48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition                            ...
strong acoustic wave reflects and propagates through ambient surroundings. Once it reaches the nozzle lip, itdisrupts the ...
ground and the porous surface L was varied (L/d = 0.5 − 1.5) using spacers placed at the corners of the surface. Aphotogra...
with less reflections was attained under the porous surface. Prior to aligning the sheet-forming optics, both laserbeams w...
∼5.5kHz. There does appear to be a slight reduction in the magnitude of the impinging tone (and its harmonics) anda signif...
a) OASPL measured using nearfield microphone         b) Unsteady pressures on the lift plate at r/d=2              Figure ...
velocity downstream of the porous surface is significantly reduced. With microjet injection (Fig. 9c), there seems tobe a ...
in the unsteadiness levels can be correlated to growth of the shear layer with and without microjet control. As weknow fro...
microjet control are effective in reducing nearfield noise and flow unsteadiness over a range of geometricalparameters, ho...
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Noise and Flowfield Characteristics of Supersonic Jet Impinging on a Porous Surface

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Noise and Flowfield Characteristics of Supersonic Jet Impinging on a Porous Surface

  1. 1. 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition AIAA 2010-2734 - 7 January 2010, Orlando, Florida Noise and Flowfield Characteristics of a Supersonic Jet Impinging on a Porous Surface Alex Wiley 1, Rajan Kumar 2 and Farrukh Alvi 3 Florida Center for Advanced Aero-Propulsion FAMU-FSU College of Engineering, Tallahassee, FL, 32310 Isaac Choutapalli 4 University of Texas - Pan American, Edinburg, TX - 78539 Control of the highly resonant flowfield associated with supersonic impinging jet has been experimentally investigated. Measurements were made in the supersonic impinging jet facility at AAPL for a Mach 1.5 ideally expanded jet. Measurements included unsteady pressures on the lift plate, acoustic measurements in the nearfield and beneath the impingement plane and velocity measurements using PIV. Results show that both passive control using porous surface and active control with high momentum microjet injection are effective in reducing nearfield noise and flow unsteadiness over a range of geometrical parameters, however, the type of noise reduction achieved by the two control techniques is different. The passive control reduces broadband noise whereas microjet injection attenuates high amplitude impinging tones. The hybrid control, a combination of two control methods reduces both broadband and high amplitude impinging tones and surprisingly its effectiveness is more that the additive affect of the two control techniques. I. Introduction S UPERSONIC impinging jets have received considerable attention in the past because of their importance in a wide range of applications from the Short/Vertical Take off Landing (S/VTOL) aircraft1-5 to the cooling of electronics6, and more recently in fuel cell applications7. The flow field of supersonic impinging jets is known to be highly unsteady especially in an S/VTOL aircraft configuration. This can have adverse effects such as high noise levels, unsteady acoustic loads and sonic fatigue on the aircraft and surrounding structures, ground erosion, ingestion of hot gases into the engine nacelle and lift loss of the aircraft. On a carrier deck, the aircraft exhaust impinges on the deflector plate and produces high noise levels and make the deck environment highly noisy and cause a serious health concern to the personnel working on the deck. It is now well known that the highly unsteady behavior of the impinging jets is due to a feedback loop between the flow and acoustic fields; a schematic of which is shown in Fig. 1. Beginning at the nozzle lip, a small instability in the thin shear layer grows into a large scale vortical structure as it convects downstream towards the ground. Upon hitting the ground, a Figure 1. A schematic of feedback loop associated with impinging jets 1 Research Assistant, Department of Mechanical Engineering, AIAA Student Member 2 Research Scientist, Department of Mechanical Engineering, AIAA Member 3 Professor, Department of Mechanical Engineering, AIAA Associate Fellow 4 Assistant Professor, Department of Mechanical Engineering, AIAA Member 1 American Institute of Aeronautics and AstronauticsCopyright © 2010 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
  2. 2. strong acoustic wave reflects and propagates through ambient surroundings. Once it reaches the nozzle lip, itdisrupts the thin shear layer which begins a new cycle thereby completing the feedback loop. Although a substantial amount of research has been carried out in the past on supersonic impinging jets and itscontrol using various passive and active control methods8-15, the high noise levels and flow field unsteadiness is stilla cause of concern. Most of the control methods employed in the past either needed modifications to the aircraftnozzle or manipulation of shear layer near the nozzle exit and were therefore impractical or less effective. Forexample, Elavarsan et al.10 attenuated the feedback loop by placing a circular plate near the nozzle exit, achieving areduction in the near-field OASPL and reasonable lift recovery. Sheplak and Spina11, with the help of high speed co-flow, shielded the primary jet from the acoustic field. Shih et al.12 successfully suppressed screech tones of non-ideally expanded jets using counter-flow at the nozzle exit. A recent approach to suppress the feedback mechanismof supersonic impinging jets using an array of high momentum microjets appropriately placed near the nozzle exithas shown highly promising results13-15. This control-on-demand technique has many advantages over traditionalpassive and active control methods and has proven to be successful over a range of geometric and flow conditions. The focus of the present study is a technique of using a porous surface just above the ground plane to achievenoise reduction as well as reduction in the flow field unsteadiness. We have also investigated a hybrid control, acombination of active control, based on microjets, near the nozzle exit and a passive control such as a porous surfacenear the ground plane. The concept of porous impinging surface at low speed axisymmetric and planner jets wasstudied by Webb and Castro16 and Cant et al.17, respectively. Murray and Seiner18 performed acoustic measurementsof a sonic jet impinging on a ground, with and without the presence of a porous surface but very little is knownabout their results. In this paper, we present a detailed experimental investigation including the unsteady pressure,near-field acoustic and whole field velocity measurements using particle image velocimetry. II. Experimental Setup A. STOVL Facility The experiments were carried out at the STOVL supersonic jet facility of the Advanced Aero-PropulsionLaboratory (AAPL) located at the Florida State University. This facility is mainly used to study jet-inducedphenomenon on STOVL aircraft during hover. It is capable of running single and multiple jets at design or off-design conditions up to M = 2.2. In order to simulate different aircraft to ground plane distances, the ground plate ismounted on a hydraulic lift and can be moved up and down. A high pressure compressed air (~160 bars) is stored inlarge storage tanks (10 m3) and is used to drive thefacility. The measurements were made at ideally-expanded conditions for a Mach 1.5 jet issuing from aconverging-diverging axisymmetric nozzle. The designMach number of the nozzle was 1.5 and was operated atNozzle Pressure Ratio (NPR, where NPR = stagnationpressure/ambient pressure) of 3.7. The test Reynoldsnumber based on exit velocity and nozzle diameter of thejet was 7 x 105. The stagnation temperature of the jet wasvaried from 300K to 420K, corresponding to atemperature ratio, TR =1.0 and 1.4 (where, TR =stagnation temperature / ambient temperature)representing cold and hot conditions, respectively(though only results from TR=1.0 will be discussed inthis article). The throat and exit diameters (d, de) of thenozzle are 2.54 cm and 2.75 cm respectively. Thediverging section of the nozzle is a straight-walled with3° divergence angle from the throat to the nozzle exit. Acircular plate of diameter 25.4 cm (= 10d) was flushmounted with the nozzle exit. This plate, henceforthreferred as lift plate, represents a generic aircraftplanform and has a central hole, equal to the nozzle exitdiameter, through which the jet is issued. For the Figure 2. A Schematic of the experimental setuppurpose of this study a porous surface of porosity β = 0.29 (essentially a screen of uniform porosity) anddimensions 11d x 11d, was placed between the nozzle exit and the ground plane. The distance between the 2 American Institute of Aeronautics and Astronautics
  3. 3. ground and the porous surface L was varied (L/d = 0.5 − 1.5) using spacers placed at the corners of the surface. Aphotograph of the facility and test model used in present experiments is shown in Fig. 2. A total of sixteen microjetswere flush mounted circumferentially on the lift plate around the main jet to implement the active flow control. Thejets are issued using 400 µm diameter stainless steel tubes mounted at an inclination of 60° with respect to the mainjet axis. The supply for the microjets was provided from compressed nitrogen cylinder through a plenum chamber.The microjets were operated at a pressure of 100 psia and the combined mass flux from all the microjets was lessthan 0.5% of the primary jet mass flux. B. Measurements and Instrumentation 1) Acoustic and Unsteady Pressure Measurements Acoustic and unsteady pressure measurements were taken at four locations as shown in Fig. 3. Nearfieldacoustics were measuredusing a microphone (B& KType 4939 coupled withType 2670 preamplifierpowered using a NexusConditioning Amplifier Type2690) placed at r/d = 15(where r is the radialdistance from the jetcenterline) in the nozzle exitplane. Noise transmissionthrough the ground plane wasmeasured using a secondmicrophone placed at y/d = 5below the impingement plate.This microphone wasshielded from ambient andreflected noise using acoustic a) Microphone locations b) Pressure Transducer locationsinsulation on all nearby hard Figure 3. A photograph of experimental setup and measurement locationssurfaces. Additionally, two±5psid Kulites were flush-mounted with the lift plate atr/d=2 and r/d = 3 to measure the unsteady pressure loadsexperienced by an aircraft in hover-mode. Signals werefiltered at 30kHz using two Stanford Research SystemsSR640 Dual Low-Pass Filters before beingsimultaneously sampled at 70kHz. Processing was doneoffline using MATLAB® codes. 2) Particle Image velocimetry (PIV) Measurements Based on the unsteady pressure measurements, PIVmeasurements were also performed for select cases. Figure4 shows the experimental setup. A double-pulsed Nd.YAGlaser from Spectra-Physics with a maximum beam intensityof ~ 400 mJ/pulse was used for illumination of the flowfield. The beam is redirected from the laser via two 45omirrors to the height of the region of interest. From there itpasses through two convex spherical mirrors ofapproximately the same focal length to concentrate thebeam. The beam then passes through a convex negativecylindrical lens to spread the beam in one direction, therebycreating a laser “sheet” (see Fig. 4). Extra care was taken inseeding and laser sheet alignment in the cases where a Figure 4. Particle image velocimetry setupporous surface was present. Being that the region ofinterest was between the porous surface and the ground, a trade-off had to be made in which the better illumination 3 American Institute of Aeronautics and Astronautics
  4. 4. with less reflections was attained under the porous surface. Prior to aligning the sheet-forming optics, both laserbeams were aligned to ensure consistent illumination in the centerline plane of the jet. The main jet was seeded using a modified nebulizer while the ambient air was seeded using a Rosco® fogmachine to prevent biasing of the shear layer data on the high-speed side of the jet (both used Rosco fog fluid).Approximating the main jet at 450m/s, image pairs were recorded 1.25 µs apart to best match a particledisplacement of about 6 pixels which is optimal for the PIV algorithm used. The image pairs were captured at15Hz using a Kodak ES 1.0 Megaplus camera with a resolution of approximately 150pixels/inch. 1000 imagepairs were recorded for a given condition to resolve both the mean velocities and turbulence statistics of the flow.Afterwards, the image pairs acquired were processed using Provision II software from IDT. 3) Measurement Uncertainty The stagnation pressure (Po) and temperature (To) were measured in the settling chamber near the nozzle. Thepressure was measured using an Omegadyne PX219-200A5V pressure transducer with an accuracy of ± 0.5psia.Microjet total pressure (Pmj) was measured in a similar fashion using an Omegadyne PX303-200G5V transducerwith an experimental uncertainty of ± 0.5psig. To is monitored using a K-Type thermocouple, the signal of which isamplified using an Analog Devices AD595 monolithic thermocouple amplifier. The uncertainty of the measurementis ± 1oC. Both Kulites used to measure unsteady pressure loads were Model No. XCS-062-5D which have ameasurement uncertainty of ± 0.0125psid. The microphones used for acoustic measurements were both Bruel &Kjaer Type 4939, and the uncertainty of which is ± 1dB. Prior to the experiments, each pressure transducer wascalibrated using a Druck DPI 605 calibrator over the full-scale range of the transducer. Microphone correctionswere calibrated using a Bruel & Kjaer 4220 Pistonphone which generates 124dB at 250Hz. III. Experimental Results A. Unsteady Pressure Field As mentioned earlier, acoustic and unsteady pressure measurements were made at four different testconfigurations/conditions termed as baseline (jet impinging on a solid surface), with passive control (jet impingingon a porous surface), with microjet control (for baseline), and finally with hybrid control (both microjet and passivecontrol). The tests were performed over a range of nozzle to plate distances, at a fixed value of NPR = 3.7 and TR =1.0. In the following sections, we will first discuss the narrowband spectra obtained using FFT and then OASPL andPrms levels. 1. Narrowband Spectra Figure 5a shows the effect of passive control on the unsteady spectra as measured by the sideline microphone forh/d = 4. The results show that there is a strong impinging tone at ~7kHz along with its harmonics in the baseline a) Nearfield spectra using microphone b) Unsteady pressure spectra on lift plate Figure 5. Effect of passive control on the acoustic and unsteady pressure spectraflow at this h/d. With passive control (screen located at y/d=0.5), it appears that the impinging tone has shifted to 4 American Institute of Aeronautics and Astronautics
  5. 5. ∼5.5kHz. There does appear to be a slight reduction in the magnitude of the impinging tone (and its harmonics) anda significant reduction (nearly 5dB) is observed in broadband levels. Very similar results are seen (Fig. 5b) for kulitesensor located at r/d =2 on the lift plate except that the magnitude of impinging tones and associated broadbandlevels are higher. The observed frequency shift in impinging tone with porous surface is somewhat expected as thedistance between the nozzle and impinging surface in reduced with passive control and we know from our previousstudies that frequency of the impinging tone is strongly dependent on nozzle-to-plate distance. Next in Fig. 6a we see the effect of microjet control on the nearfield microphone spectra. With microjetinjection, there is a large reduction in the impinging tone and its harmonics and a small reduction in broadbandlevels. These results are very similar to those observed in earlier studies on impinging jet control using microjetsinjection. These results with microjet control when compared to the passive control bring out some interesting a) Effect of microjet control b) Effect of hybrid control Figure 6. Nearfield noise spectra measured using sideline microphonefeatures. While the overall reductions in noise for both control methods in terms of OASPL are about 4dB, the typeof noise reduction is quite different. The passive method using porous surface reduces mostly the broadband of thenoise spectra while a strong impinging tone and harmonics remain. Microjet injection, as has been observed before,mostly reduces the impinging tone and its harmonics. Being that the two methods primarily reduce differentcomponents of the noise spectra, a combination of the two was tested to see if their effects are additive whichwould lead to even larger reductions than either control method alone. The nearfield sound spectra of suchmeasurements termed as hybrid control are shown in Fig. 6b. The results clearly show that the impinging tones andtheir harmonics are completely eliminated and broadband levels are significantly reduced. This behavior isrepresentative and can be seen at all other cases of varying nozzle-to-ground distances, h/d and at all the fourmeasurement locations. 2. Overall Sound Pressure Levels (OASPL) Figure 7 shows a comparison of the OASPL levels as a function of nozzle-to-ground distance (h/d) for differentlocations of passive control (the spacing between the porous surface and the ground plane was varied from 0.5d-1.5d). Figure 7a shows the noise measured by the nearfield microphone while Fig. 7b shows the unsteadypressures measured at the lift-plate using kulite at r/d = 2. In general, the OASPL levels are reduced withpassive control but its effectiveness depend upon the spacing between porous surface to ground plane (y/d) andnozzle to ground plane distance (h/d). Figure 8 shows a comparison of effectiveness of different control methods inreducing nearfield noise and unsteady pressures. Both passive control using porous surface and active control withmicrojet injection show reasonable reductions in noise levels and unsteady pressures but their combined effort interms of hybrid control is even more effective. It is very interesting to observe that at most of the test conditions, theeffectiveness of hybrid control is more than the additive effect of passive and microjet control. 5 American Institute of Aeronautics and Astronautics
  6. 6. a) OASPL measured using nearfield microphone b) Unsteady pressures on the lift plate at r/d=2 Figure 7. Effect of passive control in reducing noise and unsteady pressures a) OASPL measured using nearfield microphone b) Unsteady pressures on the lift plate at r/d=2 Figure 8. Effect of various control methods in reducing noise and unsteady pressures B. Particle Image Velocimetry (PIV) PIV measurements were made at few select cases to map the velocity field associated with supersonic impingingjet and its control using both passive and active control methods. PIV measurements were obtained along astreamwise central plane at h/d = 5 for impinging jet at NPR = 3.7 and TR = 1.0 for baseline and control conditions.First we present color contour plots of the ensemble-averaged (mean) velocity (Umean) non-dimensionalized by UJ,the fully expanded jet velocity. Velocity vectors at selected locations are shown superimposed on the mean velocitycontour plots. Figures 9a – d show the velocity field for baseline and different control conditions. As mentionedearlier, measurements were conducted at NPR = 3.7, corresponding to ideally expanded jet conditions. The length ofthe vector represents the magnitude of velocity at each location. The velocity vectors at the nozzle exit show that forbaseline and different control cases, the jet exhibits a near top-hat velocity profile. The mean jet velocity at the exitplane is ≈ 430 m/s, corresponding to the fully expanded jet velocity at M = 1.5. With passive control (Fig. 9b) the jet 6 American Institute of Aeronautics and Astronautics
  7. 7. velocity downstream of the porous surface is significantly reduced. With microjet injection (Fig. 9c), there seems tobe a formation of stagnation bubble near the impingement plane at this value of h/d (=5). The appearance anddisappearance of stagnation bubble at some values of h/d with control has been noted in earlier studies as well. Thehybrid control (Fig. 9d) seems to make the jet more stable and has positive features of both passive and activecontrol. a) Baseline jet b) Passive control c) Microjet control d) Hybrid control Figure 9. Contour plots of ensemble average (mean) streamwise velocity in the central plane The streamwise rms velocity Urms, a measure of flow unsteadiness is non-dimensionalized by UJ. Figures 10a-dshows the Urms /UJ distribution for baseline and different types of flow control situations. For the baseline case (Fig.10a), we notice that near the nozzle exit Urms peaks in the shear layer and as the jet moves downstream, the level offlow unsteadiness increase, however downstream of potential core the highest levels of Urms exist near the centerlineof the jet. With passive control (Fig. 10b) for porous surface located at L/d = 1.5, we see that magnitude and extentof flow unsteadiness is in general the same up to porous surface location, however, in the region beneath theporous surface the Urms levels are much lower in magnitude and narrower in extent. Next with microjet control(Fig. 10c), we see a small increase in Urms near the nozzle exit and then relatively lower levels up to the potentialcore, beyond which the unsteadiness levels are high near the center of the jet. This initial increase and then decrease 7 American Institute of Aeronautics and Astronautics
  8. 8. in the unsteadiness levels can be correlated to growth of the shear layer with and without microjet control. As weknow from our previous studies that with microjet control, near the nozzle exit (y/d<1) the shear layer growth is a) Baseline jet b) Passive control c) Microjet control d) Hybrid control Figure 10. Contour plots of the streamwise rms velocity measured in the central planemore as compared to jet without control, whereas, at downstream locations (y/d>1), the jet with control spreadslesser than without control. The relatively higher levels of flow unsteadiness near the centerline above theimpingement plane are due to oscillations in the stagnation bubble. Finally, we look at Fig. 10d in which bothmicrojet and porous surface control are applied. This case when compared with the baseline flow, we see thatthere a significant reduction in Urms levels and extent across the entire flowfield. This significant reduction inUrms with hybrid control agrees with the acoustic and unsteady pressure measurements discussed earlier. Theseresults clearly demonstrate the effectiveness of hybrid control in reducing supersonic impinging jet noise and flowunsteadiness. IV. Conclusions Noise and flowfield characteristics of a supersonic impinging jet with a passive control using a porous surfaceand an active control with microjet injection are experimentally investigated. The results show that both passive and 8 American Institute of Aeronautics and Astronautics
  9. 9. microjet control are effective in reducing nearfield noise and flow unsteadiness over a range of geometricalparameters, however, the type of noise reduction achieved by the two control techniques is different. The passivecontrol reduces broadband noise whereas microjet injection attenuates high amplitude impinging tones. Acombination of two control methods - a hybrid control reduces both broadband and high amplitude impinging tonesand surprisingly its effectiveness is more that the additive affect of the two control techniques. A maximum of 17dBreduction is observed in unsteady pressures on the lift plate and an OASPL reduction of ~13dB is seen in thenearfield noise with hybrid control. PIV measurements show that with passive control the streamwise mean velocity is significantly reduceddownstream of the porous surface and there is a presence of stagnation bubble near the impingement surface withmicrojet injection. The flow unsteadiness is significantly reduced with hybrid control over the entire measurementregion which is consistent with the reductions observed in the nearfield acoustic and unsteady pressures. Theseresults clearly demonstrate the effectiveness of hybrid control in reducing the noise and flow unsteadiness associatedwith supersonic impinging jets and makes it an attractive flow and noise control technique for STOVL aircraft. References 1 Krothapalli, A., Rajkuperan, E., Alvi, F. S. and Lourenco, L., “Flow field and noise characteristics of a supersonic impingingjet,” Journal of Fluid Mechanics, Vol. 392, 1999, pp. 155–181. 2 Alvi, F. S. and Iyer, K. G., “Mean and unsteady flow field properties of supersonic impinging jets with lift plates,” AIAAPaper 99–1829, 1999. 3 Ho, C.-M. and Nossier, N. S., “Dynamics of an impinging jet. Part 1. The feedback phenomenon,” Journal of FluidMechanics, Vol. 105, 1981. 4 Messersmith, N. L., “Aeroacoustics of supersonic and impinging jets”, AIAA paper 95–0509, 1995. 5 Tam, C. K. W. and Ahuja, K. K., “Theoretical model of discrete tone generation by impinging jets”, Journal of FluidMechanics, Vol. 214, 1990, pp. 67–87 6 Ro, I. P., Loh, G. B., “Feasibility of using ultrasonic flexural waves as a cooling mechanism,” IEEE Transactions onIndustrial electronics, Vol. 48, No. 1, 2001 7 Greska, B., DeRoche, P., and Krothapalli, A., “A Novel Delivery Reactant System for PEM Fuel Cells,” Proceeding fromFuelCell2008, 2008. 8 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. 9 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. 10 Elavarasan, R., Krothapalli, A., Venkatakrishnan, L. and Lourenco, L., “Suppression of self-sustained oscillations in asupersonic impinging jet”, AIAA Journal, Vol. 39, No. 12, 2001, pp. 2366–2373. 11 Sheplak, M. and Spina, E. F., “Control of high speed impinging-jet resonance”, AIAA Journal, Vol. 32, No. 8, 1994,pp.1583–1588. 12 Shih, C., Alvi, F. S. and Washington, D., “Effects of counterflow on the Aeroacoustics properties of a supersonic jet,”Journal of Aircraft, Vol. 36, No. 2, 1999, pp. 451–457. 13 Alvi, F. S., Lou, H., Shih, C., and Kumar, R., “Experimental study of physical mechanisms in the control of supersonicimpinging jets using microjets,” Journal of Fluid Mechanics, Vol. 613, 2008, pp. 55–83. 14 Kumar, R., Lazic, S., and Alvi, F. S., “Control of high-temperature supersonic impinging jets using microjets,” AIAAJournal, Vol. 47, No. 12, December 2009, pp. 2800-2811. 15 Alvi, F. S., Shih, C., Elavarasan, R., Garg, G., and Krothapalli, A., “Control of Supersonic Impinging Jets UsingSupersonic Microjets,” AIAA Journal, Vol. 41, No. 7, 2003. 16 Webb, S. and Castro, I. P., “Axisymmetric jets impinging on porous walls”, Experiments in Fluids, Vol. 40, 2006, pp.951–961. 17 Cant, R., Casto, I., and Walklate, P., “Plane jets impinging on porous walls”, Experiments in Fluids , Vol. 32, 2002, pp.15–26. 18 Murray, N. and Seiner, J., “Acoustics of Jet Impingement on a Porous Plane,” American Physical Society, 60th AnnualMeeting of the Division of Fluid Dynamics , 2007. 9 American Institute of Aeronautics and Astronautics

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