PIV Measurements of the Flow Around Airfoil Models Equipped With the Plasma Actuator

16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012

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PIV measurements of the flow around airfoil models equipped with the plasma actuator

Artur Berendt1, Janusz Podlinski1*, Annie Leroy3, Pierre Audier4, Dunpin Hong4 and Jerzy Mizeraczyk1, 2

1: Centre for Plasma and Laser Engineering, The Szewalski Institute of Fluid Flow Machinery, Polish Academy of Sciences, Fiszera 14, 80-952 Gdask, Poland

2: Department of Marine Electronics, Gdynia Maritime University, Morska 81-87, 81-225 Gdynia, Poland 3: Laboratoire PRISME, Universit d'Orlans, 8 rue Lonard de Vinci, 45072, Orlans Cedex 2, France 4: GREMI, CNRS-Universit d'Orlans, UMR 7344, 14 rue d'Issoudun, 45067, Orlans Cedex 2, France

* correspondent author: [email protected]

Abstract In this paper results of 2D Particle Image Velocimetry (PIV) measurements of the airflow around NACA 0012 and NACA 0015 airfoil models are presented. Both airfoil models used in our experiments were equipped with a multi dielectric barrier discharge (DBD) plasma actuator in order to investigate its capabilities to reattach the flow separated either at the leading edge or at the trailing edge. Our investigations were carried out in a subsonic wind tunnel. The free stream velocity was fixed to 10 m/s, corresponding to a Reynolds number based on the chord of the order of 105. The results obtained for leading edge (NACA 0012) and trailing edge (NACA 0015) airflow separation control showed that actuation with this multi DBD actuator operated in continuous mode enables flow reattachment leading to postpone the airfoil stall.

1. Introduction

Nowadays, importance of air transport in the world economy is constantly growing. Unfortunately, the heavy air traffic is the source of pollutions which are harmful for human health and environment. Thus, the great research effort is directed to make aircrafts more human and environment friendly. This objective can be achieved e.g. by improving aircraft aerodynamics. Thus, in addition to conventional technologies, new solutions like the use of dielectric barrier discharge (DBD) plasma actuators for active airflow control around aerodynamic elements are under development. DBD actuators are devices using plasma generated by the surface dielectric barrier discharge for active airflow control [1-3]. The surface DBD occurs when a voltage is applied to electrodes which are set on the top and bottom sides of a dielectric material. The plasma generated by the DBD actuator induces electrohydrodynamic (EHD) flow which allows momentum addition in the natural flow near the body surface. Using DBD actuators, it is possible for example to influence the laminar to turbulent transition or separation of the boundary layer evolving around bodies. Currently, researches on DBD plasma actuators for flow control are performed in many laboratories all over the world [4-9]. Although, published experimental results showed that DBD plasma actuators are capable of modifying airflow around aerodynamic bodies, they are still not used for practical applications because of the relatively low airflow velocity generated by DBD actuators (for a single DBD actuator generated airflow typically does not exceed 5-6 m/s). Thus, more investigations leading to a better understanding of surface DBD properties and mechanism of inducing EHD flow are needed in order to improve their performance. For studies of an EHD flow induced by DBD plasma actuators and for investigations of airflow around airfoils 2-Dimensional (2D) PIV technique is commonly used. PIV method is a laser technique for velocity field measurements in fluids, developed thanks to the great advances in the field of pulsed lasers and high-speed digital cameras. This method can be applied for flow


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measurements in a very wide range of the size of measured area. This method is particularly useful for flow topology visualisation and measurements in vicinity of high voltage discharges, where classical methods like hot wire anemometry cannot be used because of possible electric arcs. Our 2D PIV measurements presented in this paper were aimed at investigations of effects of electrohydrodynamic flow induced by the multi-DBD plasma actuator on flow around airfoil models.

2. Experimental set-up

The scheme of experimental set-up for measurements of the flow around NACA airfoil models is presented in Fig. 1. It consisted of a wind tunnel test section where NACA airfoil models were placed, the 2D PIV equipment for measurements of the flow velocity fields, an AC power supply and an oscilloscope for current and voltage waveforms monitoring. The NACA 0012 and NACA 0015 airfoil models were used and equipped with the multi-DBD plasma actuator.

Fig. 1 Scheme of experimental set-up for the 2D PIV measurements of the airflow around NACA airfoil models

Wind tunnels Experiments described in this paper were carried out in two wind tunnels. The leading edge flow control investigations were conducted at the University of Orleans in a large-scale subsonic wind tunnel. The test section was 2 m in high, 2 m in wide and 5 m in long. The operating speed of the wind tunnel varies from 10 m/s to 60 m/s with an airflow turbulence level below 0.4%. A plexiglas window mounted on one side enables the PIV measurements. A frame sustaining NACA 0012 airfoil model was placed in the test section. The model could be rotated to change the angle of incidence. During our investigations, the free stream velocity was fixed to 10 m/s. The trailing edge flow control studies were performed in the wind tunnel at the Szewalski Institute of Fluid Flow Machinery. The section of the wind tunnel at the Szewalski Institute of Fluid Flow Machinery was 0.6 m wide and 0.46 m high and 1.5 m long. The NACA 0015 airfoil model was mounted in the frame which allowed to change an airfoil incidence angle. The maximum free stream airflow velocity in the wind tunnel test section is up to 100 m/s and the turbulence level is below 0.1%. The results showed in this paper were obtained at a free steam velocity of 10 m/s.


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Fig. 2 Image of the test section of the wind tunnel at the Szewalski Institute of Fluid Flow Machinery

2D PIV apparatus The 2D PIV systems (at the University of Orleans and the Szewalskie Institute of Fluid Flow Machinery) were composed of a double Nd-YAG laser system ( = 532 nm), a cylindrical telescope, a CCD camera and a PC computer. A laser sheet shaped by the cylindrical telescope was formed above the airfoil model. An oil droplets or incense smoke were used as a seeding. The images of the seeding particles following the airflow around airfoil models were recorded by a CCD camera. The CCD camera sensor size was 2048 pixels 2048 pixels (for leading edge flow control experiment) and 1600 pixels 1186 pixels (for trailing edge flow control experiment). During the trailing edge experiment the camera was equipped with an interference filter (FWHM 11.8nm around = 532.6 nm) to eliminate influence of ambient light on the recorded images. The captured pairs of PIV images were transmitted to the PC computer for a digital analysis. Digital analysis (e.g. to compute instantaneous and time-averaged flow velocity fields) was made using a Dantec Flow Manager software. 2D PIV measurements were carried out in a plane defined by the laser sheet which was set in both cases at the middle of the airfoil models in their spanwise direction. 200 pairs of PIV instantaneous flow images were taken. Then, an adaptive cross-correlation algorithm was applied to compute instantaneous flow velocity fields. The adaptive correlation method calculated velocity vectors with an initial interrogation area of the size equal to N times (N is defined by a user) the size of the final interrogation area. Then, the algorithm used the intermediary velocity vectors as information for the next (smaller) interrogation area. This procedure was repeated until the final interrogation area size was reached. Basing on instantaneous flow velocity fields time-averaged flow velocity fields and apparent flow streamlines were calculated. In our case the final interrogation area was 32 pixels 32 pixels. The overlap between neighboring interrogation areas was 25%. The spatial resolution was 90 m per pixel (leading edge flow separation control experiment) and 70 m per pixel (trailing edge flow separation control experiment).

NACA airfoil models with plasma actuators Two NACA airfoil models were prepared for the leading and trailing edge airflow separation control experiments. The surface of both airfoil models was hollowed out to create a shallow insert


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(in spanwise direction) in which the multi-DBD actuator was placed. Such an actuator placement reduced the flow disturbances induced by the actuator itself. The first airfoil model having NACA 0012 profile was used in the leading edge airflow separation control experiment. The airfoil was 1100 mm wide (spanwise) and its chord was 300 mm long (Fig. 3). The multi-DBD plasma actuator was mounted (Fig. 4) near the leading edge of the airfoil. The first discharge generated by this actuator started at position z/C = 4% (z - position in z direction, C - chord length). In the trailing edge airflow separation control experiment the NACA 0015 airfoil model was used. The airfoil was 595 mm wide (spanwise) and its chord was 200 mm long (Fig. 5). The first DBD generated by the multi-DBD actuator mounted in the airfoil started at position z/C = 52%.

Fig. 3 NACA 0012 airfoil model placed in the wind tunnel test section (experiment in University of Orleans)

Fig. 4 Schematic side view of the NACA 0012 airfoil model equipped with the multi-DBD actuator (experiment in University of Orleans)

Fig. 5 Schematic side view of the NACA 0015 airfoil model equipped with the multi-DBD actuator (experiment in the Szewalski Institute of Fluid Flow Machinery)


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Multi-DBD actuator For each NACA airfoil model different multi-DBD actuator was used. Schemes of the actuators used in the leading edge (NACA 0012) and trailing edge (NACA 0015) airflow control experiments are showed in Fig. 6a and 6b, respectively. To fit the multi-DBD actuator to the airfoil model a flexible material (Kapton tape) was used as a main dielectric barrier. All electrodes used in these actuators were made of 50 m thick copper tape. The electrode lengths were 1000 mm in the case of actuator for NACA 0012 airfoil and 500 mm in the case of actuator mounted on NACA 0015 airfoil. The floating and grounded electrodes were on the flow active side of the dielectric material and were partially insulated with Kapton tape (as it is shown in Fig. 6). The HV electrodes were on the opposite side of the dielectric barrier and were fully insulated. Therefore, a dielectric barrier discharge and a plasma were generated by the air-exposed grounded and floating electrodes. The HV electrodes were smooth (Fig. 7a), while the grounded electrodes and the floating interelectrodes were serrated (saw-like) (Figs. 7b and 7c). The floating interelectrode consisted of a series of separated saw teeth. The serrated electrodes were used because our previous results showed that with such electrodes the DBD had started at lower voltage, produced more homogenous plasma and induced EHD flow with higher velocities than the DBD with smooth electrodes [10].

a)

b)

Fig . 6 Schematic side view of the multi-DBD actuator mounted on the NACA 0012 (a) and NACA 0015 (b) airfoil models

Fig . 7 Schematic top view of the smooth HV electrode (a), the serrated grounded electrode (b) and separated saw teeth of the floating interelectrode (c)


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For all experiments, the plasma was obtained with a steady actuation performed by applying a sinusoidal signal to the insulated electrode with a high-voltage amplitude VHV =7.5 kV and a frequency FHV =1.5 kHz (plasma was produced by air-exposed grounded and floating electrodes). For these operating electrical parameters, in Fig. 8, time-averaged flow patterns produced by our multi-DBD are presented. It has to be noted that the results presented in this figure were obtained for a multi-DBD actuator with inflexible dielectric barrier material i.e. with a glass plate. Electrode positions and HV electrodes width were slightly different form these described above. As it can be seen in Fig. 8, a significant airflow was induced tangentially to the dielectric surface. It was almost continuously accelerated along the consecutive actuators (Fig. 8a). Backward flow or vortices between successive DBD sets were not observed (Figs. 8a and 8b). In this case the maximum induced airflow velocity was about 8 m/s but it could be increased up to 10 m/s when the applied voltage frequency was increased [11]. Thus, the airflow velocity generated by this multi-DBD actuator was twice higher than an airflow velocity produced by a typical single-DBD actuator.

Fig . 8 Time-averaged contour velocity map and (a) and time-averaged flow velocity vector field of the airflow produced by the multi-DBD actuator (c). VHV =7.5 kV ; FHV =1.5 kHz.. FL floating interelectrode.

3. Results

Focusing on demonstrating abilities of the multi-DBD actuator to suppress flow separation, the results presented in this paper were obtained for both airfoils placed at an incidence chosen according to aerodynamic load evolution against incidence in a freestream flow of 10 m/s (Reynolds number equal to 2105 and 1.3105 for the NACA 0012 and NACA 0015 model, respectively). In the case of leading edge flow separation, the chosen incidence was representative of full separated flow corresponding to a near post-stall configuration, and in the case of trailing


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edge flow separation, it was chosen in the range corresponding to a progressive flow separation from the trailing edge but before the full separated flow leading to a full stall configuration.

3.1 Leading edge flow separation

Active separation control in this configuration involves Reynolds number effects and according to actuator location, actuation effects can be considered as an active boundary layer tripping. Even if actuators were flush mounted at the model surface in order to limit their intrusivity, tests were performed in natural boundary layer (actuator not operated) and in tripped boundary layer using turbulators located at the leading edge just before the actuator. Then it could be assumed that a laminar and a turbulent boundary layer separation occurred respectively.

Flow patterns of the airflow near the leading edge of the NACA 0012 airfoil in natural boundary layer conditions are presented in Figs. 9-13 for an angle of incidence of 12.3 degrees. Instantaneous vector velocity field of the airflow when the multi-DBD actuator was turned off is shown in Fig. 9. In this figure vectors show flow direction while colours show velocity magnitude. Vortex formation (from z = 100 to z = 140) can be observed suggesting vortex shedding in the airfoil wake. A time-averaged (obtained from 200 instantaneous flow images) vector velocity field map, contour maps of velocity or turbulence intensity in the freestream flow direction and streamlines are presented in Figs. 10, 11 and 12. As expected, it is observed in these figures that the airflow separation occurs close to the leading edge of the airfoil and a large recirculation zone above the airfoil surface exists. Figure 12 shows high level of the fluctuating velocity in the freestream direction along the recirculation zone contour, highlighting the shear layer emitted from the flow separation line. When the multi-DBD actuator is turned on the velocity vector field is representative of an attached flow and turbulence intensity contours are homogenously low. Thus the flow full reattachment occurs (Figs. 13 and 14).

Fig. 9 Instantaneous vector velocity field of the airflow (natural boundary layer) near the leading edge. Actuator off.


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Fig. 10 Time-averaged vector velocity field of the airflow (natural boundary layer) near the leading edge. Actuator off.

Fig. 11 Time-averaged contour velocity field and the apparent streamlines of the airflow (natural boundary layer) near the leading edge of the NACA 0012 airfoil model. Actuator off.

Fig. 12 Turbulence intensity in the freestream direction and streamlines of the airflow (natural boundary layer) near the leading edge. Actuator off.


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Fig. 13 Time-averaged vector velocity field of the airflow (natural boundary layer) near the leading edge. Actuator on (VHV =7.5 kV ; FHV =1.5 kHz).

Fig. 14 Turbulence intensity in the freestream directionand streamlines of the airflow (natural boundary layer) near the leading edge. Actuator on (VHV =7.5 kV ; FHV =1.5 kHz).

Flow patterns of the airflow near the leading edge of NACA 0012 airfoil model in case of tripped boundary layer are presented in Figs. 14-17 for an angle of incidence of 11.8 degrees. It can be observed in Figs. 14 and 15 that when the multi-DBD plasma actuator is turned off the airflow separation occurs. When the actuator is turned on the airflow is not as fully reattached near the surface as for the natural boundary layer case, but flow separation is largely attenuated (Figs. 16 and 17). It may be due to the fact that the induced flow by the DBD actuator was not sufficient enough with this operating conditions in a turbulent flow.


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Fig. 14 Time-averaged contour velocity field of the airflow (tripped boundary layer) near the leading edge Actuator off.

Fig. 15 Time-averaged contour velocity field and streamlines of the airflow (tripped boundary layer) near the leading edge. Actuator off.

Fig. 16 Time-averaged contour velocity field of the airflow (turbulent boundary layer) near the leading edge. Actuator on (VHV =7.5 kV ; FHV =1.5 kHz).


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Fig. 17 Time-averaged contour velocity field and the apparent streamlines of the airflow (turbulent boundary layer) near the leading edge. Actuator on (VHV =7.5 kV ; FHV =1.5 kHz).

3.2 Trailing edge flow separation

Similar effect of actuation was observed during the trailing edge flow separation experiments. The example of obtained time-averaged contour velocity map of the airflow around the NACA 0015 airfoil model (without actuation) is presented in Fig. 18 for an angle of incidence of 14. As it can be seen, the flow above the airfoil model is separated and a large vortex exists near the trailing edge of the airfoil model. When the multi-DBD actuator is activated, almost fully reattachment of the airflow occurs and only a small vortex close to the trailing edge of the airfoil can be observed (Fig. 19).

Fig. 18 Time-averaged contour velocity map and the apparent streamlines of the airflow around the NACA 0015 airfoil model Actuator off.


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Fig. 19 Time-averaged contour velocity map and the apparent streamlines of the airflow around the NACA 0015 airfoil model. Actuator on (VHV =7.5 kV ; FHV =1.5 kHz).

4. Summary

The ability of the multi-DBD plasma actuator with floating interelectrode to influence the airflow around airfoil models was investigated. 2D PIV measurements were carried out for investigation of flows around NACA 0012 and NACA 0015 airfoil models with plasma actuator located on their surface. It enabled flow topology visualization and flow velocity measurements. The results obtained for the leading edge (NACA 0012) and trailing edge (NACA 0015) airflow separation control experiments showed that EHD flow induced by our multi DBD actuator enables reattachment of separated flow and thus is capable of postpone the airfoil stall for flows with Reynolds numbers of the order of 105. As for common applications of flow separation control by DBD actuators reported in the literature, the location of the DBD actuator remains a key parameter for optimizing control effects, as well as operating electrical parameters (frequency, amplitude, burst modulation of the high voltage for example). However, our multi-DBD actuator with floating interelectrode can be attractive for some aerodynamic applications and more investigations are needed for the DBD plasma actuators development. The new actuators should effectively influence the airflow around aerodynamic elements at higher airflow velocities and Reynolds numbers.

Acknowledgments

The research presented in this paper received funding from the European Community, Seventh Framework Programme FP7/2007-2013 under grant agreement no.: 234201 (PLASMAERO Useful PLASMas for AEROdynamic control www.plasmaero.eu).

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PIV Measurements of the Flow Around Airfoil Models Equipped With the Plasma Actuator