Active Noise Control in Jets from Conical and Contoured Supersonic Nozzles with Plasma Actuators

Transcription

1 15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference) May 2009, Miami, Florida AIAA Active Noise Control in Jets from Conical and Contoured Supersonic Nozzles with Plasma Actuators J.-H. Kim *,M.Kearney-Fischer, and M. Samimy, Gas Dynamics and Turbulence Laboratory Department of Mechanical Engineering, The Ohio State University GDTL/AARL, 2300 West Case Road Columbus, Ohio USA S. Gogineni Spectral Energies, LLC, Dayton, Ohio An active noise control work in jets from contoured and conical supersonic axisymmetric nozzles (henceforth contoured and conical jets, respectively) with plasma actuators is conducted. The Mach 1.65 contoured nozzle is designed by using the method of characteristics for shock-free jet. The Mach 1.65 conical nozzle has converging and diverging conical sections with a sharp throat. Eight actuators, distributed uniformly around the nozzle exit, are used and the jet is forced with azimuthal modes of 0-3, and ±4 and at forcing Strouhal numbers ranging from 0.18 to 4.0. Far-field acoustic is measured by a linear microphone array covering polar angles from 25 to 80 relative to the jet axis. At a far-field polar angle of 30, more noise reduction is observed at a higher azimuthal mode of 3 or ±4 in both jets. The maximum reduction in overall sound pressure level is 1.5 and 2.6 db for the contoured and conical jets, respectively. At a polar angle of 80 (near sideline), the reduction in noise level is negligible. In the conical jet, there is a competition for energy between the naturally-occurring structures and those generated by forcing, similar to the process taking place in forcing an imperfectly-expanded supersonic jet. At moderate forcing Strouhal numbers, the structures generated by forcing prevail and thus the naturally-occurring structures are suppressed or weakened. This seems to be responsible for the reduced or suppressed screech tones in the conical jet. I. Introduction Most of the research on supersonic jet noise has been carried out using contoured nozzles, which are designed by the method of characteristics for shock-free flow at the design condition. However, the nozzles used in some military aircraft have a conical diverging section with a sharp throat. Shock waves are generated inside a conical nozzle even when it is operated at the design Mach number [Back and Cuffel 1966, Liu et al. 2009, Munday et al. 2009]. As a result, the flow field and acoustic characteristics of a conical nozzle are significantly different from those of a contoured nozzle. * Research Associate, Member AIAA Graduate Research Associate, Member AIAA The Howard D. Winbigler Professor of Engineering, Fellow AIAA President, Fellow AIAA Copyright 2009 by Jin-Hwa Kim. Published by the, Inc., with permission.

2 A. Jet Flow It is generally agreed that jet noise is generated by dynamics of turbulence structures in the jet. The principal propagation direction of noise from large-scale structure dynamics (mixing noise) is about 30 to the jet axis, while the noise radiated normal to the jet axis direction is from the dynamics of smaller scales [Tam 1996]. Since noise is generated by the dynamics of flow structures, jet noise control can be done via controlling flow structures in the shear layer. The formation and development of large-scale structures in the shear layer of a round jet involve three instability modes. Controlling jet flow is closely related to these three modes, and thus an ideal actuator needs to be able to control one or more of these modes. The first instability mode is the initial shear layer instability (K-H type) due to the velocity gradient with an inflection point which is present in a shear layer. The frequency of this mode is scaled with the momentum thickness at the nozzle exit and is known to be unstable at St = f 0 0 /U j = The second instability is the jet column mode. Since the structure size affecting jet column mode is as large as the jet diameter, the frequency of this mode is scaled with the nozzle exit diameter (D). The jet column is unstable at St D =f p D/U j = , depending on the jet facility [Crow and Champagne, 1971; Zaman and Hussain, 1980; Reynolds and Bouchard, 1981; Hussain and Zaman, 1981]. The last instability in a round jet is azimuthal mode. Cohen and Wygnanski [1987] predicted using linear analysis that many azimuthal modes are unstable in the initial shear layer when the boundary layer is very thin. In the earlier research, it was demonstrated that the noise and flow fields in round Mach 0.9 [Samimy et al. 2007a, Kim et al. 2009] and 1.3 jet [Kim et al. 2008, Kim and Samimy 2009] is significantly affected by the choice of azimuthal mode at a fixed forcing frequency. B. Jet Noise In a subsonic jet, mixing noise or Mach wave radiation is a dominant jet noise source. Mach wave radiation appears when the large-scale structures travel at a supersonic speed relative to the surrounding gas. In an unheated subsonic jet, the convection velocity of large-scale structures is also subsonic so mixing noise is the main noise source. In a highly heated subsonic jet or heated high Mach number supersonic jet, Mach wave radiation can be one of the major jet noise components. It is generally agreed that the dynamics of large-scale structures are responsible for the mixing noise. Mixing noise and Mach wave radiation have a preferential direction relative to the jet axis. Supersonic jets have additional noise components due to shock cell trains in the jet plume. The main noise component in supersonic jets is well explained in Seiner [1984] and Shih and Alvi [1999]. In a shock containing jet, screech tones and broadband shock associated noise (BSAN) can occur. Screech tones are generated by a feedback loop consisting of downstream-traveling large-scale-structures/waves and upstream-traveling acoustic waves. The periodic shock train amplifies large-scale structures at a specific frequency and strong screech tones are generated by the feedback loop. The mechanism for the screech was explained by Powell [1953a&b]. An extensive review of screech noise was done by Raman [1999]. C. Objectives of the present research In the earlier research, the plasma actuators developed by the authors showed control authority in a wide range of frequencies in a high-speed jet. A significant enhancement of jet mixing/spreading was achieved in an ideally expanded Mach 1.3 jet [Samimy et al. 2007b, Kim and Samimy 2009] and a Mach 0.9 jet [Kim et al. 2009]. A moderate reduction in noise was observed in Mach 0.9 [Samimy et al. 2007a] and ideally-expanded Mach 1.3 [Kim et al. 2008] jets. In the present research, the focus will be on noise mitigation in Mach 1.65 jets with conical and contoured nozzles at the design condition. The control authority of plasma actuators will be also investigated. 2

3 In addition to the three instability modes discussed earlier, a periodic shock train occurring in a conical nozzle plays a significant role in the jet development and acoustic radiation. Since the jet with conical nozzle has a strong shock train, as in an imperfectly-expanded jet, it is expected that controlling a conical jet will be similar to an imperfectly-expanded jet. Flow control in an imperfectly-expanded Mach 1.3 jet showed reduced effectiveness of actuators when compared to perfectly-expanded jets [Kim and Samimy 2009]. II. Facility and Techniques All the experiments are conducted at the Gas Dynamics and Turbulence Laboratory at The Ohio State University. Compressed air supplied by three 5-stage reciprocating compressors, which is filtered and dried, is stored in two cylindrical tanks with a capacity of 43 m 3 up to 16 MPa. The compressed air is supplied to the stagnation chamber of a D = 2.54 cm exit diameter axisymmetric nozzle. The air is discharged horizontally through the converging-diverging nozzle. At the end of the nozzle, a boron nitride extension is attached to house eight plasma actuators. Each actuator is composed of two tungsten pin electrodes with a diameter of 1 mm. All electrodes are placed 1 mm upstream of the extension exit and flush mounted with the inside of the extension surface. The electrodes protrude through a ring groove, measuring 1 mm wide and 0.5 mm deep, to prevent the plasma from being blown off. In the present study, two nozzles with the design Mach number of 1.65 are used. The nozzle inner surface profile is either straight lines (conical nozzle) or contoured using the method of characteristics (contoured nozzle) as shown in Fig. 1. (a) Contoured nozzle (b) Conical nozzle Fig. 1 Cross-section of contoured and conical nozzle. Acoustic data is acquired at 25, 30, 35, 40, 45, 50, 60, 70, and 80 locations with respect to the jet axis in an anechoic chamber as shown in Fig. 2. All nine microphones, ¼ B&K condenser type, are facing the jet centerline so that the microphone axis is normal to the jet axis. The noise signal is sampled at 200 khz with a window size of 8192 and thus the frequency resolution in a spectrum is 24.4 Hz. All measured spectra are normalized to 80 nozzle exit diameters. The jet velocity field is measured by a LaVision PIV (particle image velocimetry) system using either one or two cameras with 2048x2048 pixel resolution. A Spectra Physics Model SP-400 dual-head Nd:YAG laser is used for the light source. The cameras and laser are synchronized by a timing unit housed in a dual-processor PC. The setup for the PIV is depicted in Fig. 3. The spatial resolution of the velocity vectors depends on the field of view, and the number of pixels used. For most of streamwise velocity filed measurements, the spatial resolution is about 2.8 mm (0.11 nozzle exit diameters). The streamwise span of PIV measurements is from 2 to 15 nozzle exit diameters. Streamwise locations less than 2 jet diameters are not measured because there is no significant activity of large-scale structures just downstream of the nozzle exit. 3

4 Fig. 2 Schematic of linear microphone array. The jet plume is seeded with Di-Ethyl-Hexyl-Sebacat (DEHS) liquid droplets atomized by a four jet LaVision atomizer. A 38.1 cm (15 ) duct is placed upstream of the jet exit to generate a co-flow. The co-flow is generated by channeling part of the air entrained by the jet through the duct without using any fans or blowers. The co-flow is seeded by a fogger to avoid spurious velocity vectors in the entrained air region. The average droplet size is about 0.25 and 0.7 m for the jet flow and co-flow, respectively. The turbulence statistics were converged using 600 to 650 image pairs [Kim et al. 2009]. Thus, about 700 images are used for all the statistics reported in this paper. The uncertainty of PIV measurements is detailed in Kim et al. [2009]. Fig. 3 Schematic of PIV setup. The plasma generating system has two high voltage Glassman DC power supplies, with an output of 10 kv and 1 Amps. Each power supply can drive four actuators at the same time, and thus up to eight actuators can be operated simultaneously. The actuators are switched using Behlke high voltage transistors. A National Instruments analog board is used to generate eight independent pulse trains to control the transistor switches. Details of the DC plasma system are in Utkin et al. [2007]. The forcing frequency, duty cycle, and azimuthal mode are controlled through LabView. The azimuthal modes of forcing to be tested are m = 0-3, 1, 2, and 4. These are the only modes available when using 8 actuators. A more detailed description of azimuthal modes is in Kim et al. [2009]. 4

5 III. Results and Discussion A. Baseline Jets The far-field baseline spectra for both jets are shown in Fig. 4. At 30 polar angle, the noise level does not show any significant difference except for a screech tone in the conical jet at St D = One minor difference is that the broadband noise level near St D = 0.5 is slightly lower for the conical jet than for the contoured jet. However, the noise level at 80 polar angle for the conical jet is considerably higher compared to the contoured jet (Fig. 4b). This is shock noise due to strong shock cells in the jet plume (Fig. 4) as will be discussed shortly. (a) 30 (b) 80 Fig. 4 Far-field baseline spectra at 30 and 80 polar angles for contoured and conical Mach 1.65 jets. The spikes at both polar angles in the conical jet are screech tones generated by a feedback loop, composed of downstream traveling flow structures and upstream traveling acoustic waves. A strong interaction between large-scale structures and periodic shock cells reinforces the strength of structures and is responsible for stronger noise generation than in shock-free or weak-shock jets. As shown in Fig. 5, the shock cells are very weak in the contoured jet, but they are very strong in the conical jet. (a) Contoured F= khz (b) Conical Fig. 5 Time-averaged streamwise schlieren images for the baseline jets. The tick mark spacing is one nozzle diameter and the first tick mark is at x/d = 0. There are two and three diamond-shock cell trains for the contoured and conical jets, respectively. In both jets, the shock train emerging from the jet center at the nozzle exit plane originates from the mating surface of the nozzle extension. Another shock train is formed from the nozzle lip in both jets. For the conical jet, there is one more weak shock train observed just downstream of the strongest shock train which originates from the nozzle lip. Figure 6 shows wall pressure profiles for both nozzles. A big jump in wall pressure in the conical nozzle suggests that a shock wave is generated just downstream of the 5

6 nozzle throat. In the contoured nozzle, both shock trains are very weak. For the conical nozzle, the shock train emanating from the nozzle lip is strongest. In Munday et al. s Mach 1.56 conical jet, the strength of shock trains initiating from nozzle lip and throat was comparable. The relative weakness of the shock train initiated from nozzle throat in the present research is thought to be due to the nozzle extension. Fig. 6 Inner wall pressure profiles for the Mach 1.65 contoured and conical nozzle at the design condition. A separate mark for each nozzle is just to indicate the location of throat. In a jet with periodic shock cells, another noise component is Broadband Shock Associated Noise (BSAN), generated by interaction of large-scale structures and shock-cells. The broadband hump near St D = 0.6 in Fig. 4 for the conical jet is BSAN. The weak shock cells in the contoured jet (seen in Fig. 5a) produce very weak BSAN. (a) Ensemble-averaged velocity, contoured jet (b) Ensemble-averaged velocity, conical jet (c) Galilean streamlines, contoured jet (d) Galilean streamlines, conical jet Fig. 7 Ensemble-averaged velocity contours and Galilean streamlines. Ensemble-averaged streamwise velocity contours and Galilean streamlines are shown in Fig. 7 for both baseline (unforced) jets. For the contoured jet, the theoretical convection velocity is used for Galilean decomposition. There are large vortical structures where closed or spiral streamlines appear. A detailed procedure of conditional sampling and obtaining Galilean streamlines is given in Kim et al. [2009] 6

7 and Kim and Samimy [2009]. The ensemble-averaged flow fields are largely the same for both jets, but there is stronger undulation of streamwise velocity in the conical jet due to the periodic shock train as shown in Fig. 5b. The Galilean streamlines show that there are less organized, random structures in the contoured jet. However, there are well organized large-scale structures in the conical jet, which is naturally formed by the feedback loop. The structure spacing (2.51 D) in Fig. 7d agrees well with the shock cell spacing in Fig. 5b. The convection velocity of large-scale structures is calculated to be about 290 m/s (U c /U j = 0.66) using the screech tone frequency of about 4.59 khz. B. Effects on overall sound pressure level A large number of data sets were taken for each jet at azimuthal modes m = 0-3, and ±4, and at St DF s ranging from 0.09 to 4.0. The effects of azimuthal modes and St DF s are investigated using OASPL (overall sound pressure level) for each data set. The OASPL is calculated for each data set for St D ranging forcing. (a) m = 0 (b) m =1 (c) m = 3 (d) m = ±4 Fig. 8 Change in OASPL in the contoured jet for m = 0, 1, 3, and ±4. The effects of forcing Strouhal numbers (St DF ) in the contoured jet are shown in Fig. 8 for m = 0, 1, 3, and ±4. In the earlier research, tones in each spectrum were removed in calculating OASPL since the actuator tone were the only tones in the measurements and are expected to be less significant in a larger or 7

8 full-sized jet [Samimy et al. 2007a]. However, all tones are included in the present research since the conical jet contains screech tones in addition to actuator tone, therefore, removing tones in the conical jet is not reasonable and comparison of actuator performance should be conducted for the same conditions. The angle shown in the figure is the polar angle relative to the jet axis as shown in Fig. 3. A lower angle indicates downstream direction and an angle of 90 represents sideline direction. The effect of St DF is observed if one follows the contour level along a radial direction at a fixed polar angle. The dependency of OASPL change on polar angles can be obtained if one follows a fixed St DF arc. There is no reduction in OASPL at an angle greater than 55 for all modes shown. At low polar angles, a slight reduction of 0.5 db is seen for lower azimuthal mode numbers and a moderate level of reduction of 1.0 to 1.5 db is observed for higher azimuthal numbers of m = 3 and ±4. The maximum reduction occurs at lower polar angles for each mode. Thus, the acoustic noise mitigation by plasma actuators is more effective in the downstream direction as observed in other Mach numbers: 0.9 [Samimy et al. 2007a] and 1.3 [Kim et al. 2008]. A significant increase of OASPL in the downstream direction at low St DF s for m = 0 and ±4 is due to actuator noise as will be further discussed later since the broadband noise level is not changed significantly. At St DF s greater than 2, there is no significant dependence on azimuthal mode, nor on St DF. Depending on azimuthal mode, the maximum noise reduction is observed at St DF = , where largescale activity is maximum for most cases. (a) m = 0 (b) m =1 (c) m = 3 (d) m = ±4 Fig. 9 Change in OASPL in the conical jet for m = 0, 1, 3, and ±4. 8

9 The changes in OASPL for the conical jet are shown in Fig. 9 for the same azimuthal modes as for the contoured jet. A reduction in OASPL is seen in a wider range of St DF and polar angles when compared to the contoured jet (Fig. 8). In the sideline direction, the increase in OASPL is less than that for the contoured jet. In the conical jet, the actuator noise level is comparable to the screen tones as will be discussed further later. Thus, the contribution of actuator tone in OASPL is not as significant as in the contoured jet. This explains why the increase in OASPL in the sideline is less in conical jet. In m = 0 and ±4, the actuator noise is very strong in the downstream direction (low polar angles) as in the contoured jet. In low azimuthal modes of 0 and 1, the maximum reduction is seen at a slightly higher polar angle of 40 - it is 30 for the contoured jet. In higher azimuthal modes of 3 and ±4, the maximum reduction for each mode occurs at lower St DF s than in m = 0 and 1 as was observed in the contoured jet. A greater reduction in OASPL is seen in conical jet for all modes when Figs. 8 and 9 are compared. The maximum mitigation at m = 3 and ±4 is greater than 2.5 db. The greater reduction is associated with reduction in natural screech tone level by forcing as will be further discussed. (a) 30, Contoured jet (b) 80, Contoured jet (c) 30, Conical jet (d) 80, Conical jet Fig. 10 Change in OASPL at two polar angles of 30 and 80 for the contoured jet. C. Effects of azimuthal modes The effects of azimuthal modes are further investigated at two representative polar angles of 30 and 80 (Fig. 10). As was shown in Figs. 8 and 9, the maximum reduction is observed at 30 for both jets. The trend of with azimuthal modes is very similar for both jets at both polar angles (compare Figs. 10a&c or 10b&d). Similar to the results in other Mach numbers of 0.9 [Samimy et al. 2007a] and 1.3 [Kim et al. 2008], the maximum reduction in OASPL increases with increasing mode number for both contoured and conical jets at 30 (Figs. 9a & c). The maximum reduction is about 1.5 (Fig. 9a) and 2.6 db 9

10 (Fig. 9c) for the contoured and conical jets, respectively. In the contoured jet, the reduction in m = 2 is comparable to m = 3 or ±4. In conical jet, modes 3 and ±4 are most effective and mode 2 is slightly less effective than these two. The significant increase in OASPL near St DF = 0.4 is due to actuator noise as will be observed in far-field spectra later (Fig. 10). The peak actuator noise is maximum for m = 0 since all actuators fire simultaneously. The next loudest actuator noise comes from m = ±4, where four actuator fire at the same time. For mode m = 1 and 3 no actuators were firing simultaneously. As a result, the actuator noise is lower for these modes. At a polar angle of 80 (near side line), no reduction is seen in contoured jet, and the reduction for the conical jet is less than 0.4 db. The OASPL was increased moderately at low St DF numbers for both jet (Figs/ 9d&d). At high St DF numbers, the effects of azimuthal mode are negligible as at 30 D. Effects on each noise component Some selected spectra are presented in this section to investigate the impact of forcing on supersonic jet noise component of screech tone, BSAN, and mixing noise. Figure 11 shows acoustic spectra at 30 for contoured and conical jets for m = 0. The small triangles on the top and bottom of each figure indicate the forcing Strouhal number. Note that the Strouhal number (St D ) is a non-dimensional farfield acoustic frequency measured by a microphone, while forcing Strouhal number (St DF ) is a nondimensional forcing frequency of the actuators. Azimuthal mode 0 For all cases, a strong forcing tone and its harmonics are seen these tones are actuator noise. At a low St DF number of 0.18, the mixing noise is not changed at all for either jet (Fig. 11a&d), and the noise level at high Strouhal numbers is increased due to forcing. At a moderate St DF number of 0.88, the effects of forcing on mixing noise are also minimal (Figs. 11b&e). The broadband noise level increased significantly at Strouhal numbers greater than the forcing Strouhal number for the contoured jet. The increase is not significant for the conical jet. At a high St DF number of 3.0, the forced spectrum for the both jets is almost identical to the corresponding baseline except for the forcing tone (Figs. 11c&e). (a) St DF = 0.18, Contoured jet (b) St DF = 0.88, Contoured jet (c) St DF = 3.0, Contoured jet (d) St DF = 0.18, Conical jet (e) St DF = 0.88, Conical jet (f) St DF = 3.0, Conical jet Fig. 11 Spectra at m = 0 at 30 polar angle for contoured and conical jets. 10

11 For the conical jet, the screech tones can be seen in the baseline profile. At a low St DF number of 0.18, the forcing tone becomes dominant and the previously dominant (baseline) screech tone is reduced by 6 db. The screech tone is reduced by 9 db at a St DF number of 0.88 and mixing noise also reduced slightly. At a high St DF number of 3.0, the screech tone is reduced slightly and the effects of forcing is minimal at this St DF number. (a) St DF = 0.18, Contoured jet (b) St DF = 0.70, Contoured jet (c) St DF = 1.80, Contoured jet (d) St DF = 0.18, Conical jet (e) St DF = 0.70, Conical jet (f) St DF = 1.80, Conical jet Fig. 12 Galilean streamlines superimposed on conditionally-averaged streamwise velocity at m = 0. Figure 12 shows Galilean streamlines superimposed on conditionally-averaged streamwise velocity fields at m = 0 for both jets. The forcing Strouhal numbers in this figure do not match those in Fig. 11, but the flow structure patterns for St DF of 0.7 and 1.8 are very similar to those of St DF of 0.88 and 3.0, respectively. As mentioned earlier, closed or spiral streamlines indicate the presence of vortical structures. At St DF = 0.18 (Fig. 12a, corresponding spectrum in Fig. 11a) for the contoured jet, the structures are random and not well organized as in the baseline jet (Fig. 7a). It seems that there is little effect of forcing on either acoustics or flow field at St DF = 0.18 for the contoured jet. Well organized, periodic structures are observed at St DF = 0.70 (Fig. 12b), which are generated by forcing. From Figs. 11b and 12b, it is hard to figure out the contribution or role of these structures in acoustic fields. At St DF = 1.8, there are random, less organized structures for the contoured jet (Fig. 12c). For m = 0, the flow field at St DF s greater than 1.0 is very similar to that for the baseline (supporting results are not shown here). The available data is insufficient to draw any correlation between the flow field and the acoustic field. For the conical jet, there is a naturally-occurring screech tone as shown in Figs. 4 and 11. Also there are very well organized large-scale structures in the jet plume as shown in Fig. 7d. These structures are naturally occurring due to the feedback loop. Forcing the conical jet under design conditions is expected to be very similar to forcing an imperfectly-expanded supersonic jet since both cases have shock trains and feedback loops in the jet plume. When an imperfectly-expanded Mach 1.3 jet was forced by the same plasma actuators, there was a competition for energy between naturally-occurring and forced structures [Kim and Samimy 2009]. A similar competition is observed in forcing the conical jet. At low and high St DF of 0.18 and 1.8 (Figs. 12d&f), the identified structures are very similar to those in the corresponding baseline (Fig. 7d). However, the forced structures prevail at St DF = 0.70 as evidenced by the in-phase structures by m = 0 mode forcing (Fig. 12e). Each forced case is compared to the baseline for the conical jet to find out how the screech tone is changed. At St DF = 0.18, the strength of structures appears to be less than that for the baseline (compare Figs. 7d and 12d). This reduced strength seems to be responsible for the reduced tone level in Fig. 11d. The structures are less energetic at St DF = 0.70 since they are smaller than those in the baseline. In this 11

12 case, a schlieren image of the shock-cell structures is required to determine if the strength of shock cells is changed or if the shock structures are deformed by forcing. Although further investigation regarding shock cell strength is needed, less energetic structures are partially responsible for the reduced tone level (Fig. 11e). At a high St DF of 1.8 (Fig. 12f), the jet large-scale structures are very similar to those in the baseline, and also the acoustic spectrum is very similar to the baseline (Fig. 11f). (a) St DF = 0.18, Contoured jet (b) St DF = 0.88, Contoured jet (c) St DF = 3.0, Contoured jet (d) St DF = 0.18, Conical jet (e) St DF = 0.88, Conical jet (f) St DF = 3.0, Conical jet Fig. 13 Spectra at m = 0 at 80 polar angle for contoured and conical jets. Figure 13 shows spectra at 80 and m = 0 for both jets. For the contoured jet, there is no effect of forcing on the broadband noise except for the actuator noise at all St DF s. At St DF = 0.18 for the conical jet (Fig. 13d), the screech tone level is reduced by about 5 db in the fundamental and second harmonics. The hump for BSAN is shifted slightly suggesting the shock cell structures are changed by forcing. A similar trend is observed at a moderate St DF of The forced spectrum at St DF = 3.0 (Fig. 13f) is almost identical to the baseline except for the forcing tone, as was observed at 30 (Fig. 11f). It seems that the jet, regardless of nozzle geometry, does not respond to the forcing at high St DF s as Figs. 11f, 12f, and 13f suggest. Azimuthal mode 3 Spectra measured at 30 for m = 3 are shown for both contoured and conical jets (Fig. 14). The forcing tone is less significant at this mode since the actuators do not fire simultaneously. At low and high St DF number of 0.18 and 3.0, little change in mixing noise is observed (Figs. 14a &c) for both jets. The broadband noise around the St DF and the peak mixing noise are reduced moderately at a St DF number of 0.88 in both jets. For the conical jet, the screech tone present in the baseline is completely suppressed at a St DF number of 0.88 (Fig. 14e). However, no suppression is observed at a higher St DF number of 3.0 (Fig. 14f) as in the contoured jet. For the conical jet, visualized flow structures are shown in Fig. 15. At St DF = 0.18 (Fig. 15a), it appears that the structures are less organized and weakened when compared to those in the baseline (Fig. 7d). A reduction of screech tone by about 9 db in Fig. 14d may be due to the weakened structures. The structures are weakened significantly when the jet is forced at St DF = 0.70 (Fig. 13b). Well organized 12

13 structures in the baseline jet seem to be suppressed by forcing so that no periodic, organized structures are observed at this forcing Strouhal number. This is consistent with the completely suppressed screech tone in Fig. 14e. At a high St DF of 1.8, the structures are very similar to those in the baseline of the conical jet. As discussed earlier, there was no change in spectra at a high St DF of 3.0 (Fig. 14f). Thus, the effects of forcing on flow and acoustic fields are negligible at a high St DF of 1.8. (a) St DF = 0.18, Contoured jet (b) St DF = 0.88, Contoured jet (c) St DF = 3.0, Contoured jet (d) St DF = 0.18, Conical jet (e) St DF = 0.88, Conical jet (f) St DF = 3.0, Conical jet Fig. 14 Far-field spectra at 30 and m = 0 for contoured and conical jets. (a) St DF = 0.18 (b) St DF = 0.70 (c) St DF = 1.8 Fig.15 Galilean streamlines superimposed on conditionally-averaged streamwise velocity at m = 3 for the conical jet. IV. Conclusions Preliminary results are reported on control of an axisymmetric jet from contoured and conical nozzles at the design condition of Mach 1.65 using plasma actuators. Forcing parameters include azimuthal mode and Strouhal number. The available forcing azimuthal modes with eight actuators are m = 0-3, ±1, ±2, and ±4. The range of forcing Strouhal numbers is 0.18 to 4.0. Far-field noise is measured by a linear array of nine microphones and the jet flow fields are measured by a PIV system. The jet from the contoured nozzle (contoured jet) shows near shock-free far-field spectra and very weak undulation of the centerline streamwise velocity. In the jet from the conical nozzle (conical jet), there are strong screech tones in the far-field spectra and a significant undulation of centerline streamwise velocity due to the feedback loop. In the baseline/unforced conical jet, well organized, periodic large- 13

14 scale structures are observed. Three shock-cell trains, originating from different nozzle inside and exit plane locations, are observed. The shock train emanating from the nozzle lip is the strongest. The contoured jet s response to control is very similar to what was observed in the subsonic Mach 0.9 and perfectly-expanded Mach 1.3 jets with the same actuators. At the far-field 30 polar angle, the maximum noise reduction increases as the mode number increases. The maximum noise reduction is about 1.5 db at m = 3 or ±4 and at a forcing Strouhal number of 0.7. However, no reduction is observed at 80 polar angle. In the conical jet, there is a competition for energy between naturally-occurring and forced structures similar to what was observed in an imperfectly-expanded jet. At moderate forcing Strouhal numbers, the forced structures prevail and, as a result, the naturally-occurring structures are suppressed. The suppression of naturally-occurring structures seems to be responsible for the reduced screech tones at moderate forcing Strouhal numbers. However, the naturally occurring structures are not suppressed by forcing at low and high forcing Strouhal numbers. Thus, the reduction in screech tone level is less (at low forcing Strouhal numbers) or negligible (at high Strouhal numbers). At the far-field 30 polar angle in the conical jet, a greater reduction occurs at the higher azimuthal modes of 3 or ±4, as in the jet with contoured nozzle. The maximum reduction is about 2.5 db at a forcing Strouhal number of 0.8. At high forcing Strouhal numbers, the effects of azimuthal modes are negligible as in the contoured jet. At 80 polar angle, the effects of forcing on noise are negligible as in the contoured nozzle. Acknowledgment The support of this work by NAVAIR (with John Spyropoulos) and by NASA Glenn Research Center (by James Bridges and Cliff Brown) is greatly appreciated. Fruitful discussion with Steve Martens of GE Global is also appreciated. References Back, L.H. and Cuffel, R.F., Detection of Oblique Shocks in a Conical Nozzle with a Circular- Arc Throat, AIAA Journal, Vol. 4, No. 12, 1966, pp Cohen, J., and Wygnanski, I., The evolution of instabilities in the axisymmetric jet. Part 1. The linear growth of disturbances near the nozzle, Journal of Fluid Mechanics, Vol. 176, 1987, pp Crow, S., and Champagne, F., Orderly Structure in Jet Turbulence, Journal of Fluid Mechanics, Vol. 48, 1971, pp Hussain, A.K.M.F., and Zaman, K.B.M.Q., The preferred mode of the axisymmetric jet, Journal of Fluid Mechanics, Vol. 110, 1981, pp Kim, J.-H, Kastner, J., and Samimy, M., Active Control of High-Subsonic Jets, AIAA Journal, Vol. 47, No. 1, 2009, pp Kim, J.-H, Adamovich, I., and Samimy, M., Active Noise Control in a Mach 1.3 Ideally- Expanded Jet with Plasma Actuators, AIAA Paper No , Kim, J.-H. and Samimy, M., Effects of Active Control on Flow Structure in a High Reynolds Number Supersonic Jet, To appear in International Journal of Flow Control, Liu, J., Kailasanath, K., Munday, D., and Gutmark, E., Investigation of Near-Field Acoustic Properties of Imperfectly Expanded Jet Flows Using LES, AIAA Paper No , Munday, D., Heeb, N., Gutmark, E., Markus O. Burak, M.O., and Eriksson, L.-E., Supersonic Jet Noise from a Conical C-D Nozzle with Forward Flight Effects, AIAA Paper No , Powell, A, On the noise emanating from a two-dimensional jet above the critical pressure, Aeronautical Quarterly, Vol. 4, 1953a, pp Powell, A., On the mechanism of choked jet noise, Proceedings of the Physical Society of London, Vol. B66, 1953b, pp

15 Raman, G., Supersonic Jet Screech: Half-Century from Powell to the Present, Journal of Sound and Vibration, Vol. 225, No. 3, 1999, pp Reynolds, W.C., and Bouchard, E.E., The effect of forcing on the mixing-layer region of a round jet, Unsteady Shear Flows, Springer, Berlin/Heidelberg/New York, 1981, pp Samimy, M., Kim, J., Kastner, J., Adamovich, I. and Utkin, Y., Active Control of a Mach 0.9 High Reynolds Number Jet for Noise Mitigation Using Plasma Actuators, AIAA Journal, Vol. 45 No. 4, 2007a, pp Samimy, M., Kim, J.-H., Kastner, J., Adamovich, I. and Utkin, Y., Active Control of High- Speed and High Reynolds Number Jets Using Plasma Actuators, Journal of Fluid Mechanics, Vol. 578, pp , May 2007b. Seiner, J.M., Advances in High Speed Jet Aeroacoustics, AIAA/NASA 9 th Aeroacoustics Conference,. AIAA , Shih, F.S., Alvi, D.M., Effects of Counterflow on the Aeroacoustic Properties of a Supersonic Jet, Journal of Aircraft, Vol. 36,, 1999, pp Tam, C.K.W., Golebiowski, M., and Seiner, J.M., On the two components of turbulent mixing noise from supersonic jets, AIAA Paper , Utkin, Y., Keshav, S., Kim, J.-H., Kastner, J., Adamovich, I., and Samimy, M., Use of Localized Arc Filament Plasma Actuators for High Speed Jet Control, Journal of Physics D: Applied Physics, Vol. 40, pp , February Zaman, K.B.M.Q., and Hussain, A.K.M.F., Vortex pairing in a circular jet under controlled excitation. Part 1. General Jet Response, Journal of Fluid Mechanics, Vol. 101, 1980, pp