noise and flowfield characteristics of supersonic jet impinging on a porous surface

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American Institute of Aeronautics and Astronautics

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

Alex Wiley1, Rajan Kumar2 and Farrukh Alvi3

Florida Center for Advanced Aero-Propulsion FAMU-FSU College of Engineering, Tallahassee, FL, 32310

Isaac Choutapalli4

I. Introduction

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.

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

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

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Figure 1. A schematic of feedback loop associated with impinging jets

48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition4 - 7 January 2010, Orlando, Florida

AIAA 2010-273

Copyright © 2010 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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strong acoustic wave reflects and propagates through ambient surroundings. Once it reaches the nozzle lip, it disrupts 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 its control using various passive and active control methods8-15, the high noise levels and flow field unsteadiness is still a cause of concern. Most of the control methods employed in the past either needed modifications to the aircraft nozzle or manipulation of shear layer near the nozzle exit and were therefore impractical or less effective. For example, Elavarsan et al.10 attenuated the feedback loop by placing a circular plate near the nozzle exit, achieving a reduction 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 mechanism of supersonic impinging jets using an array of high momentum microjets appropriately placed near the nozzle exit has shown highly promising results13-15. This control-on-demand technique has many advantages over traditional passive 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 achieve noise reduction as well as reduction in the flow field unsteadiness. We have also investigated a hybrid control, a combination of active control, based on microjets, near the nozzle exit and a passive control such as a porous surface near the ground plane. The concept of porous impinging surface at low speed axisymmetric and planner jets was studied by Webb and Castro16 and Cant et al.17, respectively. Murray and Seiner18 performed acoustic measurements of a sonic jet impinging on a ground, with and without the presence of a porous surface but very little is known about 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-Propulsion

Laboratory (AAPL) located at the Florida State University. This facility is mainly used to study jet-induced phenomenon 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 is mounted on a hydraulic lift and can be moved up and down. A high pressure compressed air (~160 bars) is stored in large storage tanks (10 m3) and is used to drive the facility. The measurements were made at ideally-expanded conditions for a Mach 1.5 jet issuing from a converging-diverging axisymmetric nozzle. The design Mach number of the nozzle was 1.5 and was operated at Nozzle Pressure Ratio (NPR, where NPR = stagnation pressure/ambient pressure) of 3.7. The test Reynolds number based on exit velocity and nozzle diameter of the jet was 7 x 105. The stagnation temperature of the jet was varied from 300K to 420K, corresponding to a temperature 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 in this article). The throat and exit diameters (d, de) of the nozzle are 2.54 cm and 2.75 cm respectively. The diverging section of the nozzle is a straight-walled with 3° divergence angle from the throat to the nozzle exit. A circular plate of diameter 25.4 cm (= 10d) was flush mounted with the nozzle exit. This plate, henceforth referred as lift plate, represents a generic aircraft planform and has a central hole, equal to the nozzle exit diameter, through which the jet is issued. For the purpose of this study a porous surface of porosity β = 0.29 (essentially a screen of uniform porosity) and dimensions 11d x 11d, was placed between the nozzle exit and the ground plane. The distance between the

Figure 2. A Schematic of the experimental setup

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ground and the porous surface L was varied (L/d = 0.5 − 1.5) using spacers placed at the corners of the surface. A photograph of the facility and test model used in present experiments is shown in Fig. 2. A total of sixteen microjets were flush mounted circumferentially on the lift plate around the main jet to implement the active 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 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 less than 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. Nearfield

acoustics were measured using a microphone (B& K Type 4939 coupled with Type 2670 preamplifier powered using a Nexus Conditioning Amplifier Type 2690) placed at r/d = 15 (where r is the radial distance from the jet centerline) in the nozzle exit plane. Noise transmission through the ground plane was measured using a second microphone placed at y/d = 5 below the impingement plate. This microphone was shielded from ambient and reflected noise using acoustic insulation on all nearby hard surfaces. Additionally, two ±5psid Kulites were flush-mounted with the lift plate at r/d=2 and r/d = 3 to measure the unsteady pressure loads experienced by an aircraft in hover-mode. Signals were filtered at 30kHz using two Stanford Research Systems SR640 Dual Low-Pass Filters before being simultaneously sampled at 70kHz. Processing was done offline using MATLAB® codes.

2) Particle Image velocimetry (PIV) Measurements Based on the unsteady pressure measurements, PIV

measurements were also performed for select cases. Figure 4 shows the experimental setup. A double-pulsed Nd.YAG laser from Spectra-Physics with a maximum beam intensity of ~ 400 mJ/pulse was used for illumination of the flow field. The beam is redirected from the laser via two 45o mirrors to the height of the region of interest. From there it passes through two convex spherical mirrors of approximately the same focal length to concentrate the beam. The beam then passes through a convex negative cylindrical lens to spread the beam in one direction, thereby creating a laser “sheet” (see Fig. 4). Extra care was taken in seeding and laser sheet alignment in the cases where a porous surface was present. Being that the region of interest was between the porous surface and the ground, a trade-off had to be made in which the better illumination

a) Microphone locations b) Pressure Transducer locations Figure 3. A photograph of experimental setup and measurement locations

Figure 4. Particle image velocimetry setup

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with less reflections was attained under the porous surface. Prior to aligning the sheet-forming optics, both laser beams 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® fog machine 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 particle displacement of about 6 pixels which is optimal for the PIV algorithm used. The image pairs were captured at 15Hz using a Kodak ES 1.0 Megaplus camera with a resolution of approximately 150pixels/inch. 1000 image pairs 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. The

pressure 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 transducer with an experimental uncertainty of ± 0.5psig. To is monitored using a K-Type thermocouple, the signal of which is amplified using an Analog Devices AD595 monolithic thermocouple amplifier. The uncertainty of the measurement is ± 1oC. Both Kulites used to measure unsteady pressure loads were Model No. XCS-062-5D which have a measurement 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 was calibrated using a Druck DPI 605 calibrator over the full-scale range of the transducer. Microphone corrections were 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 test configurations/conditions termed as baseline (jet impinging on a solid surface), with passive control (jet impinging on a porous surface), with microjet control (for baseline), and finally with hybrid control (both microjet and passive control). 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 and Prms levels.

1. Narrowband Spectra Figure 5a shows the effect of passive control on the unsteady spectra as measured by the sideline microphone for

h/d = 4. The results show that there is a strong impinging tone at ~7kHz along with its harmonics in the baseline

flow at this h/d. With passive control (screen located at y/d=0.5), it appears that the impinging tone has shifted to

a) Nearfield spectra using microphone b) Unsteady pressure spectra on lift plate

Figure 5. Effect of passive control on the acoustic and unsteady pressure spectra

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∼5.5kHz. There does appear to be a slight reduction in the magnitude of the impinging tone (and its harmonics) and a significant reduction (nearly 5dB) is observed in broadband levels. Very similar results are seen (Fig. 5b) for kulite sensor located at r/d =2 on the lift plate except that the magnitude of impinging tones and associated broadband levels are higher. The observed frequency shift in impinging tone with porous surface is somewhat expected as the distance between the nozzle and impinging surface in reduced with passive control and we know from our previous studies 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 microjet injection, there is a large reduction in the impinging tone and its harmonics and a small reduction in broadband levels. These results are very similar to those observed in earlier studies on impinging jet control using microjets injection. These results with microjet control when compared to the passive control bring out some interesting

features. While the overall reductions in noise for both control methods in terms of OASPL are about 4dB, the type of noise reduction is quite different. The passive method using porous surface reduces mostly the broadband of the noise 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 different components of the noise spectra, a combination of the two was tested to see if their effects are additive which would lead to even larger reductions than either control method alone. The nearfield sound spectra of such measurements termed as hybrid control are shown in Fig. 6b. The results clearly show that the impinging tones and their harmonics are completely eliminated and broadband levels are significantly reduced. This behavior is representative and can be seen at all other cases of varying nozzle-to-ground distances, h/d and at all the four measurement 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 different

locations 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 unsteady pressures measured at the lift-plate using kulite at r/d = 2. In general, the OASPL levels are reduced with passive control but its effectiveness depend upon the spacing between porous surface to ground plane (y/d) and nozzle to ground plane distance (h/d). Figure 8 shows a comparison of effectiveness of different control methods in reducing nearfield noise and unsteady pressures. Both passive control using porous surface and active control with microjet injection show reasonable reductions in noise levels and unsteady pressures but their combined effort in terms of hybrid control is even more effective. It is very interesting to observe that at most of the test conditions, the effectiveness of hybrid control is more than the additive effect of passive and microjet control.

a) Effect of microjet control b) Effect of hybrid control

Figure 6. Nearfield noise spectra measured using sideline microphone

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B. Particle Image Velocimetry (PIV) PIV measurements were made at few select cases to map the velocity field associated with supersonic impinging

jet and its control using both passive and active control methods. PIV measurements were obtained along a streamwise 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 velocity contour plots. Figures 9a – d show the velocity field for baseline and different control conditions. As mentioned earlier, measurements were conducted at NPR = 3.7, corresponding to ideally expanded jet conditions. The length of the vector represents the magnitude of velocity at each location. The velocity vectors at the nozzle exit show that for baseline and different control cases, the jet exhibits a near top-hat velocity profile. The mean jet velocity at the exit plane is ≈ 430 m/s, corresponding to the fully expanded jet velocity at M = 1.5. With passive control (Fig. 9b) the jet

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

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velocity downstream of the porous surface is significantly reduced. With microjet injection (Fig. 9c), there seems to be a formation of stagnation bubble near the impingement plane at this value of h/d (=5). The appearance and disappearance of stagnation bubble at some values of h/d with control has been noted in earlier studies as well. The hybrid control (Fig. 9d) seems to make the jet more stable and has positive features of both passive and active control.

The streamwise rms velocity Urms, a measure of flow unsteadiness is non-dimensionalized by UJ. Figures 10a-d shows 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 of flow unsteadiness increase, however downstream of potential core the highest levels of Urms exist near the centerline of the jet. With passive control (Fig. 10b) for porous surface located at L/d = 1.5, we see that magnitude and extent of flow unsteadiness is in general the same up to porous surface location, however, in the region beneath the porous 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 potential core, beyond which the unsteadiness levels are high near the center of the jet. This initial increase and then decrease

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

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in the unsteadiness levels can be correlated to growth of the shear layer with and without microjet control. As we know from our previous studies that with microjet control, near the nozzle exit (y/d<1) the shear layer growth is

more as compared to jet without control, whereas, at downstream locations (y/d>1), the jet with control spreads lesser than without control. The relatively higher levels of flow unsteadiness near the centerline above the impingement plane are due to oscillations in the stagnation bubble. Finally, we look at Fig. 10d in which both microjet and porous surface control are applied. This case when compared with the baseline flow, we see that there a significant reduction in Urms levels and extent across the entire flowfield. This significant reduction in Urms with hybrid control agrees with the acoustic and unsteady pressure measurements discussed earlier. These results clearly demonstrate the effectiveness of hybrid control in reducing supersonic impinging jet noise and flow unsteadiness.

IV. Conclusions Noise and flowfield characteristics of a supersonic impinging jet with a passive control using a porous surface

and an active control with microjet injection are experimentally investigated. The results show that both passive and

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 plane

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microjet control 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. A combination of two control methods - a hybrid control reduces both broadband and high amplitude impinging tones and surprisingly its effectiveness is more that the additive affect of the two control techniques. A maximum of 17dB reduction is observed in unsteady pressures on the lift plate and an OASPL reduction of ~13dB is seen in the nearfield noise with hybrid control.

PIV measurements show that with passive control the streamwise mean velocity is significantly reduced downstream of the porous surface and there is a presence of stagnation bubble near the impingement surface with microjet injection. The flow unsteadiness is significantly reduced with hybrid control over the entire measurement region which is consistent with the reductions observed in the nearfield acoustic and unsteady pressures. These results clearly demonstrate the effectiveness of hybrid control in reducing the noise and flow unsteadiness associated with supersonic impinging jets and makes it an attractive flow and noise control technique for STOVL aircraft.

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noise and flowfield characteristics of supersonic jet impinging on a porous surface

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