[American Institute of Aeronautics and Astronautics 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition - Orlando, Florida ()] 48th AIAA

American Institute of Aeronautics and Astronautics

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PIV around a NACA0012 Airfoil with a Plasma Actuator for Noise Reduction

Shunsuke Koike1, Kazunori Mitsuo2, Hiroyuki Kato2, Hiroki Ura1, Shigeya Watanabe3 Japan Aerospace Exploration Agency (JAXA), Chofu, Tokyo, 182-8522, Japan

and

Motofumi Tanaka4 Toshiba Corporation, Tokyo, 105-8001, Japan

A dielectric barrier discharge plasma actuator (PA) was applied to reduce trailing edge noise and vortices emitted from a NACA0012 airfoil model. The PA in length of 1 m was installed at 60 % of the chord length on the pressure side of the model. The velocity field around the trailing edge of the model was measured via 2D-PIV at the uniform flow velocity of 15, 30, and 40 m/s. The Reynolds numbers based on the code length of the model for the three cases were 2.0x105, 4.0x105, and 5.4x105, respectively. When the PA was off, the measured velocity and the result of the proper orthogonal decomposition showed that the strong vortices were periodically emitted from the trailing edge of the model at the uniform flow velocity of 15 m/s. These trailing vortices were dramatically reduced by the PA. As the uniform flow velocity increased, the trailing edge vortices decreased. As the result, the effect of the PA was decreased as the uniform flow velocity increased.

Nomenclature ai = mode coefficient of proper orthogonal decomposition defined by Eq. (2) C = two point correlation matrix CRww = coefficient of spatial cross correlation of w defined by Eq. (1) c = cord length of the airfoil M = number of the grid points in proper orthogonal decomposition N = number of the sample of the velocity vectors t = time U = matrix defined by Eq. (7)

U = velocity of the uniform flow

u = velocity component in the uniform flow direction, x v = velocity component in the spanwise direction, y w = velocity component in the vertical direction, z x = coordinate in the uniform flow direction defined by Fig. 3 y = coordinate in the spanwise direction defined by Fig. 3 z = coordinate in the vertical direction defined by Fig. 3

iφ = base function of proper orthogonal decomposition defined by Eqs. (2) and (3)

i (ramda) = eigen value of the two point correlation matrix C and the energy of the POD mode i

1 Researcher, Wind Tunnel Technology Center, JAXA, 7-44-1 Jindaiji-Higashi, Chofu, Tokyo, Member AIAA. 2 Associate Senior Researcher, Wind Tunnel Technology Center, JAXA, 7-44-1 Jindaiji-Higashi, Chofu, Tokyo, Member AIAA. 3 Group Leader, Fluid Dynamics Group, JAXA, 7-44-1 Jindaiji-Higashi, Chofu, Tokyo, Senior Member AIAA. 4 Specialist, Power and Industrial Systems Research and Development Center, Toshiba Corporation, 2-4, Suehiro-cho, Tsurumi-ku, Yokohama.

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

AIAA 2010-1412

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


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Superscript

= average in time

' = fluctuation value

I. Introduction ecently, it is required to reduce the airframe noise from an aircraft especially during landing and take-off phases. So, the development of the noise reduction techniques is one of the important issues for the development of

aircrafts. Dielectric barrier discharge plasma actuator (PA) is one of the promising devices for the aero acoustic noise reduction because of its fast response and its control flexibility [1, 2]. In order to investigate the basic effect of the PA on the aero acoustic noise reduction, the PA was applied to reduce trailing edge noise [3-6] from an NACA0012 airfoil. It have been reported in previous researches that the trailing edge noise can be reduced by the passive flow control devices. One of those techniques is roughness tape to disturb the boundary layer [5]. However, it is difficult to use the passive control devices effectively for the various flow conditions. Sometimes, such devices produce the noise and the drag. So, the PA which is one of the active control devices was applied to the trailing edge noise reduction as the basic noise reduction study.

In this research, first, we measured the sound pressure level via a microphone array in the various cases [7]. Then the velocity fields were measured in the selected cases via 2D-PIV. Although we mainly report the result of the velocity field in this paper, the result of the sound pressure level is briefly described here. Figure 1 shows the sound pressure level to the frequency at the uniform flow velocity of 15 m/s. The sound pressure level shown in Fig. 1 is the maximum spectrum around the trailing edge on the cord plane of the airfoil. The difference of the sound pressure level between the PA ON and OFF cases was observed ranging from 15 m/s to 30 m/s. The difference was not observed at the uniform flow velocity of 40 m/s. At the uniform flow velocity of 15 m/s, the peak spectrum was observed at about 420 Hz. At the uniform flow velocity of 30 and 40 m/s, the peak spectrum was much lower than that at 15 m/s [7]. The objective of the PIV measurement is to show and confirm that the PA reduces the trailing edge vortices that cause the peak noise observed in Fig. 1. First, the characteristics of the averaged velocity and velocity fluctuation are shown. Second, the spatial cross correlation and the result of the proper orthogonal decomposition are shown in order to discuss the relation between the velocity fields and the sound pressure level.

II. Experimental Setup and Conditions

A. Wind tunnel The experiment was carried out in 2m x 2m Low-speed Wind Tunnel (LWT2) of Wind Tunnel Technology

Center in Japan Aerospace Exploration Agency (JAXA). The LWT2 is a closed-circuit type wind tunnel which has 2m x 2m rectangular test section. In the 6m x 6m setting chamber, honeycomb and wire screen are placed to reduce flow deflection and vortex. Relatively high contraction ratio of 9.0 realizes low turbulence at the test section. Uniform flow of this wind tunnel shows relatively low turbulence of uniform flow below 0.06% at 60m/s at an appropriate size of test section [5].

B. Model A NACA0012 airfoil model was used in this study. The photo of the model with the activated PA is shown in Fig.

2. The size of the model was 1000 mm in wing span, and 200 mm in the chord length. The angle of attack of the model was -2.5 degree in all cases. So, the upper surface was the pressure surface. The model was supported by two struts. The electrical lines for the PA and the tubes for the pressure measurements were fixed to the struts.

R

U∞=15m/s

0

20

40

60

80

100

0 1000 2000 3000 4000Frequency[Hz]

SPL

[dB

(F)]

PA ON

PA OFF

Figure 1. Sound pressure level (SPL) measured viaa microphone array system at the uniform velocity of 15 m/s. The solid and the broken line show the activated and not activated plasma actuator case, respectively.


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In order to produce the trailing edge noise and vortices, two roughness tapes were attached to the model [8]. The position of the roughness tapes and the PA is shown in Fig. 3. The PA was attached on the model surface at 60 % x/c. One roughness tape was attached at 20 mm upstream from the PA on the upper surface. Another was attached at 70 % x/c on the lower surface. The width and the thickness of the roughness tape were 12 mm and 140 m, respectively.

C. Dielectric barrier discharge plasma actuator The PA used in this study is illustrated in Fig. 4. The

PA consists of a high voltage supplier and two electrodes that are separated by a dielectric barrier material. Copper tapes and Kapton® tapes were used as the two electrodes and the dielectric barriers, respectively. The total thickness and width of the PA were 380 m and 25 mm. The thickness and width of the each tape are shown in Fig. 2.When a high voltage a.c. input is supplied to the electrodes, the air around the electrodes is locally ionized and the flow is induced. In the experiments, it was confirmed that the noise from the PA could not be neglected. So, 15 kHz that was over the audio frequency was selected as the frequency of the a.c. input. The applied voltage was set at 6 kV in all cases. As shown in Fig. 2, the violet light was emitted from the PA when the device was on. At the applied voltage of 6 kV, the violet light was observed at the all position over the surface of the PA.

D. PIV system Velocity fields were measured via a conventional

2D-PIV system. The tracer particle was mist of Dioctyl sebacate (Density: 913.5kg/m3, Diameter: about 1m) produced via Laskin nozzle. The Nd:YAG laser (CFR200, Quantel Twins Inc., 200 mJ/pulse) was used as the light source. The laser sheet was induced from the upper window as shown in Fig. 2 (green sheet). The CCD camera (Image Pro Plus 4M, Lavision Inc. 2048 x 2048 pixels, 14 bit) was set at the side of the test section window. The particle image was acquired at 4 Hz. About 2000 pairs of pictures were taken in all cases. The analyzing software Davis 7.2 was used to calculate the velocity vectors. The interrogation area was 32 x 32 pixels and 50 % overlap. The interval of grid point is 0.5 mm. Vectors which have any velocity component exceeding the 5 times of the standard deviation level are omitted as erroneous vectors, and this process is repeated three times to get the final validated vectors.

E. Coordinate and experimental conditions The coordinate shown in Fig. 3 is used in this paper. The right hand system is used. The origin is defined at 25 % x/c on the chord in the center plane of the model. The direction of the uniform flow is defined as x. The spanwise and vertical directions are defined as y and z. The velocity components in the x, y, and z direction are defined as u, v, and w, respectively. Only the u and w were measured in this experiment.

Plasma actuator

Laser sheet

Plasma actuator

Laser sheet

Figure 2. Picture of the NACA0012 airfoil model and the plasma actuator. Violet line on the airfoil is the activated plasma actuator. The green sheet from the upper window is the laser light sheet for PIV.

Figure 3. Coordinate and the measurement areas of PIV. The three areas A, B, and C were measured only for the15 m/s case. Only the area C was measured for the 30 and 40 m/s cases.

Model

80μm（50μm）25 mm

80μm（50μm）25mm



Thickness 80μm（35μm）Width 5 mm

Model





Thickness 80μm（35μm）Width 5 mm

Figure 4. Cross section structure of the plasma actuator. Total thickness of the plasma actuator including the paste is 380 m. The numbers in parentheses show the thickness of the tape not including the paste.


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The experimental conditions are shown in Table 1. The velocity measurements were conducted at three uniform flow conditions, 15, 30, and 40 m/s. The Reynolds numbers based on the code length of the NACA0012 airfoil for these conditions were 2.0 x 105, 4.0 x 105, and 5.4 x 105, respectively. The velocity measurement was conducted for three areas, A, B, and C in Fig. 1 at y = 10 mm. The three areas were measured in 15 m/s cases. In other cases, only the area C was measured. In this paper, the representative length and velocity are defined as the uniform flow velocity U and the cord length c. The measurement results are normalized by these values. The operating

conditions of the PA were same in all cases. The applied voltage and the frequency of the a.c. input were 6 kV and 15 kHz.

Table 1 Experimental conditions

Case Uniform flow

velocity, m/s

PA PositionVoltage,

kV Frequency,

kHz Measurement

Area

15m/s PA OFF

15 OFF 60% x/c 6 15 A,B,C

15m/s PA ON

15 ON 60% x/c 6 15 A,B,C

30m/s PA OFF

30 OFF 60% x/c 6 15 C

30m/s PA ON

30 ON 60% x/c 6 15 C

40m/s PA OFF

40 OFF 60% x/c 6 15 C

40m/s PA ON

40 ON 60% x/c 6 15 C

III. Result

A. Averaged velocity Figure 5 shows the ensemble average of the velocity magnitude on x-z plane at the uniform flow velocity of 15

m/s when the PA was on. The white region over the PA is the region where the particle image for the PIV could not be obtained because of the violet light emitted from the PA. Because of the reflected laser light from the model, the quality of the particle image in the region close to the surface of the model was wrong. So, the velocity vector in the

Figure 5. Magnitude of velocity vector on x-z plane at the uniform flow velocity of 15 m/s. (The plasma actuator is activated.) The white region over the plasmaactuator could not be measured because of the violet lightemitted from the actuator.

Figure 6. Difference of the velocity vector between the activated and not activated plasma actuator cases on x-z plane at the uniform flow velocity of 15 m/s. The boundary layer is accelerated by the plasma actuator（ red region). The velocity in the wake region is decelerated (blue region).


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region was also eliminated. The main structure of the averaged velocity field around the trailing edge was almost same in all cases. The averaged velocity on the surface of the airfoil decreased as the flow was going to the trailing edge. The low speed wake region whose height was about 0.05c was observed.

The difference of the velocity magnitude on x-z plane between the PA ON and OFF cases at the uniform flow velocity of 15 m/s is shown in Fig. 6. Figures 7 and 8 show the profiles of the magnitude of the velocity vector and the velocity components u on the several lines. These figures lead two important points between the PA ON and OFF cases.

1. The averaged velocity in the boundary layer was

clearly accelerated by the PA at the uniform flow velocity of 15 m/s.

2. The averaged velocity in the wake region

decreased especially at the uniform flow velocity of 15 m/s when the PA was on.

Although the first point is not clear in Fig. 6, it is clearly observed in Fig. 7. The velocity profiles at x/c = 0.6 and 0.7 show that the velocity in the boundary layer in the PA ON case was higher than that in the PA OFF case. The maximum difference of the averaged velocity between the PA ON and OFF cases was about 5 % of the uniform flow velocity at x/c = 0.6. The second point is observed in Fig. 6 and Fig. 8. The averaged velocity in the wake region fairly decreased in the PA ON case. The reason why the velocity in the wake region was low in the PA OFF case can be considered as follows. As mentioned in the proper orthogonal decomposition analysis section, the trailing edge vortices on this plane were dramatically suppressed by the PA at the uniform flow velocity of 15m/s. So, the fluctuation in the wake region was also suppressed. As the result, the high speed flow did not flow into the center region of the wake. Furthermore, the transportation of the momentum in the uniform flow direction decreased. Therefore, the averaged velocity in the wake region was low when the PA was on.

Figure 7. Profile of the velocity magnitudeover the airfoil. Open symbols show the PAOFF case. Closed symbols show the PA ONcase. The boundary layer is accelerated at x/c = 0.60 (green line) and x/c=0.70 (orange line).

Figure 8. Profile of the streamwise velocity component in the wake region. Open symbols show the PA OFF case. Closed symbols show the PA ON case. The black, blue, and red lines show the profile of the velocity component at x/c = 0.775, 0.875, and 0.975.

Figure 9. Profile of the streamwise velocity component in the wake region at the uniform velocity of 15, 30, and 40 m/s. Open symbols show the PA OFF case. Closed symbols show the PA ON case. The black, blue, and red lines show the profile at the uniform flow velocity of 15, 30, and 40 m/s, respectively.


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As shown in Fig. 9, the difference of the velocity in the wake region between the PA ON and OFF cases decreased as the uniform flow velocity increased. It is difficult to observe the difference between the PA ON and OFF cases from the velocity profiles at the uniform flow velocity of 30 and 40 m/s.

B. Velocity fluctuation Figures 10 and 11 illustrate the contour figures of the standard deviation of the streamwise and the vertical

velocity components on x-z plane at the uniform flow velocity of 15 m/s. From x/c = 0.5, the fluctuation of the velocity components increased. The fluctuation of the both components was the maximum value at the point that was a little downstream of the trailing edge. In Fig. 12, the standard deviation of the both components and

2Uwu '' on the line at x/c = 0.775 are shown. It is clear that the fluctuation at the uniform flow velocity of 15 m/s

was much higher than that at the other uniform flow velocity. From this result, it can be estimated that the strong trailing edge vortices were emitted only in the 15 m/s case.

Following points are observed from Figs. 10 to 12 as the difference between the PA ON and OFF cases. 1. The fluctuation was high on the upper side at the uniform flow velocity of 15 m/s. The fluctuation

especially on the upper side was suppressed by the PA at the uniform flow velocity of 15 and 30 m/s.

2. The velocity fluctuation was suppressed by the PA especially at the uniform flow velocity of 15 m/s. The difference between the PA ON and OFF cases at the uniform flow velocity of 30 m/s was smaller than that at 15 m/s. There was hardly any difference of the velocity fluctuation between the PA ON and OFF cases at the uniform flow velocity of 40 m/s.

First point is clearly observed in Figs. 10, 11, and 12. The fluctuation on the upper side is fairly higher than that

on the lower side in Figs. 10(a), 11(a), and 12. When the PA was on, the fluctuation of the both velocity components decreased. It is also fairly observed in Figs. 10, 11, and 12 that the fluctuation on the upper side was more strongly suppressed by the PA. The difference between the PA ON and OFF cases is large on the upper side in Fig. 12.

PAPA

(a) PA OFF case

PAPA

(b) PA ON case Figure 10. Standard deviation of the streamwisevelocity components on x-z plane at the uniformflow velocity of 15 m/s.

PAPA

(a) PA OFF case

PAPA

(b) PA ON case Figure 11. Standard deviation of the vertical velocity components on x-z plane at the uniform flow velocity of 15 m/s.


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Second point is also observed in Fig. 12. The difference between the PA ON and OFF cases decreased as the uniform flow velocity increased. The difference was small at the uniform flow velocity of 30 m/s. There was hardly any difference between the PA ON and OFF cases at the uniform flow velocity of 40 m/s.

C. Spatial cross correlation From the standard deviation of the velocity components,

it is estimated that the PA suppressed the vortices from the trailing edge and suppressed the sound related to the vortices. However, the frequency of the vortices cannot be estimated from the statistical values described above. The relation between the trailing edge vortices and the spectrum of the sound pressure level cannot be discussed. Spatial cross correlation was calculated from the measured velocity data in order to clarify the characteristic of the trailing edge vortices. Here, the coefficient of the spatial cross correlation CRww defined as Eq. (1) was shown.

2/1

22

,

rxx

rxxrx

ww

wwCRww (1)

In the Eq. (1), x is the positional vector of the reference point. x+r is the positional vector of the point at which the correlation to the reference point is calculated. In Fig. 13, the contours of the CRww at the uniform flow velocity of 15 m/s are illustrated. Figs. 13(a) and (b) show the CRww in the measurement area B. Figs. 13(c) and (d) show the CRww in the measurement area C. The black points in Fig. 13 show the position of the reference point x. The distribution shows the coefficient of the spatial cross correlation CRww of the point x+r to the reference point x. Therefore, if the direction and the length of the vertical velocity fluctuation, w’, at the point x+r are similar to those at the reference point x, the CRww at the point is positive and close to one. If the direction is opposite, the CRww at the point x+r is negative and close to minus one. If there is no relation between the velocity fluctuations at the two points x and x+r, the CRww is zero. When the vortices are emitted cyclically, the magnitude of the spatial cross correlation is cyclically high in the space. Furthermore, the length between the maximum points of the CRww roughly shows the distance between the cyclic vortices. The characteristics of the spatial cross correlation CRww and Fig. 13 lead the following findings.

1. Cyclic vortices existed on the upper surface of the model at the uniform flow velocity of 15m/s when the PA was off. The cyclic vortices on the upper surface disappeared when the PA was on.

2. Cyclic vortices also existed in the wake region at

the uniform flow velocity of 15 m/s when the PA was off. The cyclic vortices in the wake were suppressed by the PA.

(a) Uu 2

(b) Uw 2

(c) 2 Uwu

Figure 12. The statistical values of the velocityfluctuation in the wake region. The open symbolsshow the PA OFF case. The closed symbols show the PA ON case. The black, blue, and red linesshow the values at the uniform flow velocity of 15,30, and 40 m/s, respectively.


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As shown in Figs. 13(a) and (c), the coefficient of the spatial cross-correlation CRww was cyclically high in the PA OFF case at the uniform flow velocity of 15 m/s. On the other hand, the CRww shown in Figs. 13(b) and (d) was high only in the region close to the reference point x. From this result, it has been confirmed that the PA suppressed the cyclic vortices on the upper surface of the model and those in the wake region. Figure 14 shows the CRww on the line passing through the reference point x in the area C for all cases. The CRww is cyclically high only in the 15m/s PA OFF case. At the other uniform velocity, the CRww is low except in the region close to the reference point x. Therefore, this figure leads following finding.

3. When the PA was off, cyclic vortices were strong at the uniform flow velocity of 15 m/s and weak at the other uniform flow velocity. The effect of the PA was highest at the uniform flow velocity of 15 m/s.

D. Proper orthogonal decomposition (POD) analysis In the Fig. 14, the line which corresponds to the 15

m/s PA OFF case has three maximum points. The first peak is located at the reference point. The distance between the first and second peaks is about 36 mm. The frequency of the vortices is estimated at 416 Hz from the distance between the first and second peak assuming that the convective velocity of the vortex is same as the uniform flow velocity, 15 m/s. The frequency, 416 Hz almost agrees with the first peak of the sound pressure level shown in Fig. 1. The distance between the first and third peaks is about 18 mm. The frequency estimated from this distance is 833 Hz. This frequency almost

PAPA

(a) PA OFF

PAPA

(b) PA ON

(c) PA OFF

(d) PA ON

Figure 13. Coefficient of the spatial cross correlation at the uniform flow velocity of 15 m/s. The black solid point shows the reference point for the calculation of the spatial cross correlation. (a) and (b) show the area B in Fig. 3. (c) and (d) show the area C.

Figure 14. Profile of the spatial cross correlation on the line passing through the reference point (z/c = 0.4). Open symbols show the PA OFF case. Closed symbols show the PA ON case. The black, blue, and red lines show the values at the uniform flow velocity of 15, 30, and 40 m/s, respectively.


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agrees with the frequency of the second harmonic wave of the first peak. Here, Proper Orthogonal Decomposition (POD) analysis was conducted in order to separate the vortices relating to the first peak of the sound pressure level from the other velocity fluctuation.

Time resolution of the 2D-PIV we used in this study was very low. So, the snapshot POD proposed by Sirovich [9] was used. Van Oudheusden et al. [10] and Graftieaux et al. [11] successfully decomposed the cyclic vortices using this method. We referred the POD method in these reports [9-11]. Here a brief description is given of the method of the POD analysis.

In this analysis, the fluctuating part of the velocity field was decomposed as Eq. (2).

M

iii tatt

1

xφxu,xuxu,xu (2)

Here, xu and t,xu denote the averaged and the fluctuation velocity vector. In this study, only two velocity

components, u and w were measured. So the t,xu consists of two velocity components. M denotes the number of

the grid points for the POD analysis. tai and xφ i denote the mode coefficients and the normalized base

function. The normalized base functions xφi are orthogonal in space. The mode coefficients tai are

uncorrelated in time.

1 xφxφ ji ji

0 ji (3)

iji tata ji

0 ji (4)

Here, a bracket, < >, and a bar over the variable, , indicate the spatial integration and the temporal averaging,

respectively. The normalized base function xφi can be obtained from the next Eq. (5).

iii φCφ (5)

Here, C denotes the two point correlation matrix. The normalized base functions xφi and i are obtained as the

eigen vectors and eigen values of the matrix C. Matrix C can be obtained from the next Eq. (6).

T

NUUC

1 (6)

Here, N is the number of the sample. Matrix U is defined as the next Eq. (7).

NM

NM

N

N

M

M

tw

tu

tw

tu

tw

tu

tw

tu

,x

,x

,x

,x

,x

,x

,x

,x

U

1

1

1

1

11

11

(7)

The eigen values, i , represents the energy contribution of

the corresponding POD mode to the total fluctuating energy. Here, mode number is defined by the next equation. M 21 (8)

If the cyclic vortices dominate the main velocity fluctuation, the eigen values i of the modes that represent such cyclic

vortices are high. Therefore, it is meaningful that the eigen values i of the each experimental case are compared to

investigate the existence and the intensity of the cyclic vortices. In Fig. 15, the eigen value i normalized by the total

fluctuating energy is shown. Horizontal axis shows the mode number i. The eigen values of the modes 1, 2, 3, and 4 at the uniform flow velocity of 15 m/s are extremely higher than

Figure 15. Eigen value spectrum calculated from POD analysis for velocity fluctuation in the area C. POD analysis was performed for the area illustrated in Fig. 16. Open symbols show the PA OFF case. Closed symbols show the PA ON case.


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those of the other modes. The i of the mode 2 is almost same as that of the mode 1. The i of the mode 4 is

almost same as that of the mode 3. It is expected from this result that the mode 1 and 2 are pair and the mode 3 and 4 are also pair. In the 15 m/s PA ON case, those eigen values decreases from those values in the 15 m/s PA OFF case. There was small difference of the eigen value i between the PA OFF and ON cases at the uniform flow velocity of

30m/s. The i of the low number mode in the PA OFF case is higher than those in the PA ON case. There was

hardly any difference of the i at the uniform flow velocity of 40 m/s.

(e) Mode 1

(f) Mode 2

(g) Mode 3

(h) Mode 4

Figure 17. POD modes of the PA ON case at the uniform flow velocity of 15 m/s. The vector shows the eigen vector of the each mode. Contour lines show the nondimensional vorticity calculated from eigen vectors.

(a) Mode 1

(b) Mode 2

(c) Mode 3

(d) Mode 4

Figure 16. POD modes of the PA OFF case at theuniform flow velocity of 15 m/s. The vector shows theeigen vector of the each mode. Contour lines show thenondimensional vorticity calculated from eigen vectors.


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The normalized base functions xφi of the mode number 1, 2, 3, and 4 at the uniform flow velocity of 15 m/s

are illustrated in Figs. 16 and 17. Figures 16 and 17 show the PA OFF and ON cases. The contour lines show the nondimensional vorticity calculated from the normalized base function. It is clear that the modes 1 and 2 are pair and the modes 3 and 4 are also pair in the PA OFF case. It is observed that the phase differences between the modes 1 and 2 or modes 3 and 4 are about 180 degree. The nondimensional vorticity is higher on the upper side than that on the lower side in the modes 1 and 2. The distances between the centers of the vortices are 36 mm in the modes 1 and 2 and 18 mm in the modes 3 and 4. These distances agree with the distances between the maximum point of the cross correlation in Fig. 14. The estimated frequencies from the distances between the vortices are 416 Hz for modes 1 and 2 and 833 Hz for modes 3 and 4. These frequencies almost agree with the frequency of the first peak of the sound pressure level and the frequency of the second harmonic wave of the first peak shown in Fig. 1. The base function distributions shown in Fig. 17 are dramatically different from those in Fig. 16. This difference means that the cyclical trailing edge vortices observed in Fig. 16 were suppressed by the PA. These results are summarized as follows.

1. The cyclical trailing edge vortices were mainly emitted from the upper (pressure) side of the airfoil at the uniform flow velocity of 15 m/s in the PA OFF case.

2. The two pairs of the POD modes were predominant in the 15 m/s PA OFF case. The frequencies estimated

from those base functions were 416 Hz and 833 Hz, respectively. The lower frequency 416 Hz agrees with the first peak of the sound pressure level. The higher frequency 833 Hz agrees with the frequency of the second harmonic wave of the first peak.

3. When the PA was activated, the predominant cyclical vortices were suppressed. So the PA suppressed the

trailing edge vortices and suppressed the noise caused by the trailing edge vortices. As described above, the PA suppressed the trailing edge vortices. It is meaningful to discuss the mechanism how the PA suppressed the trailing edge vortices finally. At the uniform flow velocity of 15 m/s, the Reynolds number based on the cord length is 2.0 x 105. Around at this Reynolds number, it has been reported that the trailing edge noise relating to the Tollmien-Schlichting wave was emitted from the airfoil model [6]. In such cases, the transition point was close to the trailing edge of the airfoil [5, 6]. In this study, the roughness tapes were used in order to produce the trailing edge noise efficiently because the trailing edge noise was small without roughness tapes. As the result, the trailing edge noise was emitted efficiently with the roughness tapes. This technique was effective for the trailing edge noise relating to the TS wave because the roughness tapes play the role of the fluctuation accepter [8]. From these information and the Fig. 13(a) and Fig. 16, it is considered that the trailing edge vortices observed in this experiment related to the TS wave. If this hypothesis is accepted, the mechanism how the PA suppressed the trailing edge noise is explained as follows. The PA accelerated the boundary layer. The transition point moved to the upstream region. The flow around the trailing edge became turbulent. As the result, the trailing edge vortices related to the TS wave decreased. The noise related to the trailing edge vortices decreased.

IV. Conclusion A dielectric barrier discharge plasma actuator (PA) was applied to reduce trailing edge noise and vortices

emitted from a NACA0012 airfoil model. The PA in length of 1 m was installed at 60 % of the chord length on the pressure side of the model. The velocity field around the trailing edge was successfully measured via 2D-PIV at the uniform flow velocity of 15, 30, and 40 m/s. The measured velocity and the POD analysis lead the following conclusions. 1) The averaged velocity in the boundary layer was accelerated by the plasma actuator. At the uniform flow velocity of 15 m/s, the maximum difference of the averaged velocity between the PA ON and OFF cases was about 5 % of the uniform flow velocity. 2) The averaged velocity in the wake region was decreased especially at the uniform flow velocity of 15 m/s in the PA ON case because the fluctuation in the wake region was suppressed by the PA. The difference of the averaged velocity between the PA ON and OFF cases decreased as the uniform flow velocity increased.


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3) The velocity fluctuation was suppressed by the PA especially at the uniform flow velocity of 15 m/s. The difference between the PA ON and OFF cases was small at the uniform flow velocity of 30 m/s. There was hardly any difference of the velocity fluctuation at the uniform flow velocity of 40 m/s. 4) The spatial cross correlation and the result from the POD analysis show that the periodical trailing edge vortices were mainly emitted from the pressure side of the airfoil at the uniform flow velocity of 15 m/s in the PA OFF case. The result from the POD analysis shows that the four POD modes predominant the velocity fluctuation in this case. When the PA was activated, the predominant vortices were suppressed. 5) The frequency of the trailing edge vortices at the uniform flow velocity of 15 m/s was estimated from the distances between the vortices in the first and second POD modes assuming the convective velocity of the vortex was the uniform flow velocity. The estimated frequency was 416 Hz. This frequency almost agrees with the frequency of the first peak of the sound pressure level that was reduced by the PA. 6) It was confirmed from the conclusions 4) and 5) that the PA suppressed the trailing edge noise to suppress the predominant trailing edge vortices.

Acknowledgments The work described in this paper was carried out at the JAXA Chofu aerospace center. The author would like to

thank the member of the LWT2 and ATS sections in the wind tunnel technology center and thank the member of the fluid dynamics group. The Toshiba Corporation cooperated in the development of the plasma actuator and its operation. The author would like to thank the member of the Toshiba Corporation.

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[American Institute of Aeronautics and Astronautics 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition - Orlando, Florida ()] 48th AIAA