1

A Parametric Investigation of Plasma Streamwise Vortex Generator Performance

Michael Wicks, Flint O. Thomas, David Schatzman, Partick Bowles, and Thomas C. Corke University of Notre Dame, Notre Dame, IN, 46556

Mehul Patel and Alan B. Cain Innovative Technology Applications Company, LLC, Chesterfield, MO, 63017

In this paper a parametric investigation of arrays of plasma streamwise vortex generators (PSVG) is presented. The arrays are flush mounted to a flat plate on which a nominally zero pressure gradient turbulent boundary layer develops upstream of the array. The investigation is focused on characterization of the influence of freestream velocity, applied voltage, length of the covered electrode and spanwise inter-electrode spacing on actuator performance. The metrics used to assess performance are measured spanwise wall normal component mean velocity gradient, ∂V /∂z , and spanwise longitudinal mean velocity gradient, ∂U /∂z , downstream of the array. The former quantity is related to direct streamwise vorticity production and the latter is related to spanwise vorticity redistribution to the streamwise component through the term ωz ∂U /∂z . In addition, a direct comparison is made between the PSVG and passive vortex generators under identical flow conditions. Results demonstrate the ability of the PSVG to harvest energy from the external flow in a manner very similar to the passive VG. This unique feature of PSVG may be exploited for flow control applications.

Nomenclature Up = plasma induced jet velocity

U∞ = freestream velocity

U = Spanwise cycle-average mean velocity

L = covered electrode length λ = spanwise inter-electrode spacing Epp = peak-to-peak excitation voltage U = streamwise mean velocity component V = wall normal mean velocity component x, y, z = streamwise, wall-normal and spanwise spatial coordinates, respectively. ∂U /∂z = spanwise spatial derivative of streamwise mean velocity ∂V /∂z = spanwise spatial derivative of wall-normal mean velocity ωx = streamwise component mean vorticity δ = 99% boundary layer thickness δ * = boundary layer displacement thickness Γ x( ) = circulation

Rex = Reynolds number

50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition09 - 12 January 2012, Nashville, Tennessee

AIAA 2012-0824

Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

2

I. Motivation and Objectives ASSIVE vortex generators (VGs) are widely used for separation control on both commercial and military aircraft. There are a wide variety of passive vane type surface-mounted vortex generators as

reviewed by Lin [1] and Bushnell [2]. These often have vane heights on the order of the local boundary layer thickness δ, or smaller. Some designs create pairs of counter-rotating streamwise vortices while others produce co-rotating vortices. Regardless of the details of their design, the production of near-wall streamwise vortices that persist over a significant downstream distance function to promote cross-stream mixing of momentum across the boundary layer. This serves to transport relatively high momentum fluid from the outer region toward the wall and the boundary layer is therefore much better able to withstand an imposed adverse pressure gradient without undergoing separation. Passive VGs have several advantages. They are simple, robust, proven devices that are easily retrofitted to the airframe. However, their major detractor is a significant drag penalty incurred in portions of the flight envelope where they are not needed. It is desirable to have the ability to generate streamwise voriticity when boundary layer separation is a possibility and have the VG effectively disappear when not needed. This is the basis of plasma streamwise vortex generators (PSVG) which are the focus of this paper.

Psvgs are flush mounted devices designed to utilize the body force associated with dielectric barrier discharge (DBD) plasma actuation to create streamwise vortices. Readers unfamiliar with application of DBD plasma actuators for aerodynamic flow control are referred to the recent comprehensive reviews on the subject by Corke et al. [3] and Moreau [4]. One of the first applications of PSVGs for aerodynamic flow control was reported by Huang et al. [5] where plasma-induced streamwise vorticity was used for aeroacoustic control of a low-speed cavity resonance. More recently, Schatzman and Thomas [6] utilized PSVGs to eliminate separation of an adverse pressure gradient turbulent boundary developing on a convex ramp. PSVGs were also found to be quite effective in eliminating unsteady vortex shedding from a circular cylinder in cross flow at high subcritical Reynolds number by Kozlov and Thomas [7] and in a tandem cylinder flow configuration Kozlov and Thomas [8]. Okita et al. [9] reported delayed separation on a NACA 0024 airfoil due to the streamwise vortex produced by a single, yawed DBD actuator that essentially functioned as a PSVG. A detailed parametric investigation into DBD vortex generators was recently reported by Jukes and Choi [10]. The plasma vortex generator used in that study consisted of a DBD plasma actuator placed at a yawed angle with respect to the oncoming flow. The plasma-induced spanwise wall jet interacted with the primary boundary layer flow to produce a streamwise vortex whose streamwise evolution was examined via particle image velocimetry in local cross-flow planes. DBD actuators producing both co-rotating and counter rotating vortices were demonstrated. The effect of plasma-induced velocity, freestream velocity, surface electrode yaw angle and length were systematically investigated. The benefits of electode length and yaw angles orthogonal to the oncoming flow were documented.

In this paper a parametric investigation of the performance of PSVG arrays is presented. The PSVG arrays are flush mounted to a flat plate on which a nominally zero pressure gradient turbulent boundary layer develops. The understanding gleaned from this parametric investigation is considered prerequisite to optimizing PSVG arrays for specific flow control applications. Unlike the study by Jukes and Choi [10], which utilized a thin, 250 µm mylar dielectric barrier, this study exploits the work of Thomas et al. [11] which demonstrated order-of-magnitude increases in the plasma-induced body force produced by plasma actuators with comparatively thick dielectric barriers. Consequently, the PSVGs used in this study utilize both Delrin and quartz dielectrics of 3.18 mm (1/8 in) thickness. Furthermore the PSVG array design utilized in this study is different than the yawed DBD actuators used in [10] as detailed in the following section. The freestream flow velocity used in the present study extends in excess of 40 m/s whereas most of the experiments results presented in [10] are for freestream velocities less than 2 m/s. Perhaps more significantly, the ratio of plasma-induced jet velocity, Up , to freestream velocity, U∞ , was as high as 0.85 in [10]. In the current study the values are more than an order of magnitude smaller.

II. Experimental Apparatus and Procedures

a. Wind Tunnel Facility The PSVG parametric investigation was performed in one of the low-turbulence, subsonic, in-draft

wind tunnels located at the Hessert Laboratory for Aerospace Research at the University of Notre Dame. The wind tunnel has an inlet contraction ratio of 20:1. A series of 12 turbulence management screens at the

P P

3

front of the inlet give rise to tunnel freestream turbulence levels less than 0.1% (0.06% for frequencies above 10 Hz). The maximum tunnel speed is approximately U∞ = 40 m/s. Experiments are performed in a test section of 0.610 m square cross-section and 1.82 m in length. One sidewall and the test section ceiling have optical access for non-intrusive laser flow field diagnostics; in this case, particle image velocimetry (PIV), and laser Doppler velocimetry (LDV).

b. PSVG Test Fixture A rendered CAD model of the plate is shown in Fig. 1a.

The PSVG test fixture consists of a flat Plexiglas boundary layer development plate of 1.2 m (48 in) length and 1 cm thickness with a rounded leading-edge containing (1), distributed sand grain roughness in order to promote rapid boundary layer transition. The last 0.2 m of the plate consists of (2), a 2.2o linear, symmetric taper down to a trailing-edge thickness of 1.6 mm. The plate is sidewall mounted and spans the wind tunnel test section. Endplates minimize the influence of the tunnel sidewall boundary layers.

As shown in (3) of Fig. 1a, PSVG actuator test fixtures are flush mounted into a cavity machined into the plate surface at a location 0.79 m (31 in) downstream of the leading-edge. This streamwise distance from the leading-edge allows for turbulent boundary layer growth prior to application of PSVG flow control. The cavity provides ample room for accommodating both the required high-voltage leads and the dielectric material that forms the PSVG. Figure 1b shows a close up of one of the PSVG arrays ready for installation into the plate cavity. A description of the PSVG geometry and details regarding construction are presented in the following section.

c. PSVG Array The basic geometry of the PSVG arrays examined in this study is shown schematically in Figure 2. As indicated in the figure, this configuration uses a common covered electrode with streamwise length, L, and is separated from the exposed electrodes by a sheet of dielectric barrier material. The multiple exposed electrodes with spanwise inter-electrode spacing λ are aligned parallel to the oncoming flow. Due to the use of a common covered electrode, plasma forms on both sides of each surface electrode and this gives rise to a series of opposing wall jets in the spanwise direction that collide, thereby generating pairs of counter-rotating vortices with size O(λ/2). These become the basis for enhanced cross-stream mixing of momentum within the boundary layer for separation control.

As noted earlier, the dielectric material used in this study was either Delrin or quartz of 3.2 mm thickness (1/8 in). In both cases, the dielectric material extends 0.57 m (22.5 in) in the spanwise direction and 20.3 cm (8 in) in the streamwise direction. The PSVG actuator arrays were constructed with interchangeable Delrin or quartz plates. Each side of the Delrin plates is protected by a layer of 2 mil

Figure 2. Schematic of the geometry of the PSVG array, front view (left) and orthographic view (right).

Figure 1. a) Rendered CAD model of boundary layer development plate, and b) close-up of PSVG array.

a)

b)

1 2

3

4

Kapton tape and an outer layer of 2 mil Teflon tape. These additional layers of thin tape have proven to extend the lifetime of the actuator, while preserving the advantage associated with use of the thick dielectric. Each plasma actuator electrode consisted of copper foil tape of 0.05 mm thickness. The streamwise length of the covered electrode was varied in this investigation. Streamwise lengths of 2.54 cm, 5.08 cm, 10.2 cm and 15.24 cm (1, 2, 4, and 6 in) were examined. In each case the span was fixed at 46.4 cm (18.25 in). The exposed electrodes extend from a common electrode over the dielectric above the covered electrode. These straight exposed electrodes are 3.2 mm (1/8 in) in width and have a length of 15.24 cm (6 in). Both the covered electrode and exposed electrode are connected to high voltage wires that lead out of the test fixture and connect to the transformers of the plasma generation circuit. The circuit used to operate the PSVG array is identical to that presented in Figure 6 of Kozlov and Thomas [7]. Unless otherwise noted, the actuators were operated with an ac carrier consisting of a positive sawtooth waveform of 2 kHz frequency and a peak-to-peak voltage that was varied from 15 to 50 kV.

d. Parameters and Actuator Performance Metrics The experimental study involved varying the following parameters: actuator peak-to-peak voltage, Epp,

freestream velocity, U∞ , covered electrode length, L, and spanwise inter-electrode spacing, λ while noting the effect on actuator performance. The range of values tested for each parameter is listed in Table 1. Since the parameter space was large, it was desirable to define a performance metric that was indicative of the actuator performance but relatively rapid to measure. To characterize the array performance, LDV measurements were made in a cross flow plane at a fixed location 76.2 mm (3 in) downstream of the PSVG array. Two-component LDV measurements were conducted using a Dantec Dynamics Fiber Flow LDV system with a Spectra Physics Stabilite 2017 Argon Ion Laser. The Fiber optic LDV system was operated in 180o backscatter mode. The Doppler bursts were measured using a BSA F60 Flow processor and BSA Flow Software Version 4.10. The LDV probe volume was traversed in the spanwise direction, z, and the x (streamwise) U , and y (wall-normal) V , mean velocity components were measured. The wall-normal location of the traverse was set to the local oncoming boundary layer displacement thickness, δ * , which for a zero pressure gradient turbulent boundary layer, is the centroid of mean spanwise vorticity. This was deemed the most appropriate wall-normal location to measure both ∂V /∂z and ∂U /∂z . Spanwise traverses were also performed at wall normal locations 2δ * and 0.5δ * and showed results consistent with those obtained at δ * and will not be presented here. The quantity ∂V /∂z is one component of the mean streamwise vorticity, ωx , and represents a metric related to the efficacy of the PSVG to produce

ωx . The second quantity ∂U /∂z is an important component of a source term in the transport equation

for streamwise vorticity, ωz ∂U /∂z . This term serves to take initially spanwise mean vorticity and turn it into the streamwise direction. In this manner both were deemed relevant quantities in characterizing the PSVG array performance.

Table 1. Parameter space for PSVG parametric study

Parameter Epp (kV) U∞ (m/s) L (in) λ (in) Dielectric

Range 20 – 50 5 – 40 1 – 6 0.5 – 1.25 Delrin/Quartz

e. PIV Data Acquisition A LaVision time-resolved PIV system was also used to non-intrusively acquire two-component velocity

data in selected cross-stream planes. The system consisted of a Litron LDY300 series Nd:YLF pulse laser, a Photron SA 1.1 high-speed camera, and dedicated computer running DaVis 8.0 vector processing software. The laser light sheet was directed through the side window of the wind tunnel. Sheet optics consisting of a combination of a spherical and a cylindrical lens were used to create a 1 mm thick light

5

sheet at the measurement plane. The high speed camera, equipped with a Nikon 105-mm lens and 2x teleconverter operating at f/2.8, was positioned on top of the test section, upstream of the measurement region, angled approximately 30° to the flow direction. This angular distortion was corrected with an appropriately aligned calibration plate, and calibrations were performed at each discrete streamwise position. At each streamwise location 1200 image pairs were captured in double-frame mode at a sample rate of 1 kHz with Δt = 200 µs. The interrogation region was approximately 640 x 640 pixels before image correction and varied from (601 to 649) x (685 to 788) pixels afterward, corresponding to physical domains of approximately 48.10 to 56.97 mm x 59.01 to 64.97 mm (depending on the calibration used). A TSI atomizer produced nominally one-micron diameter DEHS seed particles injected upstream of the wind tunnel inlet via a mixing box allowing for uniform particle dispersion in the measurement plane. The data were preprocessed by subtracting the sliding minimum over the first 200 image pairs, thereby removing a majority of image background noise. The vector processing software was operated in multipass mode in which two passes were made with 64 x 64 window size with 50% overlap, and three passes with 32 x 32 window size with 50% overlap resulting in a 76 to 81 x 86 to 96 vector map for the given interrogation region. A Peak ratio of 1.2, universal outlier detection, and interpolation were used in post-processing to remove spurious vectors.

III. Experimental Results In this section, key results of the parametric investigation into the performance of several PSVG arrays

are presented. In the first portion a comparison is made between passive trapezoidal VGs and PSVG under identical flow conditions. Consideration is also given to the appropriate scaling of ∂U /∂z and ∂V /∂z produced by both devices.

a. Comparison of Passive VGs and PSVGs: Scaling Issues In order to provide a baseline data set for comparison, the test fixture containing the PSVG was

temporarily replaced with one containing a spanwise array of five passive trapezoidal vortex generators. A series of experiments was performed to provide a comparison between ∂U /∂z and ∂V /∂z downstream of both the passive VGs and the PSVG. The passive vortex generators were of trapezoidal shape and about 6.35 mm (0.25 in) in height; smaller than the nominal local turbulent boundary thickness of 2.54 cm (1 in). Their width was 12.7 mm (0.5 in) at the front and 25.4 mm (1 in) at the back with a length of 25.4 mm (1 in). The VG angle was 25o. The center-to-center spacing was λ = 50.8 mm (2 in).

In order to characterize ∂U /∂z , the LDV probe volume was traversed in the spanwise z-direction at a fixed distance of 76.2 mm (3 in) downstream of the VGs and with the wall normal height fixed at δ * , the boundary layer displacement thickness. For the nominally zero pressure gradient boundary layer δ * represents the wall-normal location of the centroid of mean spanwise vorticity. Figure 4 presents the spanwise variation of mean velocity U(z) downstream of the passive vortex generator for external velocities ranging from 12.5 m/s ≤U∞ ≤ to 20 m/s (corresponding to the Reynolds number range of 0.672 million≤ Rex ≤ to 1.08 million). In order to account for the varying external velocity, the measurements

are presented by subtracting the spanwise cycle average mean velocity, U , and normalizing by the

external velocity, U∞ . The location z = 0 is centered on a vortex generator. Figure 4 shows that this normalization provides a reasonable collapse for all the cases.

Figure 3. PIV experimental setup in the cross-stream plane.

6

Due to the scaling demonstrated in Fig. 4, the ordinate may be denoted as F(z), and it follows that ∂U /∂z =U∞ dF / dz . That is, the spanwise velocity gradient ∂U /∂z is directly proportional to U∞ .

Figure 5 presents peak values of ∂U /∂z measured downstream of the passive VGs as a function of freestream speed. The proportionality to U∞ suggested by Figure 4 is fully confirmed.

The initial PSVG experiments were performed with an array that consisted of 17 surface electrodes, each of which were 7 cm (2.75 in) in length and 3.2 mm (1/8 in) in width with a covered electrode of length 2.54 cm (1 in). The spanwise inter-electrode electrode spacing was 2.54 cm (1in) for the array (the same as for the passive VG). For the PSVG experiment, the origin of the spanwise coordinate z = 0 was centered on one of the surface electrodes. The PSVG array was operated in a quasi-steady manner with an ac frequency of 2kHz. The peak-to-peak ac voltage amplitude was varied from 20 kV to 40 kV. Experiments were performed for a similar external velocity range as for the passive VGs. The experiment was focused on characterizing the spanwise variation in mean velocityU z( ) and V z( ) induced by the PSVG array. As

with the passive VG, the LDV probe volume was traversed in the spanwsie direction at a streamwise location 76.2 mm (3 in) downstream of the PSVG array with the height of the probe volume above the wall again fixed at the oncoming boundary layer displacement thickness, δ * . As a representative example of the results obtained, Fig. 6 presents the variation of scaled mean velocity U(z) downstream of the PSVG operated at 40kV and for external velocities ranging from 10 m/s to 30 m/s (corresponding to the Reynolds number range of 0.54 million to 1.61 million). The ordinate scaling is the same as that applied to the passive VG data in Fig. 4; the spanwise cycle average mean velocity, U , is subtracted and the result is

normalized by the external velocity, U∞

. As was the case for the passive VGs, Figure 6 shows that this

scaling also does an excellent job of collapsing the spanwise variation of U z( ) downstream of the PSVG.

Comparison of Figures 4 and 6 reveals that the functional form of F(z) = U z( )− U( ) /U∞is different

for the passive VGs and the PSVG. Unlike the passive VGs where positive and negative excursions from the cycle mean are comparable, Fig. 6 shows larger negative deviations than positive for the PSVG. These negative peaks are centered between the surface electrodes and are associated with local upwelling of low velocity fluid from the near wall region associated with the colliding plasma induced spanwise

Figure 4. Scaled spanwise variation of mean velocity downstream of the passive VGs.

J

J

J

J

J

J

J

0

200

400

600

800

1000

1200

0 5 10 15 20 25 30

Peak

-to-p

eak

dU/d

z (1

/sec

)

External Velocity (m/s)

Figure 5. Maximum amplitude of ∂U / ∂z for passive VGs as a function of external velocity.

7

Figure 6. Spanwise variation of scaled streamwise component mean velocity downstream of the PSVG array (40kV Case)

wall jets. Positive excursions are centered over the surface electrodes and are associated with transport of comparatively high velocity fluid toward the electrodes by the plasma induced body force. However, the collapse shown in Figure 6 for the PSVG results indicates that ∂U / ∂z is proportional to the external flow speed, U

∞, as was the case for the passive VGs.

Figure 7 presents an example of measured ∂U / ∂z for the PSVG operated at 40 kV and an external flow speed of 15 m/s corresponding to the Rex = 810,000 case. This figure shows the large spanwise periodic mean velocity gradient set up by the PSVG due to the generation of spanwise counter-rotating streamwise vortices.

Figure 8 compares peak values of ∂U / ∂z as a function of external velocity for the passive VGs and the PSVG. The similarity is quite remarkable with the PSVG producing somewhat larger values over most of the range of external velocities tested. Note, however, the reduction in peak ∂U / ∂z for the PSVG measurement at U

∞= 25 m/s (Rex = 1.34 X 106). This saturation effect is unique to the PSVG and will be

described in detail in the following section. It is conjectured that this reduction is associated with the reduced residence time L/U

∞ of the flow over the PSVG electrodes (where L denotes the electrode length)

at higher external velocities.

B

B

B

B

B

BB

B

B

B

BBBB

BB

B

B

BB

B

B

B

BBBB

B

B

B

BB

B

B

BBBBBB

B

B

BB

B

B

B

BBB

BB JJ

J

J

J

JJ

J

J

J

JJJ

JJ

J

J

J

JJ

J

J

J

JJJ

J

J

J

J

JJ

J

J

J

JJJJ

J

J

J

JJ

J

J

J

JJJJJ

HH

H

H

H

HH

H

H

H

HHH

H

H

H

H

H

H

H

H

H

H

HHH

H

H

H

H

H

H

H

H

H

HHHH

H

H

H

H

H

H

H

H

HHHHH

FFF

F

F

FF

F

F

F

FFF

F

F

F

F

F

FF

F

F

F

FFF

F

F

F

FF

F

F

F

FFFF

F

F

F

F

F

F

F

F

F

F

FFF

F ÑÑ

Ñ

Ñ

ÑÑÑ

Ñ

Ñ

Ñ

ÑÑÑ

Ñ

Ñ

Ñ

Ñ

Ñ

Ñ

Ñ

Ñ

Ñ

Ñ

ÑÑ

Ñ

Ñ

Ñ

Ñ

ÑÑ

Ñ

Ñ

Ñ

ÑÑÑÑ

Ñ

Ñ

Ñ

Ñ

ÑÑÑ

Ñ

Ñ

Ñ

ÑÑÑ

Ñ

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

-60 -40 -20 0 20 40 60

Z (mm)

B Re = 0.54 X 10^6

J Re = 0.81 X 10^6

H Re = 1.07 X 10^6

F Re = 1.34 X 10^6

Ñ Re = 1.61 X 10^6

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

BB

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

B

BB

B

B

B

B

B

B

B

BB

B

BB

B

BB

-400

-300

-200

-100

0

100

200

300

400

500

-60 -40 -20 0 20 40 60

dU

/ d

z (

1/s

ec)

Z (mm)

Figure 7. Spanwise variation of ∂U / ∂z downstream of the PSVG array (U∞ =15m / s , 40kV )

B

B

B

B

B

B

B

J

J

J

J

J

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30

Pe

ak d

U/d

z X

10

^-3

(1

/se

c)

External Velocity (m/s)

B Passive VG

J PSVG

J

Figure 8. Comparison of peak-to-peak ∂U / ∂z versus U

∞for the passive VG and PSVG.

8

Figure 9 presents corresponding LDV measurements of the wall-normal mean velocity V (z) scaled in

the same manner as the streamwise component shown previously in Figure 6. These results are for a 2.54 cm (1 in) covered electrode, a 2.54 cm (1 in) inter-electrode spacing and an applied voltage of 40kV. With the exception of peak positive V which shows some reduction at the higher freestream speeds, there is again a reasonable collapse of the profiles suggesting that ∂V /∂z is also proportional to external flow speed, U∞ . This plot shows that locations over the surface electrodes (e.g. z = -25.4 mm, 0, +25.4 mm) are

associated with negative V . This is associated with the plasma body force drawing fluid toward the wall. In contrast, locations midway between adjacent electrodes are characterized by positive V peaks that arise from the upwelling associated with colliding wall jets from adjacent electrodes.

Figure 10 presents the peak-to-peak value of ∂V /∂z measured downstream of the PSVG as a function of external velocity. As was suggested by the scaling shown in Fig. 9, this figure shows that ∂V /∂z is initially proportional to U∞ . A saturation effect comes into play for the highest U∞ tested. This is to be expected from Fig. 9 where it may be noted that the positiveV peaks located between the electrodes are reduced in amplitude at the highest flow speeds giving a deviation from the similarity scaling that occurs at the lower speeds.

Figure 11a presents an example of the spanwise variation of ∂V /∂z for the case of U∞ = 20 m/s and Epp = 40 kV. The associated schematic shown Figure 11b shows that the measured ∂V /∂z pattern is fully consistent with the existence of pairs of counter-rotating streamwise vortices downstream of the PSVG. In the figure negative ∂V /∂z corresponds to clockwise streamwise vortices while positive ∂V /∂z corresponds to counter clockwise vortices. Consistent with figure 11a the locations of zero vorticity are centered over the electrodes and midway between adjacent electrodes.

BBB

B

B

B

B

B

B

BBBBBBB

B

B

BB

B

B

BBBBB

B

B

B

BB

B

B

BBBBBB

B

B

BB

B

B

BBBB

BB JJJ

J

J

JJ

J

J

JJJJJJ

J

J

J

J

J

J

J

JJJJJ

J

J

J

J

J

J

JJJJJJ

JJ

J

J

J

J

J

JJJJJJ

HHHH

H

HH

H

HHHHHH

HH

H

H

H

H

H

H

HHHH

HH

H

HHH

HHHHHHH

HH

H

HHH

H

HHHHH

H FFF

F

FFFF

FFFFF

FF

F

F

FFF

F

FFFF

FFFFFFF

FFFFFF

FFFFFF

FF

FFF

FFF

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

-60 -40 -20 0 20 40 60

B 15 m/s

J 20 m/s

H 25 m/s

F 30 m/s

BBB

B

B

B

B

B

B

BBBBBBB

B

B

BB

B

B

BBBBB

B

B

B

BB

B

B

BBBBBB

B

B

BB

B

B

BBBB

BB JJJ

J

J

JJ

J

J

JJJJJJ

J

J

J

J

J

J

J

JJJJJ

J

J

J

J

J

J

JJJJJJ

JJ

J

J

J

J

J

JJJJJJ

HHHH

H

HH

H

HHHHHH

HH

H

H

H

H

H

H

HHHH

HH

H

HHH

HHHHHHH

HH

H

HHH

H

HHHHH

H FFF

F

FFFF

FFFFF

FF

F

F

FFF

F

FFFF

FFFFFFF

FFFFFF

FFFFFF

FF

FFF

FFF

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

-60 -40 -20 0 20 40 60

B 15 m/s

J 20 m/s

H 25 m/s

F 30 m/s

z (mm) Figure 9. Spanwise variation of scaled wall-normal component mean velocity

downstream of the PSVG array (40kV Case). Spanwise coordinate z in mm.

B

B

B

B B

B

0

50

100

150

200

250

300

0 5 10 15 20 25 30 35

Peak

-to-p

eak

dV/d

z (1

/sec

)

U (m/s)

Figure 10. Peak-to-peak ∂V /∂z versus U ∞ for the PSVG (40 kV case).

9

It is apparent from the results presented in this section that the PSVG’s extract energy from the primary flow as is the case for passive vortex generators. In previously published applications of DBD plasma actuators for separation control, authority at fixed applied voltage gradually degrades with increased flow speed (i.e. Reynolds number). This is not the case for the PSVGs. The experiments have shown that the magnitude of streamwise vorticty produced by a PSVG array actually increases with external velocity, U∞ . Figures 8 and 10 show that the PSVG do, however, exhibit a saturation effect at sufficiently high U∞ . This is obviously not the case for the passive VG. In the remaining section of this paper a parametric study is presented in order to examine how to drive the PSVG saturation to higher velocities and in the course of this investigation the details of the vortex formation process are also examined.

b. Effect of Applied Voltage, Epp In order to examine the effect of peak-to-peak applied voltage on PSVG performance an actuator array was used in which both the covered electrode length and inter-electrode spacing was fixed at 2.54 cm (1-inch). Delrin was used as the dielectric. The peak-to-peak voltage was varied from 20 to 45 kV and the peak ∂V /∂z was measured via LDV at a location 7.62 cm (3 inch) downstream of the array for freestream velocities in the range 5 m/s ≤U∞ ≤ 30 m/s. The measurements were obtained at y = δ*. Results are presented in Figure 12. This figure shows that the effectiveness of the PSVG in terms of streamwise vorticity production increases with both applied voltage and external velocity. The range of external velocity for which ∂V /∂z∝U∞ increases with applied voltage which, has the effect of shifting the PSVG saturation to higher freestream speeds. It should be noted that for the Delrin dielectric material used for this experiment, Epp = 45 kV represented an effective upper applied voltage limit.

J

J

J

J

J

J

JJ

J

JJJJ

J

J

JJ

J

JJ

JJ

JJJ

J

J

JJ

J

JJ

J

J

JJJ

JJ

J

J

J

J

JJ

J

JJJJ

-150

-100

-50

0

50

100

150

-50 -40 -30 -20 -10 0 10 20 30 40 50

dV/d

z (1

/sec

)

z (mm)

Figure 11. a) Spanwise variation in ∂V /∂z for U∞ = 20 m/s and Epp = 40 kV. (Electrodes are located at z = -50.8 mm, -25.4 mm, 0, +25.4 mm, 50.8 mm) and b) Schematic of the counter –rotating srtreamwise vortices produced by the PSVG array.

a)

b)

10

B

B BB

J

J

J

J

H H

HH H

F

FF

FF F

P

P

P

P

P

P

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30

peak

-to-p

eak

dV/d

z (1

/sec

)

U (m/s)

B 20 kV

J 25 kV

H 30 kV

F 35 kV

P 40 kV

45 kV

Figure 12. Measured peak-to-peak ∂V /∂z as a function of U∞ for a range of applied voltages (L = λ = 2.54 cm).

c. Effect of Surface Electrode Length Figure 13 presents measured peak-to-peak ∂U /∂z for different PSVG arrays with covered surface

electrode lengths of 1, 2, 4 and 6 inches (2.54, 5.08, 10.16 and 15.24 cm). The spanwise inter-electrode spacing was fixed at 1 inch (2.54 cm) and the applied voltage was fixed at 35 kV. The dielectric material was Delrin. This figure clearly shows the benefit of increased actuator covered electrode length on PSVG performance. In particular, at fixed applied voltage, the effect of electrode length is to drive the PSVG saturation to higher flow speeds and extend the range of velocity for which ∂U /∂z is proportional to U∞ .

This allows larger values of ∂U /∂z to be reached.

Figure 14 presents measured peak-to-peak ∂V /∂z for different streamwise electrode lengths. The applied voltage is fixed at 35 kVpp. The effect of increased electrode length is to drive the PSVG saturation to higher freestream speeds and to thereby increase the velocity range over which ∂V /∂z ∝U∞ . Again the benefit of covered electrode length is apparent.

B

B

BB

B B

J

J

J

JJ

J

H

H

H

H

H

F

F

F

F

F

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30 35

peak

-to-p

eak

dU/d

z (1

/sec

)

U (m/s)

B 1 inch

J 2 inch

H 4 inch

F 6 inch

Figure 13. Peak-to-peak ∂U /∂z for different electrode lengths, L ; Epp = 35 kV.

B

B BB

B BJ

J

J

J

JH

H

H

H H

F

F

F

F

F

0

100

200

300

400

500

600

700

0 5 10 15 20 25 30 35

peak

-to-p

eak d

V/dz

(1/se

c)

U (m/s)

B 1inch

J 2 inch

H 4 inch

F 6 inch

Figure 14. Peak-to-peak ∂V /∂z for different electrode lengths, L ; Epp = 35 kV.

11

d. Effect of Spanwise Inter-electrode Spacing A series of experiments were performed in order to assess the effect of spanwise inter-electrode spacing

on PSVG performance. For these experiments a series of four PSVG arrays were constructed for installation on the test plate in which in the inter-electrode spacing was set at values λ = 0.75, 1.0, 1.25 and 1.5 inches (1.91, 2.54, 3.18, and 3.81 cm, respectively). In each case, the covered electrode length was kept fixed at L = 2 inches (5.08 cm). Spanwise LDV traverses were performed at a wall normal location equal to the local displacement thickness, δ*, corresponding to the centroid of boundary layer mean vorticity.

Figure 15 presents measured values of ∂U /∂z)max as a function of PSVG inter-electrode spacing, λ.

Results for three applied voltages are shown. In each case, there is clearly an optimum spacing of approximately λOPT ≈ 1.25 in. (3.2 cm). Note that there is a rather sudden reduction in PSVG performance for λ > λOPT.

Figure 16 presents measured values of ∂V /∂z)max as a function of PSVG inter-electrode spacing, λ.

These data were acquired simultaneously with those presented in Figure 15. Figure 16 clearly suggests that optimum performance of the PSVG in terms of mean streamwise vorticity production occurs for values of λ between 1 and 1.25 inches (2.54 and 3.2 cm). Again there is a very rapid drop off in performance for l > 1.5 inches (3.8 cm).

B

B

B

B

B

JJ

J

J

J

H

H

H

H

H

0

200

400

600

800

1000

1200

1400

0.25 0.5 0.75 1 1.25 1.5 1.75

dU/d

z)m

ax (

1/se

c)

Spanwise inter-electrode spacing (inches)

B 25 kV

J 30kV

H 35 kV

Figure 15. Variation of dU / dz)MAX with spanwise

inter-electrode spacing, λ .

B

B

B

B

B

JJ

J

J

J

H

H

HH

H

0

50

100

150

200

250

300

350

0.25 0.5 0.75 1 1.25 1.5 1.75

dV/d

z)m

ax (

1/se

c)

Spanwise inter-electrode spacing (inches)

B 25 kV

J 30 kV

H 35 kV

Figure 16. Variation of ∂V /∂z)MAX with spanwise

inter-electrode spacing, λ .

12

The optimum value of spanwise inter-electrode spacing for both ∂U /∂z)MAX and ∂V /∂z)MAX is

approximately equal to the local 99% boundary layer thickness on the plate upstream of the PSVG array. This is clearly shown in Figure 17, which presents ∂U /∂z)MAX as a function of λ /δ . Peak values of

∂U /∂z)MAX occur near λ /δ = 1.

The question naturally arises whether

this is coincidental or whether the optimum spanwise inter-electrode spacing actually scales with the approach boundary layer thickness. In order to investigate this, the boundary layer thickness upstream of the PSVG array was thickened by attaching a semi-circular bump (15.9 mm in diameter) near the plate leading edge followed by a downstream section of surface roughness extending 10.2 cm in the streamwise direction. This is shown in the photograph in Figure 18. This passive treatment gave rise to a separation bubble followed by turbulent reattachment, which increased the boundary layer thickness just upstream of the PSVG array by approximately 80%. The PSVG arrays with inter-electrode spacing values λ = 0.75, 1.0, 1.25 and 1.5 inches (1.905, 2.54, 3.18, and 3.81 cm, respectively) were again examined by LDV surveys to see if λOPT scaled with the new boundary layer thickness, δ. Sample results are shown in Figure 19, which presents normalized values of ∂U /∂z)MAX for both the original flat plate and

leading edge tripped cases as a function of λ /δ . In both cases the external velocity was fixed at U∞ = 20 m/s and the applied voltage was 35 kV. Note that the thicker boundary layer case peaks at a smaller value of λ /δ . Although not shown here, similar results were found to hold for ∂V /∂z)MAX . This suggests

that the optimum value for spanwise inter-electrode spacing does not scale with the approach boundary layer thickness and is instead determined by the governing equations for the electromagnetic field.

e. Streamwise Vortex Formation and Evolution In order to clarify the mechanism of vortex formation by the PSVG array and its downstream evolution,

PIV measurements were made in the cross-flow plane at several representative streamwise locations both over the PSVG actuator and at downstream locations. The quartz-based actuator was operated at Epp = 40 kV with L = 5.08 cm (4 in) and λ = 2.54 cm (1 in). The freestream velocity was fixed at U∞ = 15 m/s.

λ /δFigure 17. Peak ∂U /∂z)MAX as a function of λ /δ ;

(U∞ =20 m/s).

Figure 18. Boundary layer development plate with leading edge semi-circular bump followed by distributed sand grain roughness.

13

Sample PIV results are presented in Figure 20, which presents mean streamwise vorticty, ωx , in the cross-flow plane. The PIV image plane is centered on the gap between two adjacent surface electrodes, which are shown as black rectangles at the bottom of each frame. As a reference, the local boundary layer thickness corresponds to approximately 25-30 on the ordinate scale. The top two images are taken over the PSVG electrodes; the leftmost image 5.08 cm (2 in) upstream of the trailing edge of the actuator and the rightmost image at the actuator trailing edge. The gradual formation of pairs of organized counter-rotating streamwise vortices within the boundary layer is apparent. The bottom two images show the evolution of the streamwise vortices downstream of the PSVG array. The leftmost image is obtained 5.08 cm (2 in) downstream of the PSVG and the rightmost image 10.16 cm (4 in) downstream. The counter-rotating pairs of vortices remain well defined well downstream of the PSVG actuator. The primary effect of streamwise distance seems to be growth of the vortices and diffusion of the vortex core within the turbulent boundary layer.

�������������������������������� �� ��

��������������������� �� ���

x"="2"in"downstream"of"PSVG"

XVG"="4"in"(downstream"of"TE)"

x"="4"in"downstream"of"PSVG"

Figure 20. PIV measurements of ω x in selected z-y cross-flow planes.

λ /δFigure 19. Comparison of normalized ∂U /∂z for baseline and tripped case.

14

From PIV measurements of the cross-stream mean velocity components, the circulation Γ ≡

V •d

∫ was numerically evaluated over a fixed rectangular contour bounding a given

vortex at several streamwise locations. By Stoke’s theorem, this is equivalent to the mean streamwise vorticity flux through the cross-stream area bounded by the integration contour. In this manner, the streamwise development of ω x over the PSVG actuator and its subsequent evolution at locations downstream could be characterized. Figure 21 presents typical results for the streamwise variation of Γ x( ) for adjacent clockwise (CW) and counter-clockwise (CCW) vortices. The streamwise coordinate x is centered at the downstream edge of the PSVG so negative values indicate locations over the actuator and positive values are downstream locations. The actuator operating conditions are identical to those corresponding to Fig. 20; L = 4 inch, Epp = 40 kV with U∞ =15m / s . The symmetric development of Γ x( ) exhibited by the CW and CCW vortices is apparent. Figure 21 also shows that the circulation increases approximately quadratically with x over the actuator and reaches a peak value at the trailing edge. Commencing at the trailing edge of the actuator a plateau of peak circulation persists up to 2 inches downstream (x = L/2) followed by a gradual reduction in Γ x( ) farther downstream. It is

conjectured that this reduction in Γ is associated with both turbulent diffusion of the vortices (as suggested by Figure 20) as well as vorticity redistribution within the turbulent boundary layer.

B B

B

B

B B B

B B

J

J J

J

J

J J

J

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

-4 -3 -2 -1 0 1 2 3 4 5

Circ

ulat

ion

(m^2

/sec

)

X (inches)

B CCW

J CW

Figure 21. Streamwise evolution of Γ x( ) for adjacent CW and CCW vortices. PSVG

actuator location is denoted by shaded region on the abscissa.

IV. Discussion The experimental results presented in this paper have demonstrated that PSVGs harvest energy from the

external flow in a manner very similar to that exhibited by passive VGs. This increase in actuator effectiveness with flow speed makes the PSVG unique among DBD plasma flow control devices. Unlike

15

passive VGs, there is a saturation in the effectiveness of the PSVG at sufficiently high freestream velocity. However, this study has also demonstrated that this saturation can be driven to higher freestream speeds by increases in both actuator covered electrode length and applied voltage. For example, Figure 22 shows measured ∂U /∂z downstream of a PSVG array using quartz dielectric with 10.2 cm (4 inch) covered electrodes operated at Epp = 50 kV. Note that the actuator saturation has been effectively eliminated over the operating range of the wind tunnel.

Furthermore, comparison with the passive VG results presented previously in Figure 8 reveals that the ∂U /∂z)max produced by the PSVG exceeds that of the passive VG at corresponding flow speeds. Figure

23 presents corresponding results for ∂V /∂z)max for the same quartz PSVG actuator operated at Epp = 50

kV. Although actuator saturation is apparent, it has been pushed to the maximum wind tunnel speed. In general, PSVG saturation for ∂V /∂z)max occurs at a lower speed than for ∂U /∂z)max . Comparison of

Figures 22 and 23 also revels that for any given flow speed peak-to-peak ∂U /∂z > ∂V /∂z . In fact, this propensity of PSVG to produce large values of ∂U /∂z)max can also be exploited for flow control.

Large values of ∂U /∂z can be utilized to produce streamwise vorticity, ωx , through the term,

ωz∂U /∂z , which serves to take initially spanwise oriented vorticity and turn it into the streamwise direction. This is the basis of a “hybrid PSVG actuator” concept (HPSVG) in which spanwise vorticity is artificially introduced just upstream of the PSVG array. This vorticity is subsequently turned into the streamwise direction by the large PSVG-induced ∂U /∂z . It is envisioned that the spanwise vorticity can be introduced just upstream of the PSVG array by a spanwise oriented DBD plasma actuator using either steady or unsteady streamwise blowing. However, for the initial proof-of-concept experiment reported here, a very small scale mechanical ramp was used to create a separated shear layer very near the plate wall. That is, instead of using a spanwise plasma actuator, additional ωz was produced upstream of the PSVG

array by means of a ramp of height, h ≈ δ * , where δ * is the local boundary layer displacement thickness, as shown schematically in Figure 24. Figure 24 also serves to illustrate the concept of vortex turning that is key in the hybrid PSVG (HPSVG) concept.

J

J

J

J

J

J

0

500

1000

1500

2000

2500

3000

10 15 20 25 30 35 40 45

peak

-to-p

eak

dU/d

z (1

/sec

)

U (m/s)

Figure 22. Peak-to-peak ∂U /∂z for a PSVG with quartz dielectric; Epp = 50 kV.

J

J

J

J

J

J

0

100

200

300

400

500

600

700

10 15 20 25 30 35 40 45

peak

-to-p

eak

dV/d

z (1

/sec

)

U (m/s)

Figure 23. Peak-to-peak ∂V /∂z for a PSVG with quartz dielectric; Epp = 50 kV.

16

A proof-of-concept study was performed in order to investigate the ability of HPSVG to augment streamwise vorticity. The LDV system was used to perform non-intrusive velocity measurements along the spanwise z-axis. A second-order accurate central difference approximation was used to calculate the spatial derivatives ∂U /∂z and ∂V /∂z downstream of the PSVG array. If the HPSVG works as shown schematically in Fig. 24, then one expects significant increases in ∂V /∂z and comparably modest changes in ∂U /∂z . The maximum and spatially averaged peak-to-peak amplitudes of ∂U /∂z and ∂V /∂z are reported in Table 1. In both cases the covered electrode length, L = 5.08 cm (2 inch), the spanwise inter-electrode spacing was λ = 2.54 cm (1 inch) and Epp = 35 kV. As expected, there seems to be relatively little change in ∂U /∂z between the two configurations, with ~20 % increase on average with a maximum increase of about 30 %. However, Table 1 shows a very significant increase in the magnitude of ∂V /∂z . Remarkably, there is a maximum increase of about 80 % in peak-to-peak amplitude of ∂V /∂z ; that is, the HPSVG configuration has greatly increased actuator effectiveness in terms of direct production of streamwise vorticity. It is important to note that these results have been obtained without any effort to optimize this experimental configuration. As previously mentioned, the optimization of the HPSVG configuration may yield deeper insights into increasing actuator authority and should be subject of future study.

Table 1: Peak-to-Peak amplitude and increase of actuator metrics.

∂U /∂z)MAX (1/sec) ∂V /∂z)MAX (1/sec)

PSVG 1006.5 243.31 HPSVG 1241.3 390.49 Average Increase 23.3 % 60.5 % Maximum Increase 29.4 % 79.9 %

Figure 24. Schematic of HPSVG concept: a) is generated by the ramp element, b) is turned such that

generated by PSVG is augmented, c) The resultant magnitude of is increased.

American Institute of Aeronautics and Astronautics

17

Acknowledgments This work was supported by Innovative Technology Applications Company (ITAC), LLC under a Small Business Innovation Research (SBIR) Phase II Contract No. N00014-11-C-0267 issued by the U.S. Department of the Navy. The authors would like to thank Dr. Judah Milgram and Mr. John Kinzer of the Office of Naval Research (ONR) for their support and encouragement during this effort. The authors would also like to acknowledge Mr. Eras Noel for his important contributions to data acquisition.

References 1Lin, L., “Review of Research on Low-Profile Vortex Generators to Control Boundary Layers,” Prog. Aerosp Sci,

38:389-420, 2002. 2Bushnell, D., “Longitudinal Vortex Control-Techniques and Applications,” Aeronaut J.,96:293, 1992.

3Corke, T. C., Post, M. L., and Orlov, D.M., “Single Dielectic Barrier Discharge Plasma Enhanced Aerodynamics: Physics, Modeling and Applications,” Experiments in Fluids, Vol. 46, No. 1, 2009, 1-26.

4Moreau, E., “Airflow Control by Non-Thermal Plasma Actuators,” Journal of Physics D: Applied Physics, Vol. 40,

No. 3, 2007, pp. 605-636. 5Huang, X., Chan, S., and Zhang, X., “An Atmospheric Plasma Actuator,” IEEE Transactions on Plasma Science,

Vol. 35, No. 3, 2007, pp. 693–695.

6Schatzman, D. M., and Thomas, F. O., “Turbulent Boundary Layer Separation Control Using Plasma Actuators,” AIAA Journal, Vol. 48, No. 8, 2010, pp. 1620–1634.

7Kozlov, A. V. and Thomas, F. O., “Flow Control via Two Types of Dielectric Barrier Discharge Plasma Actuation,” AIAA Journal, 49, 9, pp. 1919-1931, 2011.

8Kozlov, A. V. and Thomas, F. O., “Plasma Flow Control of Cylinders in a Tandem Configuration,” AIAA Journal, 49, 10, pp. 2183-2193, 2011.

9Okita, Y., Jukes, T., Choi, K-S, Nakamura, K., “Flow Reattachment Over an Airfoil Using Surface Plasma Actuator,” AIAA paper 2008-4203.

10Jukes, T. N. and Choi, K-S, “Dielectric Barrier-Discharge Vortex Generators: Characterization and Optimization for Flow Separation Control,” Experiments in Fluids, DOI 10.1007/s00348-011-1212-0, 2011.

11Thomas, F. O., Corke, T. C., Iqbal, M., Kozlov, A. and Schatzman, D., “Optimization of SDBD Plasma Actuators

for Active Aerodynamic Flow Control” AIAA Journal, Vol. 47, No. 9, 2009, pp. 2169-2178.