Wind Tunnel Experiments on a NACA0015 Airfoil Equipped with Vectorizable Dielectric Barrier Discharge Plasma Actuators

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1 Paper AIAA Wind Tunnel Experiments on a NACA0015 Airfoil Equipped with Vectorizable Dielectric Barrier Discharge Plasma Actuators Carlo A. Borghi 1, Andrea Cristofolini 2, Gabriele Neretti 3 and Paolo Seri 4 University of Bologna, Bologna, 40136, Italy Alessandro Talamelli 5 and Alessandro Rossetti 6 University of Bologna, Forlì, 47121, Italy This work reports an experimental investigation performed in wind tunnel facility on a NACA0015 airfoil equipped with a set of vectorizable fluid-dynamic DBD plasma actuators. The ability of the actuators to recover the stall condition with free stream velocities in the range 5-23 m/s has been investigated. The discharge has been generated by feeding the actuators with a sinusoidal voltage waveform with a frequency of 15 khz and voltages up to 7.5 kv peak. The actuator positioned on the leading edge exhibits highest ability in the stall recovery. Unsteady actuation obtained by alternatively switching on and off the discharge is more effective in the stall recovery when compared with a steady actuation. The frequency that optimizes the stall recovery has been found to be a function of the velocity with the power of 1.5. This result leads to the determination of a constant Strouhal number if the boundary layer thickness is used as characteristic length. Finally the lift coefficient, obtained when the plasma actuator has been turned on, has been found to be a linear function of the applied voltage. T I. Introduction HE interest in flow control has showed a quick growth in the last fifteen years 1 due to the rapid development of sensor and actuators technologies. Even though passive control techniques (with no energy input) are still very interesting for the applications, active control strategies have recently received more attention since they can be used and programmed in a selective way operating the control only when it is effectively requested. One of these techniques can significantly modify the boundary layer around aerodynamic surfaces. Many experimental and numerical works have been done to both understand the basic physical phenomena involved in the EHD interaction 2-13, and evaluate the aerodynamic effects on surfaces Plasma actuators are attractive because they present high dynamic characteristics due to the absence of moving parts and short actuation times. Moreover they are characterized by low weights, they are easy to build and they generate negligible aerodynamics interferences when they are non-active. In the aeronautical domain, these actuators can be used to prevent or to induce flow separation, to reduce drag and to enhance the lift of airfoils. The peculiar construction of an EHD aerodynamic actuator induces jets in the direction tangential to the wall. These jets can modify the aerodynamic boundary layer within the first millimeters above the surface. A plasma aerodynamic actuator, able to generate a vectorizable jet, could simulate the presence of a fluid dynamic actuator as a synthetic jet. The use of these devices in airfoils delays the stall and enhances the lift 17,18. In last years several investigations have dealt with Plasma Synthetic Jet Actuators (PSJA) built by means of DBDs or Sliding Discharges in annular and linear configurations PSJAs could thus mimic synthetic jets enhancing the dynamic response and lowering the weight of the whole actuating system. In a previous work 27 a vectorized plasma DBD actuator able to 1 Professor, Dept. of Electrical, Electronic and Information Eng., Viale Risorgimento, 2, AIAA Member. 2 Professor, Dept. of Electrical, Electronic and Information Eng., Viale Risorgimento, 2, AIAA Member. 3 Assistant Professor, Dept. of Electrical, Electronic and Information Eng., Viale Risorgimento, 2, AIAA Member. 4 Ph.D. Student, Dept. of Electrical, Electronic and Information Eng., Viale Risorgimento, 2. 5 Professor, Dept. of Industrial Engineering, Via Fontanelle, 40, AIAA Member. 6 Researcher, Dept. of Industrial Engineering, Via Fontanelle, 40, AIAA Member. 1

2 produce an induced jet in a desired direction has been experimentally investigated. The vectorization is obtained by varying both the applied voltages and the electrical connections of the electrodes. In this work, a set of these vectorizable actuator arrays has been mounted on the surface of a standard NACA0015 airfoil and tested in a subsonic wind tunnel facility. Flows with free stream velocity ranging from 5 to 23 m/s have been considered. Angles of attack up to 26 have been used to obtain the complete stall in all flow conditions. Position and orientation of the induced jet, unsteady actuation frequency and supplying voltage have been varied to evaluate the stall recovery entity of the whole profile by means of a six components balance measurements. Particular attention has been paid to the unsteady actuation frequency in order to obtain a relation between this frequency and fluid dynamic parameters. A. Fluid-dynamic setup II. Experimental setup The experiments have been performed in the open-return type low speed wind-tunnel of the CICLoPE laboratory of the University of Bologna. The flow is supplied by a 39 kw axial-fan, capable to set and keep a stable velocity in the test section 0.1 m/s with a maximum speed of about 55 m/s. The test chamber, 1.9 m long, has a rectangular section, with a height of 0.90 m and a width of 0.60 m. The tunnel is equipped with a series of screens and a honeycomb to provide a good quality flow in the test section in terms of homogeneity and free stream turbulence. The tests have been performed on a standard NACA 0015 profile with a 0.31 m chord and a span of 0.33 m. Two circular Plexiglas end plates with a diameter of 0.38 m are mounted at the tips of the wing to avoid the formation of streamwise vorticity and to minimize three dimensional effects. In Fig. 1 a picture of the NACA0015 airfoil positioned within the wind tunnel facility. On the surface of the profile, the electrodes used to generate the discharge are clearly visible. Tell-tails have been positioned on the surface of the airfoil to visualize stall regime and recovery regimes. The reference velocity was measured in the free-stream by means of a Prandtl tube connected to a pressure transducer Setra 239 with a nominal full scale output of 2500 Pa, and an accuracy of +/- 0.14% FS. Atmospheric pressure, air temperature and humidity were continuously acquired to calculate the actual air density and viscosity and the Reynolds number. The lift produced by the profile was measured by means of a six components DELTATECH, P123.TRIAX-S-A- 500N balance based on strain gauges technique. The different components can be measured in three orthogonal directions with a nominal full scale output of : 350 N for Fx, 700 N for Fy, 300 N for, Fz and 100 Nm for Mx, My and Mz, with a resolution of 0.2 N. To have good precision in setting a correct angle of attack for the airfoil, a step-by-step % phase VEXTA electric motor with a resolution of /step was used. This motor is connected to the airfoil through a rod. All this is placed on top a sliding mechanism which can adjust the airfoil in the lateral direction. This sliding mechanism is for calibration purposes only and is connected to the balance. Data have been sampled by a National Instrument NI- USB bit 250 ks/s High Performance Fig. 1. Naca0015 airfoil equipped with plasma actuators, mounted within the wind tunnel facility. multifunction acquisition board, controlled by a Labview program, written in order to rule automatically the whole setup so far described. 2

3 B. Electrical setup The electrode configuration of a single vectorizable actuator array is shown in Fig. 2. The dielectric slab has been made by 250 µm thick Mylar sheet between two layers of 90 µm thick Kapton tape. This composite dielectric slab has been made to obtain high dielectric strength, thermal and chemical stability, flexibility and ease of implementation. The electrodes are made of copper strips 300 mm wide (z-direction) and 0.04 mm thick (ydirection). In the upper side of the dielectric sheet, two upper electrodes ( Up 1 and Up 2 ) have been placed at a distance of 15 mm in x-direction. These electrodes are 5 mm large in x-direction. In the lower part of the dielectric sheet, four lower electrodes ( Low 1, Low 2, Low 3 Fig. 2: Schematic of the electrode configuration. and Low 4 ) have been placed at a distance of 3 mm in x- direction. These electrodes are 7 mm large (x-direction) and are not exposed to the air to avoid plasma formation. Upper and lower electrodes are overlapped of one millimeter in x-direction on both sides to decrease the electric tip effect. Hence this actuator system consists of two units. Each upper electrode, the dielectric and the two lower electrodes facing the upper one, constitutes a single actuator unit. The two units (Up 1 - Low 1 - Low 2 and Up 2 Low 3 Low 4), are placed in a sequential array to form the multiple vectorized aerodynamic DBD plasma actuator. A set of 8 units have been used to generate 7 jets over the airfoil surface. The Kapton-Mylar-Kapton sheet, due to its flexibility, has been adapted and attached to the surface of the NACA0015 profile (Fig. 3). Fig. 3. Sketch of the NACA0015 airfoil equipped with the DBD plasma actuators. Fig. 4. Actuators and jets positions over the airfoil. Jets position and reference directions of the induced jets are shown in Fig. 4. Actuators have been positioned over the NACA0015 profile as the Actuator 5 corresponds to the leading edge. As reported in the figure, a 0 angle corresponds to a jet induced in the direction going from the leading edge toward the trailing edge (i.e. in the downstream direction). A fully electrical and fluid-dynamic characterization of a single oriented jet (fig. 1) in still air is reported in the work of Neretti et al 27. In the work velocity profiles have been achieved by Pitot tube measurements and jet orientation and morphology by using Schlieren diagnostic. The vectorization of the jet is obtained by supplying the upper electrodes by means of sinusoidal voltages at Fig. 5. Schematic of the electric connections utilized. the same frequency and different amplitudes. The frequency has been set to 15 khz. The electrical connections used in the present work are 3

4 schematized in figure (Fig. 5). Different electrical connections of upper electrodes, voltage values and subsequent jet orientation are listed in Table 1. Fig. 6. Velocity profiles (left hand side) and Schleiren images (right han side) for induced jet orientations of 0 (a), 45 (b) and 90 (C). 4

5 Table 1. Jet orientations dependence to the electrical connections Jet orientation [ ] Up 1 Up 2 Low 1 Low 2 Low 3 Low kvp floating floating ground floating floating 45 6 kvp 4.2 kvp floating ground ground floating 90 6 kvp 6 kvp floating ground ground floating kvp 6 kvp floating ground ground floating 180 floating 6 kvp floating floating ground floating Velocity profiles and Schlieren images for jet orientations of 0, 45 and 90 degrees are shown in Fig The 0 angle corresponds to the typical wall jet induced by a single DBD plasma actuator. A velocity up to 4 m/s and a jet width of about 1 mm can be detected ad a distance of 10 mm downstream the high voltage electrode. The nature of the vectorized jet is clearly visible in the Schlieren images b) and c) where an inclination of 45 and 90 are respectively displayed. When an inclination of 90 is generated, a velocity up to 3.5 m/s and a jet width of about 6 mm are reached at a distance of 5 mm from the dielectric surface. The supply system consists of a one H-bridge branch connected to a transformer cascade, used to increase the voltage to the desired value (Fig. 7). The sinusoidal voltage can be varied between 0 and 7.5 kv peak, with a frequency between 1 and 25 khz. The maximum active power is 200 W. The ac supply has been set to feed the actuator with the fixed voltage V1. The variable voltage V2 has been obtained by means of a resistive voltage divider constituted by two resistances R and R 0 (see Fig. 3) connected with the ac supply generator. If the actuator impedance magnitude is much bigger than the resistance R0, the voltage V2 can be calculated as: V 2 = R 0 R 0 + R V 1 Eq. 1 Fig. 7. Electrical supply system scheme. The voltage V2 is controlled by varying the value of the variable resistance R. According to (1), V2 results to be in phase with the fixed voltage V1. This assumption was verified by measuring simultaneously the two voltages. III. Results and discussion A. Jets orientation effect In this section the results obtained by using jets in different positions for all orientations (0, 45, 90, 135 and 180 ) will be presented. A free stream velocity of 11 m/s (Re=220000) and an angle of attack equal to 19 have been chosen. In these conditions the profile is subjected to a complete stall. Referring to Fig. 4, jets 1, 2, 3, 7 and 8 show negligible effects in the stall recovery mechanism. The percentage recovery of the lift for jets 4, 5 and 6 operated in continuous mode are reported in Fig. 8 a) as a function of the jet orientation. The percentage lift recovery has been defined as the percentage increase in lift due to the DBD actuator at a given stall angle. Except the notable exception that occurs when the actuator 5 produces a 90 jet, which clearly shows a boundary layer separation, the dependence of the lift recovery on the orientation of the jet seems to be rather weak. The lift coefficient recovery increases when the same actuators are operated in discontinuous mode. Fig. 8 b), the percentage lift recovery for jets 4, 5 and 6 operated with a 25 Hz commutation frequency and a 50% duty cycle is shown. Also in this case, the C L recovery does not appear to strongly depend on the orientation of the jet, at least for angle less than 135. It is remarkable the different behaviour of the actuator 5 and a jet orientation of 90 which in this case keeps the flow fully attached. The low sensitivity of the control on the jet orientation suggests that the stall recovery mechanism is not governed by the momentum supplied to the flow by the DBD discharge, but rather to the ability of the actuator 5

6 to produce vorticity in the boundary layer. The frequency at which the vortices are produced depends mainly on the frequency of the on-and-off cycle with which the actuator is fed. Fig. 8. Percentage recovery of the lift for jets 4, 5 and 6, with a free stream velocity of 11 m/s and angle of attack of 19. Steady plasma actuation (a) and unsteady operation with f=25 Hz (b). In order to get an insight on this phenomenon, the actuator 5 has been fed with different cycle frequencies. For all the tests, a 50% duty cycle has been utilized. The percentage C L recovery of jet 5 as a function of the on-off cycle frequency, is reported in Fig. 9 a) and b) with 5 and 15 m/s free stream velocity respectively. Fig. 9 a) and b) have been obtained considering an angle of attack of 26 and a 0 jet orientation. The percentage C L recovery has been here defined as the ratio of the lift coefficient of the airfoil obtained by means of the DBD actuator for an angle of attack of 26, and the maximum lift coefficient attainable by the same airfoil at the same free stream velocity without the aid of the DBD actuator. For each speed, the figure points out that a different C L recovery is obtained as a function of the cycle frequency. The figure allows an optimal frequency f opt to be identified, at which the C L recovery is maximum. The behavior of the percentage C L recovery vs. on off frequency curves is similar to the one depicted in Fig. 9 for all the free stream velocity investigated: an initial steep increase of the lift coefficient can be observed at low frequencies; the recovery then stabilizes, exhibiting a rather weak dependence on the on-off frequency. This feature makes the identification of the optimum frequency quite difficult in some cases f!"# = v 1.55 Eq a) v=5 m/s 90 b) v=15 m/s C L recovery [%] C L recovery [%] f [Hz] f [Hz] Fig. 9. Lift coefficient recovery obtained by using the jet 5 ant 0 with an angle of attack of 26 and by using different cycle frequencies. Free stream velocity equal to 5 m/s (a) and 15 m/s (b). 6

7 Fig. 10. Unsteady frequency that optimizes the CL recovery as a function of the free stream velocity. Data obtained by actuating Jet 5 with 0 orientation. number is Fig. 10 reports the optimal frequency obtained for different free stream velocity. Even though a linear regression fit quite well the given data set, yielding a coefficient of determination R 2 = 0.978, it is quite evident that fitting the data by means of a power law produces a better result, with R 2 = The exponent value resulting from the power shows that the optimal frequency is roughly proportional to v 3/2. This observation suggests that the characteristic length L c to be considered when evaluating the Strouhal number is the laminar boundary layer thickness δ, which depends on the inverse of the square root of the Reynolds number, and thus on the inverse of the square root of the free stream velocity. Assuming L c = δ, and considering the experimentally observed optimal frequency, the Strouhal St = fl c v = f optδ v ~constant Eq. 3 Fig. 11 a) shows the comparison between the normalized Strouhal number (St L ) calculated using the chord length as the characteristic length, and normalized Strouhal number (St δ ) evaluated using the thickness of the boundary layer for actuator 5 with a 0 orientation. The Strouhal numbers shown has been normalized to their average value. St pu a) Strouhal number, jet orientadon 0 1,50 1,30 1,10 0,90 0,70 0,50 St L pu St delta pu speed [m/s ] speed [m/s ] Fig. 11. Strouhal number in per unit as a function of the free stream velocity, with a jet orientation of 0 (a) and 45 (b). In Fig. 11 b) the same comparison is shown for actuator 5 with a 45 orientation. In both figures, St δ shows a constant behavior over the entire range of the investigated velocities. Data beyond free stream velocity of 14 m/s has not been reported, as the optimas frequency was not clearly identifiable. In Fig. 12, the maximum percentage recovery of the lift coefficient CL due to the actuator 5 is shown as a function of the free stream velocity for different jet orientation. As can be observed, the jet orientation does not dramatically affect the stall recovery. The most notable differences can be observed at low speeds, where the jet angle with zero produces a higher recovery of the other two. This difference, however, decreases as the free stream velocity increases. St pu b) Strouhal number, jet orientadon 45 1,3 1,2 1,1 1 0,9 0,8 0,7 0,6 St L pu St delta pu 7

8 Fig. 12. Maximum percentage recovery of the C L as a funstion of the free stream velocity for 0, 45 and 90 orientation of the jet 5. The effect of the jet 5 actuated with a 0 angle, with free stream velocity of 5, 11 and 19 m/s can be observed in Fig. 13. The blue line represents the lift coefficient of the profile for an angle of attack ranging from 0 to 26 and with the plasma actuator turned off. The red square indicates the lift coefficient for a 26 angle of attack obtained when the plasma actuator is turned on with a peak voltage of 6kV and the on/off frequency that maximizes the stall recovery. In the first graph (a), with a free stream velocity of 5 m/s (Re=100000) the plasma actuator guarantees a pronounced recovery, as well as the lift coefficient curve before the stall angle (17 ) can be prolonged for continuity until an angle of attack of 26. At 11 m/s (Re=220000) the stall recovery is still high and almost complete. At 19 m/s (Re=38000), the stall recovery is only partial. The lift coefficient the airfoil can attain in deep stall regime depends on the voltage applied to the actuator. In order to investigate on this dependence the test article has been set up at an angle of attack of 26, for free stream velocity ranging from 9 to 19 m/s. Actuator 5 has been operated with an on-off frequency of 20 Hz, and a 0 jet orientation. The lift coefficient obtained in the described conditions has been reported in Fig. 14. As can be noted, the lift coefficient depends linearly on the applied voltage. For actuators operating in continuous regimes, velocity linearly depends on the applied voltage 27. The results shown in Fig. 14 suggest that a similar relation exists between the applied voltage and the vortex propagation velocity and size. This aspect need further investigations. IV. Conclusion A standard NACA0015 airfoil equipped with a set of vectorizable fluid-dynamic DBD plasma actuators has been positioned in a wind tunnel facility. The ability of the actuators to recover the stall condition with free stream velocities in the range 5-23 m/s has been investigated. The DBD actuators have been fed with a Fig. 13. C L curves for with actuators turned off (blue) for angle of attack ranging from to 0-26 and free stream velocities of 5, 11 and 19 m/s. Fig. 14. Lift coefficient as a function of the applied voltage obtained with the jet 5 at a 0 orientation, for free stream velocity in the range 9-19 m/s. 8

9 sinusoidal voltage source set a fixed frequency of 15 khz and a variable voltage between 0 to 7.5 kv peak. The vectorization of the induced jets has been obtained by generating two opposite tangential wall jets of well-defined thrusts. Two jets with the same thrust, generates an overall perpendicular induced jet. A first set of experiments has been carried out to evaluate the effect of the jet orientation in the stall recovery. To do this, a free stream velocity of 11 m/s and an angle of attack of 19 have been chosen. Jets 4, 5 and 6 (the closest to the leading edge) present higher influence in the stall recovery. By actuating the jets with an on/off frequency of 25 Hz, highest performances have been obtained, especially for the jet 5, located on the leading edge. In order to point out a better insight of the unsteady actuation, the airfoil has been inclined with an angle of attack of 26 and tested with velocities in the range 5-23 m/s. The jet 5 has been actuated at 0, 45 and 90 orientations with several on-off frequencies to find the one that optimizes the stall recovery. The optimal frequency has been found to be a function of the free stream velocity at the power of 1.5, for both 0 and 45 jet orientations. This result suggests a relation between the on-off frequency and the thickness of the boundary layer. As evidence of this a constant Strouhal number has been calculated when the boundary layer thickness has been used as characteristic length (instead of the chord length). Maximum stall recovery obtained by the jet 5 with 0, 45 and 90 as a function of the free stream velocity underlines that the 0 configuration is the more effective both for higher recovery values both because a lower power consumption. An evaluation of the lift coefficient obtained with the jet 5 at 0 supplied with different voltages has been performed with free stream velocity in the range 9-19 m/s. 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