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Planform-view visualizations near the inboard plasma actuator are presented in Figs. 19 and 20 for 8 and 14 deg, respectively. These images illustrate the flow reversal and separation region that occur past the separation ramp. The tufts past the ramp are observed to be significantly unsteady and oscillate in a direction opposite to the flow, which is indicative of a region of flow reversal. As the plasma actuator is pulsed, the tufts past the ramp are observed to become mostly parallel to the flow, and there is a remarkable reduction in the unsteadiness and oscillations of the tufts. The region of “steady” tufts close to the surface indicates that the flow is mostly attached in the region past the separation ramp. By examining the flow over the windward surface, the effect of the plasma actuator on the lift coefficient can be assessed. The plasma actuator has the effect of reattaching the flow for some distance downstream of the reflex line of the separation ramp. For the flow to be reattached, the flow close to the reflex line of the separation ramp needs to accelerate around the ramp and a local low pressure is expected to occur. This induced low-pressure region near the windward-surface separation ramp will cause a reduction in the net pressure difference over the wing, and as a result, a decrease in lift is expected. In addition, if the flow is reattached for most of the region aft of the ramp, the effect of the plasma actuator would be to reduce the effective camber of the wings, thereby decreasing the overall lift. Because flow visualizations demonstrate that the plasma actuator reattaches the flow over a region past the reflex line of the separation ramp, the results correlate well with force measurements that show a decrease in the lift coefficient. IV. Conclusions Wind-tunnel experiments were conducted to investigate the aerodynamic effect of plasma actuators mounted at the onset of a windward-surface separation ramp near the trailing edge of a 1303 47-deg sweep UAV model. Results presented are proof-of-concept investigations on the use of the technique for aerodynamic control of an air vehicle. Force measurements show that the plasma actuator has the effect of decreasing the lift coefficient compared with the baseline (control off) case. Results also show that an unsteady (pulsed) plasma actuator has a significant effect on the lift coefficient, whereas a steady actuator (100% duty cycle) has a negligible effect. Three windward-surface separation ramps with backward ramp angles of 20, 30, and 40 deg were examined. Significant reductions in the lift coefficient were obtained using the 20- and 30-deg ramp configurations. For some conditions, reductions of 15 to 25% in the lift coefficient were obtained. With ramp angles higher than 20 deg, a duty cycle of 25% yielded the most significant decreases in the lift coefficient, whereas a 12.5% duty cycle produces the best results for a 20-deg ramp. Further experiments need to be performed to verify the optimal actuator parameters at higher ramp angles. The ramp angle was found to be a critical design parameter for determining the effectiveness of windward-surface plasma actuators. As the ramp angle increases, the associated adverse pressure gradient past the ramp increases accordingly. At large ramp angles, up to 40 deg in the current study, a large adverse pressure gradient and strong separation overwhelm the actuator effect on the flowfield and render the plasma actuator ineffective. Flow visualizations for the baseline case showed that the flow separated past the separation ramp and there was a region of flow reversal. In contrast, visualizations of pulsed plasma actuators showed that a region of the flow past the separation ramp was reattached. The partial/complete reattachment of the flow past the ramp is conjectured to produce a low-pressure region around the ramp and thus a decrease in lift. In addition, if the flow is attached aft of the ramp up to the trailing edge, the windward-surface plasma actuator will have the effect of reducing the effective camber of the wing and inducing a reduction in the lift coefficient. The effect of windward-surface plasma actuators was examined herein at 15 m=s (29 kt). A recent investigation by Patel et al. [14] investigated the scalability and effectiveness of leading-edge separation control on airfoils using SDBD plasma actuators. Their experiments demonstrated that the SDBD plasma actuator was effective in reattaching the flow for chord Reynolds numbers up to 1:0 106 and freestream speeds up to 60 m=s (117 kt). They also showed that the optimum unsteady actuator frequency fmod minimized the actuator voltage needed to reattach the flow, such that F fmod Lsep=U 1. In addition, Patel et al. indicated that at the optimum frequencies, the minimum voltage required to reattach the flow was weakly dependent on the chord Reynolds number and strongly dependent on the poststall angle of attack and leading-edge radius. Although moment measurements were not directly measured, it can be indirectly inferred from the reduction of the local lift coefficient that rolling and pitching moments could be generated by placing the control at different parts of the air vehicle. These induced moments can potentially be used to control and alter the dynamics of the air vehicle. The flow over the windward surface of different wing planforms is expected to resemble a 2-D flow with a weak or negligible crossflow component. These characteristics of the windward flow make the windward surface a very attractive location to successfully implement plasma actuators for aerodynamic control of many different air vehicles. Acknowledgments This work was supported by Orbital Research, Inc., under a Small Business Innovation Research Phase II Contract No. FA8650-04-C- 3405 issued by the U.S. Air Force Research Laboratory (AFRL). The authors would like to thank Charles F. Suchomel, AFRL Program Monitor, for his insightful remarks and support of this work. References [1] Patel,M.P.,Ng,T.T.,Vasudevan,S.,Corke,T.C.,andHe,C.,“Plasma Actuators for Hingeless Aerodynamic Control of an Unmanned Air Vehicle,” AIAA Paper 2006-3495, June 2006. [2] Corke, T. C., Cavalieri, D., and Matlis, E., “Boundary Layer Instability on a Sharp Cone at Mach 3.5 with Controlled Input,” AIAA Journal, Vol. 40, No. 5, 2002, pp. 1015–1018. [3] Roth, J. R., Sherman, D. M., and Wilkinson, S. R., “Electro- hydrodynamic Flow Control with a Glow-Discharge Surface Plasma,” AIAA Journal, Vol. 38, No. 7, 2000, pp. 1166–1172. [4] Corke, T., Jumper, E., Post, M., Orlov, D., and McLaughlin, T., “Application of Weakly Ionized Plasmas as Wing Flow Control Devices,” AIAA Paper 2002-0350, Jan. 2002. [5] Post, M. L., and Corke, T. C., “Separation Control on High Angle of Attack Airfoil Using Plasma Actuators,” AIAA Journal, Vol. 42, No. 11, 2004, pp. 2177–2184; also AIAA Paper 2003-1024, Jan. 2003. [6] Post, M. L., and Corke, T., “Separation Control Using Plasma Actuators: Stationary & Oscillating Airfoils,” AIAA Paper 2004-0841, Jan. 2004. [7] Corke, T. C., He, C., and Patel, M. P., “Plasma Flaps and Slats: An Application of Weakly-Ionized Plasma Actuators,” AIAA Journal (to be published); also AIAA Paper 2004-2127, 2004. [8] Jacob, J., Rivir, R., Campbell, C., and Estevedoreal, J., “Boundary Layer Flow Control Using AC Discharge Plasma Actuators,” AIAA Paper 2004-2128, June 2004. [9] Patel, M. P., Sowle, Z. H., Corke, T. C., and He, C., “Autonomous Sensing and Control of Wing Stall Using a Smart Plasma Slat,” AIAA Paper 2006-1207, Jan. 2006. [10] Corke, T. C., Mertz, B., and Patel, M. P., “Plasma Flow Control Optimized Airfoil,” AIAA Paper 2006-1208, Jan. 2006. [11] Enloe, L., McLaughlin, T., VanDyken, R., Kachner, Jumper, E., and Corke, T. C., “Mechanisms and Response of a Single Dielectric Barrier Plasma Actuator: Plasma Morphology,” AIAA Journal, Vol. 42, No. 3, 2004, pp. 589–594. [12] Enloe, L., McLaughlin, T., VanDyken, R., Kachner, Jumper, E., Corke, T. C., Post, M., and Haddad., O., “Mechanisms and Response of a Single Dielectric Barrier Plasma Actuator: Geometric Effects,” AIAA Journal, Vol. 42, No. 3, 2004, pp. 595–604. [13] Corke, T. C., and Post, M., “Overview of Plasma Flow Control: Concepts, Optimization, and Applications,” AIAA Paper 2005-0563, Jan. 2005. [14] Patel, M. P., Ng, T. T., Vasudevan, S., Corke, T. C., Post, M. L., McLaughlin, T. E., and Suchomel, C. F., “Scaling Effects of an Aerodynamic Plasma Actuator,” AIAA Paper 2007-0635, Jan. 2007. LOPERA ET AL. 1895PDF Image | Aerodynamic Control Using Windward-Surface Plasma Actuators on a Separation Ramp
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