of 60°with respect to the main jet axis. The supply for the microjets was provided from compressed nitrogen cylinderthrough a plenum chamber. The microjets were operated at a pressure of 100 psia and the combined mass flux fromall the microjets was less than 0.5% of the primary jet mass flux.B. Unsteady pressure and near-field noise measurements Unsteady pressure measurements on the lift plate were obtained using high frequency response, miniature (1.6mm dia.) KuliteTM pressure transducers of ±5 psid range mounted at two locations at x/d = 2 and 3 from the nozzlecenterline. Near field acoustic measurements were made using a 0.635 cm (1/4”) diameter B&K microphone placedat x/d = 15 from the nozzle centerline, 90° with respect to the jet axis. The pressure transducers and microphonewere carefully calibrated prior to each set of experiments. The pressure and acoustic signals were acquired throughhigh speed National Instruments digital data acquisition cards using LabviewTM and were processed offline. Thetransducer signals were conditioned using StanfordTM filters (Model No. SR650) and simultaneously sampled at 70kHz. Standard FFT analysis was used to obtain spectra and overall Sound Pressure Levels (OASPL) from thesemeasurements. A total of 100 FFT’s of 4096 samples each were averaged in order to obtain statistically reliableestimate of the narrow-band spectra.C. Phase-locked PIV Flow field measurements using phase locked Particle Image Velocimetry (PIV) were made for few chosen testconditions. For PIV measurements, a dual cavity digitally sequenced Nd:YAG laser (Spectra Physics PIV400) wasused. A light sheet of approximately 1 mm thickness was created by suitable combination of spherical andcylindrical lenses and was made to pass through the centerline of the jet. The PIV images were acquired at a rate of15 Hz using a CCD camera (Kodak ES1.0) with a resolution of 1008 (H) x 1018(V) pixels, where each pixel size is9 x 9 µm2. The camera was positioned at 90 deg to the jet axis. The pulse separation between the two laser pulseswas kept at 1–1.2 µs. In the present experiments, the jet was seeded with sub-micron (~0.3 µm) Fog fluid (a solutionof glycol and water) droplets generated by a modified Wright nebulizer, which supplied the particles to the main jet.The ambient air was seeded by a ROSCO 1600 fog generator. An image-matching approach for digital processing,similar to that used in previous experiments at AAPL (Krothapalli et al. [7] and Alvi et al. [19]), was used to extractparticle displacement and the velocity field. In this processing scheme, the interrogation window is defined by theparticle displacements , ranging from 3 to 4 pixels and the interrogation window used in the present study wasnominally set to 20 x 20 pixels. However, the adaptive scheme used in the processing ensured that a minimum of 10pairs of particle images were matched in each interrogation cell by resizing the interrogation window as required.The data was processed on a N x M points regularly spaced mesh. (PLEASE INSERT N AND M HERE) Theflowfield at every point is calculated using a least-square fitting algorithm based on a second-order polynomial. Thistechnique results in a second-order accuracy in calculating the flowfield at each point in the flow. The details of thistechnique are available in Lourenco and Krothapalli [21]. The near field microphone signal was used forthe purposes of phase-locking PIV measurements. Figure 4 shows a schematic of the phase-locking setup. The rawsignal from the microphone was first recorded unfiltered and processed online using LabVIEW® codes (same FFTparameters as in the previous section). Thereafter the signal was narrowly band-pass filtered around the impingingtone as measured from the spectra using a Stanford Research Systems SR650 High Pass/Low Pass filter. The signalwas then sent to a Model 88 Laser Lock made by Hendrick and Associates. This signal was divided such that itcorresponded to the frequency of the laser and the camera. The signal was then fed into custom timing hub matchedto a National Instruments® PCI-6602 timing card and an IMAQ 1422 image acquisition card. Phase-delays in thetrigger signal to the laser/camera were included using proVISION-XS software from IDT® which was also used toacquire and process the images. Finally, a delay on the divided signal was applied to generate the necessary phase 3 American Institute of Aeronautics and Astronautics
  4. 4. trigger for the synchronized camera and laser strobe. The laser frequency of 15 Hz requires the pulsation frequencyto be a multiple of 15. The full cycle of a pulse was sampled in 12 phases with equal intervals. At every phase, 300image pairs were obtained. Phase-averaged velocity fields were computed by taking the mean of these instantaneousfields. The measurement uncertainty is estimated to be about 1% and 10% in phased-averaged velocity and randomturbulence measurements, respectively, with a 95% confidence level. The global mean quantities were calculatedfrom phase-averaged values. Figure 4. Schematic of the Phase-Locking System III. Results and Discussion As stated in the introduction section, the main objective of this study is to understand the basic characteristics ofthe supersonic impinging jets with and without microjet control. Unsteady pressures, near-field noise and phaselocked PIV measurements were made with jet operating at NPR = 3.7 for two values of nozzle-to-plate distance h/d,4 and 4.5. Measurements were carried out both with and without microjet control operating at a fixed pressuresupply of 100 psia.A. Unsteady Pressures and Near-field Noise Figure 5 shows the near-field noise spectra measured using microphone located at 15d from the nozzle exit. The 4 Figure 5. Near-field noise spectra obtained using microphone located at 15d from centerline in the nozzle exit American Institute of Aeronautics and Astronautics plane. (a) h/d = 4, (b) h/d = 4.5
  5. 5. figure includes the data obtained at two values of h/d, 4 and 4.5 with and without control. As stated earlier, h/d = 4 isthe case where microjet control was found to be very effective in reducing the noise levels and h/d = 4.5 is the casecorresponding to minimal effect of control. As clearly seen in Fig. 5a, at h/d = 4; the spectra without control consistof a sharp impinging tone (5.8 kHz) along with its harmonics and high broadband levels. With microjet control, theamplitude of dominant impinging tone is significantly reduced (nearly 22 dB) along with a reduction in broadbandlevels. At h/d = 4.5, the baseline (without control) spectra shows multiple discrete impinging tones (2.8kHz, 4.1kHz,6.5kHz, 9.3kHz) and their harmonics and the broadband levels are even higher than those at h/d = 4. Also, it is to beobserved that at h/d = 4, the spectra shows a continuous decrease in SPL with frequency beyond 10kHz exceptcouple of low amplitude harmonics of impinging tone, whereas, at h/d = 4.5 the SPLs continue to be high even athigher frequencies (beyond 10 kHz). This behavior at high frequencies can be attributed to the broadbandamplification that normally occurs when strong tones are present in the spectra. With microjet control at h/d = 4.5,there is no reduction in the dominant impinging tone and its harmonics and a marginal reduction in broadbandlevels, however, most of the other tones present in the baseline spectra are eliminated. Figure 6. Narrowband pressure spectra at the lift plate; (a) h/d = 4, (b) h/d = 4.5 The narrowband pressure spectra for the two values of h/d, 4 and 4.5, measured at the lift plate is shown in Fig.6. The features of pressure spectra at both h/d values without control are very similar showing multiple highamplitude impinging tones and their harmonics. The frequencies of impinging tones observed in the lift plate spectraare identical to that of near-field microphone spectra indicating a global nature of the noise producing mechanisms.With microjet control at h/d = 4, the amplitude of all the tones is significantly reduced (a maximum of 32 dBreduction along with a major reduction in broadband levels, whereas at h/d = 4.5, the dominant impinging tone andits harmonics remain virtually unaffected. The kulite pressure sensor mounted in the lift plate is very sensitive andseems to pickup even minor vibrations in the plate. These results clearly demonstrate that the baseline (withoutcontrol) pressure spectra at the lift plate and the spectra of near-field noise at the two values of h/d show fewidentical and some different features. Also, the effect of microjet control in reducing pressure unsteadiness and near-field noise at two h/d is very different.B. Mean velocity field Global information regarding the evolution of the impinging jet flowfield and velocity field along a streamwisecentral plane was obtained using PIV. PIV measurements were made at both values of h/d, 4 and 4.5 with andwithout microjet control. The mean velocity field for the baseline flow corresponding to h/d = 4 and 4.5 is shown inFig. 7. The results are presented in the form of contour plots of the ensemble-averaged (mean) streamwise velocity(u) normalized with fully expanded jet velocity (Ujet). Velocity vectors at selected locations and the streamlines inthe ambient and in the shear layer of the jet are shown superposed on the mean velocity contour plots. As mentionedearlier, measurements were made at NPR =3.7, corresponding to ideally expanded jet conditions. However, a weakperiodic shock cell structure can be observed in both the cases. This may be due to weak shocks generated by thenozzle lip and a significant entrainment of ambient air resulting in a mildly under-expanded jet. The velocity vectors 5 American Institute of Aeronautics and Astronautics
  6. 6. a) h/d = 4 b) h/d = 4.5 Figure 7. Mean velocity distribution in the central plane of the impinging jet.close to the nozzle exit show top-hat velocity profile in both cases. The streamlines exhibit flow entrainment into theshear layer of the jet as it flows downward and formation of the wall jet at the impingement plate. In general, theglobal flow features of the impinging jet flowfield is very similar at h/d = 4 and 4.5 and doesn’t show anyconsiderable change between the two cases. With microjet control (not shown here), the velocity flowfield wasnearly the same suggesting that most of the mean flow properties of the jet remain unaltered with control. These a) h/d = 4 b) h/d = 4.5 Figure 8. Mean vorticity distribution in the central plane of the impinging jet.features are very similar to those observed in earlier studies. The impinging jet flowfield is known to be highly resonant and associated with instability waves in the shearlayer which grow into large-scale vortical structures, so it is important to analyze the vorticity field associated withimpinging jets. The ensemble-averaged vorticity contour plots measured along the jet central plane for h/d =4 and4.5 are shown in Fig. 8. The contours show the azimuthal component of the normalized vorticity, Zdj/Ujet, where djis the diameter of the nozzle at throat and Ujet is the fully expanded jet velocity at Mach 1.5. Compared to h/d = 4,the vorticity levels for h/d = 4.5 are higher and the size of the vortex structures seems to be bigger. Although theFigs.7 and 8 show the global flowfield of the impinging jet at two h/d, we need to further investigate to get betterunderstanding of the associated flow physics. 6 American Institute of Aeronautics and Astronautics
  7. 7. a) ϕ = 0º b) ϕ = 60º c) ϕ = 120º d) ϕ = 180º e) ϕ = 240º f) ϕ = 300ºFigure 9. Phase-averaged impinging jet flowfield at h/d = 4.5; Left hand side: azimuthal vorticity; Right hand side: streamwise velocity in the center plane. 7 American Institute of Aeronautics and Astronautics
  8. 8. C. Phase locked velocity field In order to better understand the flow physics associated with impinging jets with and without control at the twovalues of h/d, phase locked velocity measurements were carried out using PIV. The phase averaged flowfieldenables the construction of time evolution of different flow properties. The near field microphone signal was usedfor the purposes of phase-locking. As seen earlier in Fig. 4, at h/d=4 there is a single sharp tone at 5.8 kHz and weused this tone to phase-lock PIV at this h/d. A total of 300 image pairs were captured at every 30° resulting in 13datasets at 12 phases (0° and 360° overlap). At h/d = 4.5, there were four distinct tones in the spectra at 2.8, 4.1, 6.5and 9.3 kHz, the dominant of which being at 6.5 kHz. While phase-locked PIV was performed at 4.1, 6.5 and 9.5kHz, only the data collected at the strongest tone of 6.5kHz will be discussed in this paper. Figure 9 shows thecontour plots of the phase averaged streamwise velocityand azimuthal vorticity at h/d 4.5. The figure includesequally spaced (60º apart) six phase angles. Thestreamwise velocity and azimuthal vorticity contours areshown on right and left hand side of each plot. Thevorticity level flood lines and flow streamlines aresuperposed as well. These figures demonstrate some ofthe salient features of impinging jet flow field at h/d =4.5. The figures clearly show the presence of multiplewell defined vortical structures in the shear layer at eachphase. The instability initiates at the nozzle exit, growsinto a large scale vortex which convects downstreamand eventually after impingement on the ground movesalong the wall jet. As these vortices roll down and createa local suction, there is a significant entrainment of theambient air into the shear layer of the jet which leads tothe spreading of the jet. It is interesting to observe fromthe vorticity contours that the vorticity in the shear layer Figure 10. Phase-averaged impinging jet flowfieldis negative (clockwise rotation) whereas in the wall jet it at h/d = 4, ϕ = 0ºis positive (counterclockwise rotation) and there is aniso-surface close to the impingement surface where the vorticity is zero Similar to Fig. 9a for h/d = 4.5 and ϕ = 0º, the phase averaged contour plots of streamwise velocity and out-of-plane vorticity for h/d = 4 and ϕ = 0º is shown in Fig. 10. In comparison to h/d = 4.5, the contour plots for h/d = 4show relatively less coherent and weaker structures in the shear layer and the vorticity level in the shear layer isrelatively less for h/d = 4. To calculate the number, size, strength and other parameters associated with vortices, one has to preciselyidentify the location of these vortices in the flowfield. There are a number of techniques that have been used in theliterature to extract the vortices from the associated velocity and vorticity field. The simple one being using surfacesof constant vorticity magnitude (Figs. 9 and 10) but it does not differentiate between the shear and vortical motion.Other complex techniques involve the analysis of the local velocity gradient tensor and calculation of eigen values.To identify the vortical structures in the shear layer of the impinging jet, we have calculated the parameter “Swirling డ௨భ డ௨భStrength” as described by Adrian et al. (2000). The velocity gradient tensor in the central plane is given by డ௫ డ௫మCAN WE PUT A SYMBOL FOR VEL GRAD TENSOR HERE?? ቎డ௨భ డ௨మ ቏ మ డ௫భ డ௫మ 8 American Institute of Aeronautics and Astronautics
  9. 9. where the subscripts 1 and 2 correspond to streamwise and cross-stream directions respectively. Identification ofvortical structures is done by plotting regions where the imaginary portion of the complex eigen value is positive, b) h/d = 4 b) h/d = 4.5 Figure 11. Swirl strength distribution in the central plane of the impinging jet, ϕ = 0º.i.e. λci>0. The swirl-Strength is then the magnitude of this parameter. Figure 11 shows the contour plots of swirlstrength for h/d = 4 and 4.5 at ϕ = 0º. The swirl strength analysis clearly brings out the differences between the twoflowfields. Compared to h/d = 4, there are three pairs of coherent, well defined and symmetrical vortical structures ath/d = 4.5. The magnitude of the swirl strength for the vortices for h/d = 4.5 is much higher than at h/d = 4, indicatingthat the vortices are much stronger in the former case.As mentioned in the experimental setup section, the active control involved sixteen equally spaced 400 µm microjetsaround the periphery of the jet operating at a pressure of 100 psia. The effect of microjet control on the swirlstrength at h/d = 4 and 4.5 is shown in Fig. 12. At h/d = 4, the coherence of vortices is completely broken withcontrol and one observes a number of small scale structures instead of well defined equal pairs of large structures.However, at h/d = 4.5 with microjet control the strength of vortices is relatively reduced but the cohesiveness is stillmaintained. The shape of vortices near the nozzle exit is somewhat disturbed but as the flow moves downstream, 9 American Institute of Aeronautics and Astronautics a) h/d = 4 b) h/d = 4.5 Figure 12. Effect of control on swirl strength distribution, ϕ = 0º.
  10. 10. those roll up and become once again coherent. The outcome of the flowfield results with control agrees with those ofacoustic measurements. These results clearly suggest that there is a need to further increase the strength of microjetcontrol by increasing the supply pressure, so that microjets penetrate deep into the shear layer and break thecoherence of these large scale structures. IV. Conclusions The understanding of the flow physics associated with supersonic impinging jets is of great importance for thedesign of a robust control technique to suppress the high levels of noise associated with these jets. Previous studiesindicate that the effectiveness of microjet based control has a strong dependence on the nozzle-to-plate distance, sowe chose two values of this parameter of unequal effectiveness in this study. The experimental results described inthis paper include unsteady pressures, nearfield acoustics, and velocity and vorticity fields. Pressure and noisespectra showed the presence of multiple, high amplitude, discrete impinging tones along with high broadband levels.Time averaged velocity and vorticity measurement provided the whole field data and indicated different levels ofvorticity for the two flowfields but it did not provide enough information on the role of vortices. Phase-averagedvelocity and vorticity measurements brought out clear differences between the two flowfields, the difference in thestrength and size of vortices in the shear layer. The swirl strength levels further elucidated on the vortex statistics ofthe two cases and the effect of control in weakening those vortical structures. Acknowledgments We would like to thank the Florida Center for Advanced Aero-propulsion (FCAAP), a statewide center ofexcellence for supporting this research. References[1] Donaldson, C. DuP and Snedeker, R. S., “A study of free jet impingement. Part 1. Mean properties of free and impinging jets,” Journal of Fluid Mechanics, Vol. 45, No. 2, 1971, pp. 281–319.[2] Lamont, P. J. and Hunt, B. L., “The impingement of underexpanded axisymmetric jets on perpendicular and inclined flat plates,” Journal of Fluid Mechanics, Vol. 100, 1980, pp. 471–511.[3] Powell, A., “The sound-producing oscillations of round underexpanded jets impinging on normal plates”, Journal of Acoustic Society of America, Vol. 83, 1988, pp. 515–533.[4] Tam, C. K. W. and Ahuja, K. K., “Theoretical model of discrete tone generation by impinging jets”, Journal of Fluid Mechanics, Vol. 214, 1990, pp. 67–87.[5] Messersmith, N. L., “Aeroacustics of supersonic and impinging jets”, AIAA paper 95–0509, 1995.[6] Alvi, F. S. and Iyer, K. G., “Mean and unsteady flow field properties of supersonic impinging jets with lift plates,” AIAA Paper 99–1829, 1999.[7] Krothapalli, A., Rajkuperan, E., Alvi, F. S. and Lourenco, L., “Flow field and noise characteristics of a supersonic impinging jet,” Journal of Fluid Mechanics, Vol. 392, 1999, pp. 155–181.[8] Henderson, B., Bridges, J. and Wernet, M., “An experimental study of the oscillatory flow structure of tone producing supersonic impinging jets,” Journal of Fluid Mechanics, Vol. 542, 2005, pp. 115–137.[9] Powell, A., “On edge tones and associated phenomena,” Acoustica, Vol. 3, 1953, pp.233–243.[10] Karamcheti, K., Bauer, A. B., Shields, W. L. Stegen, G. R. and Woolley, J. P., “Some features of an edge tone flow field,” NASA SP 207, 1969, pp. 275–304. 10 American Institute of Aeronautics and Astronautics
  11. 11. [11] Kweon, Y. –H., Miyazato, Y., Aoki, T., Kim, H. –D. and Setoguchi, T., “Control of supersonic jet noise using a wire device,” Journal of Sound and Vibrations, Vol. 297, 2006, pp. 167–182.[12] Elavarasan, R., Krothapalli, A., Venkatakrishnan, L. and Lourenco, L., “Suppression of self-sustained oscillations in a supersonic impinging jet”, AIAA Journal, Vol. 39, No. 12, 2001, pp. 2366–2373.[13] Sheplak, M. and Spina, E. F., “Control of high speed impinging-jet resonance”, AIAA Journal, Vol. 32, No. 8, 1994, pp.1583–1588.[14] Shih, C., Alvi, F. S. and Washington, D., “Effects of counterflow on the aeroacustic properties of a supersonic jet,” Journal of Aircraft, Vol. 36, No. 2, 1999, pp. 451–457.[15] Alvi, F. S., Shih, C., Elavarasan, R., Garg, G. and Krothapalli, A., “Control of supersonic impinging jet flows using supersonic microjets,” AIAA Journal, Vol. 41, No. 7, 2003, pp. 1347–1355.[16] Lou, H., Shih, C. and Alvi, F. S., “A PIV study of supersonic impinging jet,” AIAA Paper 2003–3263, 2003.[17] Lou, H., Alvi, F. S. and Shih, C., “Active and adaptive control of supersonic impinging jets,” AIAA Journal, Vol. 44, No. 1, 2006, pp. 58–66.[18] Kumar, R., Lazic, S., and Alvi, F. S., “Active control of high temperature supersonic impinging jets,” AIAA Paper 2008–360, 2008.[19] Alvi, F. S., Lou, H., Shih, C., and Kumar, R., “Experimental study of physical mechanisms in the control of supersonic impinging jets using microjets,” Journal of Fluid Mechanics, Vol. 613, 2008, pp. 55–83.[20] Alkislar, M. B., Krothapalli, A., and Butler, G. W., “The effect of streamwise vortices on the aeroacoustics of a Mach 0.9 jet,” Journal of Fluid Mechanics, Vol. 578, 2007, pp. 139–169. 11 American Institute of Aeronautics and Astronautics