role of coherent structures in supersonic jet noise and its control

American Institute of Aeronautics and Astronautics

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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. Introduction 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) aircraft to turbine blade cooling. The flow field of the 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, and ingestion of hot gases into the engine nacelle and a lift loss of the aircraft during hover. 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. 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 methods, and their effectiveness has been detailed in the literature1-10, but the problem is still far from being resolved due to the complex flow field associated with these jets. It is very 1 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.

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Figure 1. A schematic of feedback loop for impinging jet


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important to understand the details of the flow field of these jets with and without a particular control scheme for the development 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 its understanding has its roots in the pioneering research of Powell, who explains the feedback loop associated with edge tones generated by high speed jets. A number of the general features of the feedback loop associated with impinging tones are similar to that elucidated by Powell4 for edge tones. In a similar manner, as noted by Tam and Ahuja5 and detailed by Krothapalli et al.6, the feedback loop in the impinging jet is initiated as instability waves in the shear layer of the jet at the nozzle lip. These instability waves grow in size into large-scale vortical structures as the jet travels downstream. In the case of impinging jets, the ground plane acts as a physical obstruction similar to the “edges” in edge tones. Upon impingement, these vortices generate large pressure fluctuations, which in turn travel upstream in the ambient flow in the form of acoustic waves. Upon reaching the nozzle exit, these acoustic waves excite the shear layer and complete the feedback loop. Most of the control systems studied so far including high momentum microjet control have tried to suppress this feedback loop and seem to work efficiently over a limited range of geometrical and flow conditions. In particular, effectiveness of microjet control in reducing noise shows a strong dependence on the nozzle-to-plate distance, nozzle pressure ratio and temperature ratio of the impinging jet. Figure 2 (data taken from Ref. 11) shows the variation of effectiveness of microjet control (in terms of reduction in overall sound pressure levels, ∆OASPL) with nozzle-to-plate distance. The results clearly show that at certain values of h/d, microjet control is very effective in reducing OASPL (e.g., at h/d = 3.5, ∆OASPL = ~12 dB for 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). These results hence pose a number of questions such as why the control technique which is so effective at some test condition is not so effective at other test condition. Do the flow features of jet change drastically with a small change in nozzle-to-plate distance? Does the strength of the feedback loop or evolution of large scale structures vary with h/d? It is very important to answer these questions to design a robust and highly effective control technique to mitigate high noise levels associated with supersonic impinging jets. In this study we have taken a closer look at these results and have made an attempt to answer some of these questions. In addition we have made phase locked particle image velocimetry measurements at two values of h/d, one corresponding to the case where microjet control is very effective and the other where it is not so.

II. Experimental Setup

A. Test 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

Figure 2. Effectiveness of microjet control, NPR = 3.7

Figure 3. A photograph of the STOVL facility


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(10 m3) and is used to drive the facility. The measurements were made at both design and off-design conditions of 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, corresponding to ideally expanded jet condition. The test Reynolds number based on exit velocity and nozzle diameter of the jet was 7 x 105. For these experiments, the stagnation temperature of the jet was kept constant at 300K, 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 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. 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. Unsteady pressure and near-field noise measurements Unsteady pressure measurements on the lift plate were obtained using high frequency response, miniature (1.6

mm dia.) KuliteTM pressure transducers of ±5 psid range mounted at two locations at x/d = 2 and 3 from the nozzle centerline. Near field acoustic measurements were made using a 0.635 cm (1/4”) diameter B&K microphone placed at x/d = 15 from the nozzle centerline, 90° with respect to the jet axis. The pressure transducers and microphone were carefully calibrated prior to each set of experiments. The pressure and acoustic signals were acquired through high speed National Instruments digital data acquisition cards using LabviewTM and were processed offline. The transducer signals were conditioned using StanfordTM filters (Model No. SR650) 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 were averaged in order to obtain statistically reliable estimate of the narrow-band spectra.

C. Phase-locked PIV Flow field measurements using phase locked Particle Image Velocimetry (PIV) were made for few chosen test

conditions. For PIV measurements, a dual cavity digitally sequenced Nd:YAG laser (Spectra Physics PIV400) was used. A light sheet of approximately 1 mm thickness was created by suitable combination of spherical and cylindrical lenses and was made to pass through the centerline of the jet. The PIV images were acquired at a rate of 15 Hz using a CCD camera (Kodak ES1.0) 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 deg to the jet axis. The pulse separation between the two laser pulses was kept at 1–1.2 µs. In the present experiments, the jet was seeded with sub-micron (~0.3 µm) Fog fluid (a solution of 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 extract particle displacement and the velocity field. In this processing scheme, the interrogation window is defined by the particle displacements , ranging from 3 to 4 pixels and the interrogation window used in the present study was nominally set to 20 x 20 pixels. However, the adaptive scheme used in the processing ensured that a minimum of 10 pairs 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 ) The flowfield at every point is calculated using a least-square fitting algorithm based on a second-order polynomial. This technique results in a second-order accuracy in calculating the flowfield at each point in the flow. The details of this technique are available in Lourenco and Krothapalli [21]. The near field microphone signal was used for the purposes of phase-locking PIV measurements. Figure 4 shows a schematic of the phase-locking setup. The raw signal from the microphone was first recorded unfiltered and processed online using LabVIEW® codes (same FFT parameters as in the previous section). Thereafter the signal was narrowly band-pass filtered around the impinging tone as measured from the spectra using a Stanford Research Systems SR650 High Pass/Low Pass filter. The signal was then sent to a Model 88 Laser Lock made by Hendrick and Associates. This signal was divided such that it corresponded to the frequency of the laser and the camera. The signal was then fed into custom timing hub matched to a National Instruments® PCI-6602 timing card and an IMAQ 1422 image acquisition card. Phase-delays in the trigger signal to the laser/camera were included using proVISION-XS software from IDT® which was also used to acquire and process the images. Finally, a delay on the divided signal was applied to generate the necessary phase


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trigger for the synchronized camera and laser strobe. The laser frequency of 15 Hz requires the pulsation frequency to be a multiple of 15. The full cycle of a pulse was sampled in 12 phases with equal intervals. At every phase, 300 image pairs were obtained. Phase-averaged velocity fields were computed by taking the mean of these instantaneous fields. The measurement uncertainty is estimated to be about 1% and 10% in phased-averaged velocity and random turbulence measurements, respectively, with a 95% confidence level. The global mean quantities were calculated from phase-averaged values.

III. Results and Discussion As stated in the introduction section, the main objective of this study is to understand the basic characteristics of

the supersonic impinging jets with and without microjet control. Unsteady pressures, near-field noise and phase locked 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 pressure supply 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

Figure 4. Schematic of the Phase-Locking System

Figure 5. Near-field noise spectra obtained using microphone located at 15d from centerline in the nozzle exit plane. (a) h/d = 4, (b) h/d = 4.5


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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 is the case where microjet control was found to be very effective in reducing the noise levels and h/d = 4.5 is the case corresponding to minimal effect of control. As clearly seen in Fig. 5a, at h/d = 4; the spectra without control consist of a sharp impinging tone (5.8 kHz) along with its harmonics and high broadband levels. With microjet control, the amplitude of dominant impinging tone is significantly reduced (nearly 22 dB) along with a reduction in broadband levels. 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 be observed that at h/d = 4, the spectra shows a continuous decrease in SPL with frequency beyond 10kHz except couple of low amplitude harmonics of impinging tone, whereas, at h/d = 4.5 the SPLs continue to be high even at higher frequencies (beyond 10 kHz). This behavior at high frequencies can be attributed to the broadband amplification 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 broadband levels, however, most of the other tones present in the baseline spectra are eliminated.

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 high amplitude impinging tones and their harmonics. The frequencies of impinging tones observed in the lift plate spectra are 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 dB reduction along with a major reduction in broadband levels, whereas at h/d = 4.5, the dominant impinging tone and its harmonics remain virtually unaffected. The kulite pressure sensor mounted in the lift plate is very sensitive and seems to pickup even minor vibrations in the plate. These results clearly demonstrate that the baseline (without control) pressure spectra at the lift plate and the spectra of near-field noise at the two values of h/d show few identical 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 streamwise

central plane was obtained using PIV. PIV measurements were made at both values of h/d, 4 and 4.5 with and without microjet control. The mean velocity field for the baseline flow corresponding to h/d = 4 and 4.5 is shown in Fig. 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 in the ambient and in the shear layer of the jet are shown superposed on the mean velocity contour plots. As mentioned earlier, measurements were made at NPR =3.7, corresponding to ideally expanded jet conditions. However, a weak periodic shock cell structure can be observed in both the cases. This may be due to weak shocks generated by the nozzle lip and a significant entrainment of ambient air resulting in a mildly under-expanded jet. The velocity vectors

Figure 6. Narrowband pressure spectra at the lift plate; (a) h/d = 4, (b) h/d = 4.5


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close to the nozzle exit show top-hat velocity profile in both cases. The streamlines exhibit flow entrainment into the shear layer of the jet as it flows downward and formation of the wall jet at the impingement plate. In general, the global flow features of the impinging jet flowfield is very similar at h/d = 4 and 4.5 and doesn’t show any considerable change between the two cases. With microjet control (not shown here), the velocity flowfield was nearly the same suggesting that most of the mean flow properties of the jet remain unaltered with control. These

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 shear

layer which grow into large-scale vortical structures, so it is important to analyze the vorticity field associated with impinging jets. The ensemble-averaged vorticity contour plots measured along the jet central plane for h/d =4 and 4.5 are shown in Fig. 8. The contours show the azimuthal component of the normalized vorticity, ΩZdj/Ujet, where dj is 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 the Figs.7 and 8 show the global flowfield of the impinging jet at two h/d, we need to further investigate to get better understanding of the associated flow physics.

a) h/d = 4 b) h/d = 4.5

Figure 7. Mean velocity distribution in the central plane of the impinging jet.

a) h/d = 4 b) h/d = 4.5

Figure 8. Mean vorticity distribution in the central plane of the impinging jet.


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a) ϕ = 0º b) ϕ = 60º

c) ϕ = 120º d) ϕ = 180º

e) ϕ = 240º f) ϕ = 300º

Figure 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.


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C. Phase locked velocity field In order to better understand the flow physics associated with impinging jets with and without control at the two

values of h/d, phase locked velocity measurements were carried out using PIV. The phase averaged flowfield enables the construction of time evolution of different flow properties. The near field microphone signal was used for 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 we used this tone to phase-lock PIV at this h/d. A total of 300 image pairs were captured at every 30° resulting in 13 datasets 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, 6.5 and 9.3 kHz, the dominant of which being at 6.5 kHz. While phase-locked PIV was performed at 4.1, 6.5 and 9.5 kHz, only the data collected at the strongest tone of 6.5 kHz will be discussed in this paper. Figure 9 shows the contour plots of the phase averaged streamwise velocity and azimuthal vorticity at h/d 4.5. The figure includes equally spaced (60º apart) six phase angles. The streamwise velocity and azimuthal vorticity contours are shown on right and left hand side of each plot. The vorticity level flood lines and flow streamlines are superposed as well. These figures demonstrate some of the salient features of impinging jet flow field at h/d = 4.5. The figures clearly show the presence of multiple well defined vortical structures in the shear layer at each phase. The instability initiates at the nozzle exit, grows into a large scale vortex which convects downstream and eventually after impingement on the ground moves along the wall jet. As these vortices roll down and create a local suction, there is a significant entrainment of the ambient air into the shear layer of the jet which leads to the spreading of the jet. It is interesting to observe from the vorticity contours that the vorticity in the shear layer is negative (clockwise rotation) whereas in the wall jet it is positive (counterclockwise rotation) and there is an iso-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 = 4 show relatively less coherent and weaker structures in the shear layer and the vorticity level in the shear layer is relatively less for h/d = 4.

To calculate the number, size, strength and other parameters associated with vortices, one has to precisely

identify the location of these vortices in the flowfield. There are a number of techniques that have been used in the literature to extract the vortices from the associated velocity and vorticity field. The simple one being using surfaces of 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 Strength” as described by Adrian et al. (2000). The velocity gradient tensor in the central plane is given by CAN WE PUT A SYMBOL FOR VEL GRAD TENSOR HERE??

Figure 10. Phase-averaged impinging jet flowfield at h/d = 4, ϕ = 0º


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where the subscripts 1 and 2 correspond to streamwise and cross-stream directions respectively. Identification of vortical structures is done by plotting regions where the imaginary portion of the complex eigen value is positive,

i.e. λci>0. The swirl-Strength is then the magnitude of this parameter. Figure 11 shows the contour plots of swirl strength for h/d = 4 and 4.5 at ϕ = 0º. The swirl strength analysis clearly brings out the differences between the two flowfields. Compared to h/d = 4, there are three pairs of coherent, well defined and symmetrical vortical structures at h/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, indicating that 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 microjets around the periphery of the jet operating at a pressure of 100 psia. The effect of microjet control on the swirl strength at h/d = 4 and 4.5 is shown in Fig. 12. At h/d = 4, the coherence of vortices is completely broken with control 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 still maintained. The shape of vortices near the nozzle exit is somewhat disturbed but as the flow moves downstream,

b) h/d = 4 b) h/d = 4.5

Figure 11. Swirl strength distribution in the central plane of the impinging jet, ϕ = 0º.

a) h/d = 4 b) h/d = 4.5

Figure 12. Effect of control on swirl strength distribution, ϕ = 0º.


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those roll up and become once again coherent. The outcome of the flowfield results with control agrees with those of acoustic measurements. These results clearly suggest that there is a need to further increase the strength of microjet control by increasing the supply pressure, so that microjets penetrate deep into the shear layer and break the coherence of these large scale structures.

IV. Conclusions The understanding of the flow physics associated with supersonic impinging jets is of great importance for the

design of a robust control technique to suppress the high levels of noise associated with these jets. Previous studies indicate that the effectiveness of microjet based control has a strong dependence on the nozzle-to-plate distance, so we chose two values of this parameter of unequal effectiveness in this study. The experimental results described in this paper include unsteady pressures, nearfield acoustics, and velocity and vorticity fields. Pressure and noise spectra 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 of vorticity for the two flowfields but it did not provide enough information on the role of vortices. Phase-averaged velocity and vorticity measurements brought out clear differences between the two flowfields, the difference in the strength and size of vortices in the shear layer. The swirl strength levels further elucidated on the vortex statistics of the 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 of

excellence for supporting this research.

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role of coherent structures in supersonic jet noise and its control

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