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ACTIVE CONTROL OF HIGH TEMPERATURE SUPERSONIC IMPINGING JETS
Rajan Kumar1, Sladana Lazic2 and Farrukh S. Alvi3
Florida A & M University and Florida State University, Tallahassee, FL 32310 The Supersonic impinging jets such as those occurring in short takeoff and vertical landing (STOVL)
aircraft, generate a highly oscillatory flow with high unsteady loads on the nearby structures and the landing surfaces. These high-pressure and acoustic loads are also accompanied by a dramatic loss in lift during hover, severe ground erosion of the landing surface and hot gas ingestion into the engine inlets. In the past we have examined impinging jets at cold conditions; the present study is a step toward examining this flowfield and assesses the effectiveness of microjet control at increasingly realistic conditions. An ideally expanded, Mach 1.5 primary jet was heated up to a total temperature of ~500K and issued from an axisymmetric nozzle. Temperature and pressure measurements were made on lift plate, representative of the undersurface of an aircraft and on the ground plane, over a range of nozzle-to-plate distances (representing aircraft hover conditions). In addition to temperature and pressure, near-field noise was measured using a microphone. Velocity field of impinging jets for both cold and hot conditions was mapped using particle image velocimetry. The results show that the temperature recovery factor is strongly dependent on the temperature ratio and nozzle to plate distance. The hover lift loss at high temperatures is significantly high, as large as 76% of the primary jet thrust at small nozzle to plate distances. The pressure fluctuations generated by hot impinging jets are substantially higher than cold jets and persist over a larger extent of nozzle to plate distances. The activation of microjet control shows a substantial reduction in pressure fluctuations both in terms of overall sound pressure levels and the attenuation of discrete, impinging tones. PIV results explain the increase in lift loss due to high entrainment velocities for hot impinging jets.
I. Introduction any examples of flow impingement of a jet on a solid surface can be found in engineering applications. To name a few: the launch of a rocket, takeoff and landing of a STOVL aircraft and the trust vector control of a solid rocket motor or an aircraft exhaust. For an efficient design of such systems, it is important to understand
the flow field associated with impinging jets. In particular, STOVL aircraft during hover produce high temperature impinging jets on the landing surface. These lift producing jets results in the high temperature, turbulent and highly oscillatory flow. This leads to severe ground erosion of the landing surface, lift loss due to entrainment of high speed flow near the nozzle exit, very high unsteady loads on the nearby structures and hot gas ingestion into the engine inlets. High levels of overall sound pressure levels (OASPL) associated with high temperature supersonic impinging jets are a cause of concern due to sonic fatigue failure of the aircraft structure and a major source of noise pollution for the personnel in the aircraft vicinity.
Flow field properties of a supersonic impinging jet has been investigated by many researchers in the past, including, Donaldson and Snedeker1, Lamont and Hunt2, Powell3, Tam and Ahuja4, Messersmith5, Alvi and Iyer6, Krothapalli et al.7 and more recently by Henderson et al.8. These studies clearly demonstrated the unsteady behavior of impinging jets and the presence of high amplitude discrete impinging tones. Krothapalli et al.7 demonstrated that generation of large scale structures in the jet shear layer induce high entrainment velocity near the nozzle exit, and in turn significant lift loss during hover. It is now well known that the highly unsteady behavior of the impinging jets is due to a feedback loop between the fluid and acoustic fields, which leads to these adverse effects. There have been many attempts to suppress the feedback loop using passive as well as active control methods, for example, Karamcheti et al.9 successfully suppressed edge tones by placing two plates normal to the jet centerline. Elavarsan et
1 Research Associate, Department of Mechanical Engineering 2 Research Associate, Department of Mechanical Engineering 3 Professor, Department of Mechanical Engineering, Senior Member AIAA
M
46th AIAA Aerospace Sciences Meeting and Exhibit7 - 10 January 2008, Reno, Nevada
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al.10 attenuated the feedback loop by placing a circular plate near the nozzle exit and have shown a reduction in the near-field OASPL and a reasonable lift loss 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. All the above techniques have shown reasonable reductions in the noise levels but only over a limited range of geometric and flow parameters, and require major modifications in the aircraft design.
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 results13-16. This control-on-demand technique has many advantages over traditional passive and active control methods and has been proved to be successful over a range of geometric and flow conditions. With the help of PIV measurements, Lou et al.14 have shown that the mechanism at work is the introduction of streamwise vorticity at the expense of azimuthal vorticity of the main jet. A reduction in primary shear layer instability, attenuation of upstream propagating acoustic waves and disruption of spatial coherence between large scale structures and acoustic field lead to an overall attenuation in the feedback loop. More recently, Alkislar et al.17 have demonstrated the role of the streamwise vortices on the aeroacustics of a subsonic (M = 0.9) axisymmetric jet by employing both the microjets and the chevrons near the nozzle exit. Their results reveal that the counter-rotating streamwise vortex pairs generated by microjets reside at the high speed side of the shear layer. These experiments have conclusively demonstrated the effect of microjet control in reducing the undesirable effects of impinging jets. However, these prior experiments have only been conducted for cold jets. From the application point of view, it is very important to study the effect of temperature on the impinging jet flowfield and demonstrate effectiveness of microjet control at high temperatures.
The primary objective of the present study is to characterize the properties of high temperature impinging jets and to examine the effectiveness of high momentum microjet control. An ideally expanded supersonic impinging Jet (M=1.5) was heated up to a total temperature of 480K and was issued from a converging-diverging nozzle. Temperature and pressure measurements were made on the lift plate and ground plane over a range of geometric and flow parameters. Near-field noise measurements were made using a microphone and to study the broad flow features of impinging jet and its control, flow visualization using shadowgraph technique was carried out at few selected test conditions. The velocity field for both free and impinging jets was generated using particle image velocimetry (PIV) at few selected test conditions. This paper provides the description of experimental setup, details of measurements made, typical results and discussion followed by few concluding remarks.
II. Experiments
A. Test Facilities and Models 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. More details of the facility can be found in Krothapalli et al.7.
A schematic of the test model and measurement apparatus used in present experiments is shown in Fig. 1. The measurements were made on an ideally expanded jet issuing from a converging-diverging axisymmetric nozzle. 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. The design Mach number of the nozzle was 1.5 and was operated at a Nozzle Pressure ratio (NPR, where NPR = stagnation pressure/ambient pressure) of 3.70, corresponding to ideally expanded jet. 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. An aluminum plate with dimensions 1 m x 1 m x 25 mm represents the ground plane and is mounted on the hydraulic lift directly under the nozzle exit (Fig.1).
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B. Measurements and Instrumentation
1. Temperature, pressure and near field noise measurements The temperature distribution on the ground plane and lift plate are measured using K-type thermocouples. The
ground plate and lift plate are instrumented with eight and six thermocouples respectively, the precise locations are
given in Fig. 1. The flow induced lift loss was estimated by measuring the static pressure distribution on the lift plate with 19 pressure ports along a radial line. The static pressures were measured by scanning each port with a ScanivalveTM connected to a ±1 psid range ValidyneTM strain gauge transducer. Unsteady pressure measurements on the ground plane and lift plate were made using high frequency response, miniature (1.6 mm dia.) KuliteTM pressure transducers of 100 psia and ±5 psid range, respectively. Based on the information obtained from temperature distribution, for temperature ratio TR of 1.0 and 1.2 (where, TR=Stagnation temperature/ambient temperature in K), the transducer on the ground plane was mounted at x/d = 0 (jet centerline) and x/d = 2, and for TR = 1.4, it was placed at 5.08 cm (x/d = 2) away from the centerline. Unsteady pressure field on the lift plate was measured using two transducers mounted at x/d = 2 and 3, from the nozzle centerline.
In addition to temperature and pressure measurements, near field acoustic measurements were made using a 0.635 cm diameter B&K microphone placed at x/d = 15 away from the nozzle centerline, in a direction 90° with respect to the jet axis (Fig. 1). All sensors: thermocouples, pressure transducers and microphone were carefully calibrated prior to the data acquisition. In order to minimize the sound reflections during the unsteady and acoustic measurements, parts of rig and near-by metal surfaces were covered with thick acoustic foam. The temperature, pressure and acoustic signals were acquired through high speed National Instruments digital data acquisition cards using LabviewTM and were processed offline using MatlabTM software. The transducer outputs were conditioned using StanfordTM filters and simultaneously sampled at 70 kHz. Standard FFT analysis was used to obtain spectra and overall Sound Pressure Levels (OASPL) from these measurements. A total of 100 FFT’s of 4096 samples each
x
x = -1.5d, -0.9d, -0.6d, 0, 0.3d, 0.6d, 1.2d, 1.8d
(c)
Thermocouples
de
Microjet array
Static Pressure Ports
Unsteady Pressure Transducers
(b)
x/d=2
x/d=3
d
de 10d
15d
h x
θ
Nozzle
Microphone Lift Plate
Ground Plane
d = 2.54 cm de = 2.75 cm
(a)
y
Figure 1. Schematic of the experimental setup. (a) Overall arrangement. (b) Lift plate showing locations of microjets, static pressure ports and unsteady pressure transducers. (c) Ground plane showing locations of thermocouples.
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were averaged in order to obtain statistically reliable narrow-band spectra.
2. Particle image velocimetry (PIV) The velocity field data at the central plane was obtained using particle image velocimetry. The details of the PIV
technique and associated hardware used in the present experiments are similar to Krothapalli et al.7 and Lou et al.14. Briefly, a double-pulsed Nd.YAG laser from Spectra-Physics with beam intensity of 400 mJ was used for the illumination of flow field. A light sheet of about 1mm thickness was created using a combination of optics (spherical and cylindrical lenses). A schematic of the experimental and optical arrangement of the planer PIV are shown in Fig. 2. The primary cold jet was seeded using very small (≈ 0.3 μm) Rosco fogTM fluid droplets generated using a modified Wright Nebulizer. At high temperatures (>350°F), these fog fluid droplets turned into smoke and therefore added the challenge of seeding the flow. This was overcome by using extra virgin Olive oil (smoke point ≈ 410°F). The ambient air was seeded with smoke particles (1-5 μm) produced by a Rosco 1600 fog generator. The PIV images were acquired at a rate of 15 Hz using a CCD camera (Kodak ES 1.0) with a resolution of 1008(H) x 1018(V) pixels where each pixel size is 9 x 9 μm2. The camera was positioned at 90° to the jet axis. The time between the two laser pulses was kept between 1-1.2 μs. An image matching approach for digital processing and a novel velocity data processing algorithm, similar to that used in previous experiments at AAPL (Krothapalli et al.7
and Lou et al.14), was used for data processing. The details of this technique are available in Lourenco et al.18.
3. Flow visualization Flow visualizations were obtained using a conventional mirror based, single pass shadowgraph method.
White-light Xenon pulsed flash lamp with a 5-10 µs pulse duration was used as a light source. Two 31.75 cm diameter 1st surface spherical mirrors with 254 cm focal length were mounted on the either side of the jet. A Kodak Megaplus ES1.0 digital camera, similar to the one used for PIV, was used for image acquisition.
C. Measurement Uncertainties The measurement uncertainty in an experiment depends on the individual accuracy of various measuring
instruments used. The uncertainty associated with the temperature and the mean static pressure measurements are ±1°C and 0.002 psi respectively. The rms values of unsteady lift plate and ground plane pressures are accurate within ±0.02 psi and ±0.2 psi respectively. The microphone OASPL was measured with an accuracy of ±0.5 dB. Mean and turbulent velocity fields were computed by processing a large number of image pairs (1000) resulting in repeatability of better than 1% in the mean velocity and 5% in the turbulent intensity measurements.
D. Test Conditions The experiments were conducted at a nozzle pressure ratio (NPR) of 3.7, which corresponds to a nearly ideally
expanded jet flow. The jet stagnation temperature was varied from 300K to 480K corresponding to temperature ratio TR of 1.0 to 1.6, and was controlled within ±2°C. The test Reynolds number based on exit velocity and nozzle diameter of the jet was 7 x 105. The nozzle to ground plane distance h was varied from 2d to 12d.
A total of sixteen microjets were flush mounted circumferentially on the lift plate around the main jet to implement the active flow control. The schematic of mounting arrangement is shown in Fig. 1. 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.
Nd:YAGlaser
Nozzles
Lift plate
Ground plate Hydraulic
lift
ImageAcquisition
CCD Camera
Light sheet
Figure 2. Schematic of the experimental and optical arrangement of the planer PIV
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III. Results and Discussion The focus of the present study is to characterize the properties of hot impinging jets and to examine the
effectiveness of high momentum microjet based control for hot jets. Results of temperature distributions on the ground plane are followed by static pressure measurements to estimate lift loss during hover. The flow field associated with hot impinging jets is mainly characterized with unsteady pressure data, near-field acoustic measurements and velocity field obtained using PIV. Limited flow visualization results at selected test conditions will be presented to study overall/global flow features associated with the impinging jet flow.
A. Temperature Distributions on the Impingement Plane The measured temperatures are expressed in a dimensionless form well known as the recovery factor, r. It is
defined as
( )
dTTTr 01 −
+= (1)
Where T is the wall temperature, T0 is the stagnation temperature of the jet and Td is the dynamic temperature of the jet. The value of Td in eq. 1 is estimated from the following relation (Goldstein et al.19).
( )[ ]( )[ ] 02
22
21121
2T
MM
CU
TP
jd −+
−==
γγ
(2)
Figure 3 shows the stagnation recovery factor, r0, as a function of h/d at various temperature ratios with and without microjet control. The recovery factor at the stagnation point for subsonic impinging jet from Goldstein et al.19 has also been shown here for comparison. The cited data corresponds to M = 0.47 and Re = 1.24 x 105. In the figure, each set of profile, correspond to a different TR, and is offset by a value of 0.1 from the previous profile and a reference line corresponding to r0 = 1.0 for each profile is shown for clarity. Open and filled symbols are used to represent data corresponding to baseline and with control measurements, respectively.
At a temperature ratio TR = 1.0, the stagnation recovery factor is close to unity at small values of h/d (2 to 2.5) and increases thereafter. This increase in recovery factor at large h/d is due to increased mixing of warmer air, as the jet static temperature is initially below the ambient temperature. As the jet flows downstream, increasingly larger amounts of warmer ambient air is entrained and the static temperature of the jet increases with increase in h/d and there is a net heat addition into the jet. Similar observations were made by Goldstein et al.19 in their study on impinging circular jets. With the microjet control, the stagnation recovery factor remains close to unity over a large extent of h/d (up to 5) and is lower as compared to no control case. Lou et al.14 with the help of PIV measurements have shown that microjet based control significantly reduces the global mixing of the primary jet flow. Therefore, it is expected that with microjet control there is reduction in entrainment of warmer ambient air, as the strong azimuthal vortices responsible for high entrainment are weakened, leading to lower entrainment and temperature recovery.
At higher temperature ratios, the stagnation recovery factor is close to unity for small h/d values and reduces thereafter. This decrease in the recovery factor at large h/d is once again attributed to a greater mixing of ambient air, which is relatively cooler as compared to static temperature in the jet. In this case, as the jet flows downstream, more amount of cold air is entrained and the jet static temperature reduces with increase in h/d. This in turn leads to lower total temperatures as compared to initial state. The ambient air close to the jet gets warmer due to enhanced mixing. There is a net outflow of heat flux from the jet to the ambient. The stagnation recovery factor with control at higher temperature ratios are higher as compared to without control once again emphasizing on the reduction of entrainment with control.
Figure 3. Effect of temperature ratio on the stagnation recovery factor
ro = 1.0
TR = 1.0
TR = 1.2
TR = 1.4
TR = 1.6
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B. Hover lift loss The jet induced negative lift force on the lift plate, known as hover lift loss (Krothapalli et al.7 and Alvi et al.13),
has been estimated from the measurements of radialy distributed static surface pressures on the lift plate. As mentioned earlier, a total of 19 pressure ports along a radial line were used to obtain the integrated forces. Lift loss variation as a function of h/d at two temperature ratios TR = 1.0 and 1.4, representative of cold and hot flow conditions, are shown in Fig. 4. The negative lift force so generated is normalized with the primary jet thrust estimated using isentropic relations. The results clearly show that there is a substantial amount of lift loss, as high as 53% at TR = 1.0 and h/d = 1.5 (Fig. 4a), and it decreases monotonically with h/d. The activation of microjet control leads to large reduction in lift loss. For example, at h/d = 1.5, the lift loss is reduced by 21%, from 53% without control to 32% with control. This reduction in lift loss with microjet control is expected as the azimuthal vortices responsible for high entrainment in the jet near-field, which lead to the vacuum pressures at the lift plate, are now eliminated/reduced. These results are very similar to those reported in the previous studies (Alvi et al.13).
At TR = 1.4 (Fig. 4b), the magnitude of lift loss is significantly higher (up to 76%) as compared to cold jet, and the effectiveness of control in recovering this lift loss is even better (nearly 35% of lift loss is recovered at h/d = 1.5). These results are initially alarming as the high temperature impinging jets such as those occurring in practical conditions induce very high lift loss during hover. However, at the same time, the effect of control is very encouraging that the microjet based control leads to dramatic recovery of lift loss, and the recovery is better at higher temperatures.
C. Unsteady Pressure Field Unsteady pressure measurements on the lift plate were made at x/d = 2 and 3 from the nozzle centerline and on
the ground plane at x/d = 0, 2 for TR = 1.0 & 1.2, and at x/d = 2 for higher temperature ratios. The intensity of unsteady pressure fluctuations on the lift plate and ground plane as a function of h/d are presented in terms of Prms (expressed in terms of dB, using a 20 µPa reference). Near-field acoustic measurements were obtained using a microphone located at x/d = 15 from the jet centerline and are presented in terms of overall-sound-pressure-levels (OASPL). The pressure fluctuation intensities at TR = 1.0 and 1.4 for ground plane and lift plate along with the OASPL from microphone are shown in Figs. 5a and b, respectively. At TR = 1.0 (Fig. 5a), the rms pressure levels on the ground plane are the highest, followed by lift plate and near-field microphone. As observed in previous studies on impinging jets7,13,15, the magnitude of Prms is strongly dependent on nozzle to plate distance, in general high at small h/d (h/d = 2 to 6), decreasing at larger h/d. This is expected as the feedback loop associated with impinging
(a) TR = 1.0 (b) TR = 1.4
Figure 4. Variation of lift loss with nozzle to plate distance
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jets at small h/d has been observed to be very strong. At TR = 1.4 (Fig. 5b), the trend of unsteady pressure fluctuations with h/d is very similar to that observed at TR = 1.0, except that the magnitudes at TR= 1.4 at each h/d are significantly higher (up to 9dB) than those at TR = 1.0.
The variation of Prms on lift plate at x/d = 2 as a function of h/d at various temperature ratios are shown in Fig. 6. An increase in the jet temperature from TR=1.0 to 1.6 gives rise to significant increase (up to 9 dB) in the overall pressure fluctuation levels. It may also be observed that at TR = 1.4 and 1.6, high pressure fluctuations persist over a larger extent of h/d as compared to lower temperature ratios, suggesting the existence of stronger feedback loop over a larger extent of h/d at high temperatures. The effectiveness of microjet control in terms of reduction in Prms on the lift plate at various temperature ratios as a function of h/d is shown in Fig. 7. This figure clearly shows that the fluctuating loads on the lift plate are significantly reduced with the activation of microjets at all temperature ratios. In fact, the reduction in Prms is significantly higher (up to 15dB) at high temperature ratios. Also the effectiveness of control at high temperatures is significant over a larger extent of nozzle to plate distances.
The narrowband pressure spectra for the unsteady pressures at lift plate, ground plane and near-field microphone at TR = 1.0 at h/d = 3.0 are shown in Fig. 8. The spectra clearly show discrete, high amplitude, multiple
Figure 6. Effect of temperature ratio on the pressure fluctuation intensities
Figure 7. Reduction in pressure fluctuation intensities at various temperature ratios
(a) TR = 1.0 (b) TR = 1.4
Figure 5. Pressure Fluctuation intensities at various measurement locations
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impingement tones associated with acoustic feedback loop. Moreover, these tones are identical for all three pressure transducers and the microphone, suggesting a common source. The effect of temperature on the unsteady pressure spectra on lift plate at x/d = 2 and h/d = 4.0 is shown in Fig. 9. Note that the tonal frequencies are strongly dependent on the jet temperature, where an increase in temperature ratio from 1.0 to 1.6 results in an increase in the tone frequency. Moreover, the amplitude of these discrete tones also increases with increase in temperature ratio. The change in broadband levels with temperature ratio is less significant. These features of pressure spectra explain why an active control technique, which efficiently attenuates the feedback loop, such as the one adopted in present experiments is able to significantly reduce the unsteady loading associated with impinging jets.
A comparison of unsteady pressure spectra with and without microjet control on the lift plate and those obtained at near-field using microphone at TR=1.4 is shown in Fig. 10. As you may observe in both the figures that the dominant tones associated with acoustic feedback loop are significantly reduced and some of them are even eliminated with control, clearly demonstrating the effectiveness of microjet based active control in disrupting the
Figure 9. Effect of temperature ratio on narrowband frequency spectra
Figure 8. Narrowband frequency spectra at TR=1.0
(a) Lift Plate (b) Microphone
Figure 10. Effect of control on narrowband pressure spectra, TR=1.4, h/d=4.0
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feedback loop. In addition to reduction in strong tones, broadband levels have also been considerably reduced with the activation of control.
D. Flow Visualization A conventional shadowgraph technique was used to visualize the broad flow features associated with impinging
jet flow and its control using microjets. Instantaneous shadowgraph images at TR = 1.0 and h/d = 4 with and without
control are shown in Figs. 11a and b, respectively. It may be observed that the impinging jet flow field (Fig. 11a) consists of large-scale, azimuthal vortical structures in the jet shear layer along with multiple strong acoustic waves traveling up and down. These impinging and reflecting acoustic wave are the source of high amplitude impingement tones observed in pressure spectral analysis. The presence of strong vortices in the jet shear layer explains the temperature recovery and lift loss behavior of the impinging jets. With the activation of microjets (Fig. 11b), the flow features are significantly different. First of all, the strength of large scale azimuthal vortices in the shear layer has been significantly reduced so that they are barely visible in the shadowgraph image. The strong acoustic waves, clearly seen in Fig. 11a, have now been completely eliminated with the activation of control (Fig. 11b). A close and careful examination of the image in Fig.11b reveals the existence of streamwise streaks in the primary jet, which suggest the presence of streamwise vortices. These qualitative measurements are very much in line with the results discussed above based on the quantitative measurements.
E. PIV Results Global information on the evolution of jet and
velocity at each point in the flowfield could be obtained using particle image velocimetry (PIV). In the present study, PIV measurements were carried out along a streamwise central plane for impinging jets at TR = 1.0 and 1.4, representing cold and hot conditions, respectively. The results are presented in the form of contour plots of mean velocity distribution (Vmean). Velocity vectors are shown superposed on mean velocity contour plots at few selected locations for reference.
The contour plots of mean velocity distribution for impinging jets corresponding to h/d = 5, at
Figure 12. Mean velocity distributions in the central plane of impinging jet; Right half: TR = 1.0, Left half: TR = 1.4
Vref = 500 m/s
(a) No Control (b) With Control
Figure 11. Instantaneous shadowgraph images at TR=1.0, h/d=4.0
Waves propagating upstream
Waves propagating downstream
No waves
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TR=1.0 and 1.4 are shown in Fig. 12. As mentioned earlier, these measurements were conducted at NPR = 3.7, corresponding to the ideally expanded jet conditions. The velocity vectors at the nozzle exit show that for both cold as well as hot jets, the jet exhibits a near top hat velocity profile. At TR = 1.0 (right half of Fig. 12), the mean jet velocity at the exit plane is ≈ 430 m/s corresponding to the fully expanded jet velocity at M = 1.5, whereas, at TR = 1.4 (left half of Fig. 12), the mean jet velocity at the exit is ≈ 550 m/s, slightly higher than the fully expanded jet velocity at T0 = 420K. The length of the vector represents the magnitude of velocity at each location. The entrainment velocities near the exit plane for the hot jet (TR = 1.4) are much higher than the cold jet (TR = 1.0), resulting in the larger amount of suction at the lift plate for hot jets as compared to cold jets. This results in a much higher lift loss for hot impinging jets as observed earlier in Fig. 4.
The effect of microjet control on the mean velocity distributions of impinging jets at TR = 1.0 and 1.4 is shown in Figs. 13a and b, respectively. In each of these figures, left half of the plot shows the velocity field corresponding to ‘no control’ and right half ‘with control’. In general, the velocity flowfield with and without control are globally similar and show nearly the same flow features. The noteworthy exception being that the jet without control appears to spread less than that with control near the nozzle exit. However, at downstream distances (y/d >2), the jet without control spread slightly more than the jet with control. These results are consistent with those observed in earlier studies on cold impinging jets14-15.
IV. Conclusions The flow and acoustic field created by high temperature supersonic impinging jets such as those occurring in the
STOVL aircraft is highly complex. There have been many attempts in the past to understand this highly oscillatory flow field and associated feedback loop responsible for adverse performance effects. A number of experimental studies have been conducted to disrupt this feedback loop using supersonic microjets at isothermal conditions. The present study is an attempt to characterize the hot impinging jets, primarily to study the effect of temperature on feedback loop and associated flow field characteristics and to demonstrate the effectiveness of microjet control over a range of temperature ratios.
The experimental results described in this paper include temperature distributions on the ground plane, lift loss characteristics, unsteady pressures on the lift plate and ground plane, acoustic measurements in the near-field, velocity field for both cold and hot conditions and few typical flow visualization images to see broad flow features of the jet flowfield. The results show that stagnation recovery factor strongly depends on the temperature ratio and nozzle to plate distance. The mixing characteristics play an important role in the variation of stagnation recovery
a) TR = 1.0 b) TR = 1.4 Figure 13. Effect of microjet control on mean velocity field; Left half: No Control, Right half: With Control
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factor and in turn heat transfer characteristics. The jet induced lift loss at higher temperature ratios is significantly higher as compared to the cold jet and the microjet control provides substantial recovery of lift loss (nearly 35%) at higher temperatures.
The unsteady pressure fluctuations of high temperature impinging jets are significantly higher than those for cold jets and continue to be high over a larger extent of nozzle to plate distances. This suggests that the feedback loop in case of hot jets is much stronger and persist over larger parametric space. The activation of microjets leads to dramatic reductions (up to 15dB) in the rms pressure fluctuation levels on the ground plane and lift plate, and the near-field sound pressure levels. Microjet control not only attenuated or sometimes even eliminated the discrete, high-amplitude, impinging tones but also reduced the broadband noise levels considerably. Velocity field measurement results show that entrainment velocities near the exit plane for the hot impinging jet are much higher than the cold impinging jet, resulting in much higher lift loss, consistent with the pressure measurements. Instantaneous shadowgraph images clearly show the disappearance of large scale structures in the jet shear layer with the activation of microjet control.
Acknowledgements This research was supported by a grant from AFOSR, monitored by Lt. Col. R. Jefferies and Dr. J. Schmisseur. We are grateful for this support. The authors would also like to thank the staff of AAPL, especially Dr. I. Choutapalli and Mr. R. Avent, for their help in conducting these experiments.
References 1Donaldson, 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. 2Lamont, 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. 3Powell, 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. 4Tam, 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. 5Messersmith, N. L., “Aeroacustics of supersonic and impinging jets”, AIAA paper 95-0509, 1995. 6Alvi, F. S. and Iyer, K. G., “Mean and unsteady flow field properties of supersonic impinging jets with lift plates,” AIAA
Paper 99-1829, 1999. 7Krothapalli, 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. 8Henderson, 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. 9Karamcheti, 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. 10Elavarasan, 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. 11Sheplak, M. and Spina, E. F., “Control of high speed impinging-jet resonance”, AIAA Journal, Vol. 32, No. 8, 1994,
pp.1583-1588. 12Shih, 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. 13Alvi, 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. 14Lou, H., Shih, C. and Alvi, F. S., “A PIV study of supersonic impinging jet,” AIAA Paper 2003-3263, 2003. 15Lou, 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. 16Arakeri, V. H., Krothapalli, A., Siddavaram, V., Alkislar, M. B., and Lourenco, L. M., “On the use of microjets to suppress
turbulence in a Mach 0.9 axisymmetric jet,” Journal of Fluid Mechanics, Vol. 490, 2003, pp. 75-98. 17Alkislar, 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. 18Lourenco, L. M., and Krothapalli, A., “True resolution PIV: a mesh-free second order accurate algorithm,” Proceedings of
10th International Symposium on Applications of laser Techniques in Fluid Mechanics, Lisbon Portugal, 2000. 19Goldstein, R. J., Behbahani, A. I. and Heppelmann, K. K., “Streamwise distribution of the recovery factor and the local heat
transfer coefficient to an impinging circular air jet,” International Journal of Heat Mass Transfer, Vol. 28, No. 8, 1986, pp. 1227-1235.
