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PATEL ET AL. 517 flap (plasma actuators applied on the wing leading edge and trailing edge, respectively) on the aerodynamic performance of a NACA 0015 were discussed [2]. The present work is focused on formulating self-governing methods to enable “closed-loop” operation of a plasma slat. Much of this work is focused on reducing the power levels of the plasma actuator for practical air vehicle applications, hence due consideration was given to the design of feedback control approaches that enable a continuous self-governing plasma actuator using a simple commercial off-the-shelf (COTS) pressure sensor. Wind tunnel experiments were conducted on a slowly pitching 2-D NACA 0015 airfoil to validate two different feedback control techniques designed for autonomous control. The designs are generic enough to be applied to any flow-control application where smart sensing and control of incipient flow separation is desired. The following sections provide a brief discussion of the different types of wing stall, plasma actuators, feedback control, and results from wind tunnel experiments. Wing Stall and Control Wing Stall The stall of a wing is a very complex problem and still remains one of the most important phenomena in aerodynamics. It occurs when the wing is unable to generate sufficient lift to keep the air vehicle in the air which can happen if the vehicle speed is too slow and/or if the angle of attack is too sharp, causing the flow to separate at the leading edge. Flow separation occurs when the boundary layer lifts off or separates from the surface of the wing under the influence of viscous forces and adverse pressure gradients acting within the boundary layer or due to geometrical aberrations [3,4]. Such low velocity and high conditions are usually encountered during takeoff and landing. Because of the large energy losses associated with boundary-layer separation, the performance of lifting surfaces is often deteriorated; hence the use of effective high-lift systems is crucial during such conditions. One way of delaying wing stall is by using a high-lift device such as a leading-edge slat. It consists of a moving surface on the lower side of the wing that extends out ahead of the wing leading edge. Its primary effect is to increase stall and CL;max by allowing air from the high-pressure lower surface to circulate over the upper surface, which energizes the boundary layer and prevents flow separation, and therefore stall. Other examples of mechanical high-lift devices include a droop nose, Krueger flap, slotted Krueger flap, and a slotted slat. These hinged high-lift devices are effective, to varying degrees, in extending the stall and CL;max of the airfoil, however, they also add parasitic drag, weight, cost, and mechanical complexity to the high- lift system. Flow control offers alternative methods of separation control using low-power hingeless devices. Some of the previously demonstrated flow-control methods for controlling flow separation include periodic excitation [5,6], passive and active vortex generators [7,8], and pulsed jet actuators [9]. In this work, a relatively new type of flow-control device, an aerodynamic plasma actuator [10], is used for controlling leading-edge flow separation and wing stall using feedback control. Because the present work deals with formulation based on the detection of flow characteristics (bubble formation, adverse pressure gradient, etc.) associated with separated flows, it seems relevant to first touch on the subject of wing stall. Types of Stall Decades of research in understanding the different stalling characteristics and their relation to the state of the boundary layer has led to a generalization of three types of stall that are widely accepted today for low-speed flows, albeit their demarcation calls for a more careful examination. These include 1) the thin-airfoil stall, where there is a gradual loss of lift at low lift coefficients as the turbulent reattachment point moves rearward; 2) the leading-edge stall, where there is an abrupt loss of lift, as the angle of attack for maximum lift is exceeded, with little to no rounding over the lift curves; and 3) the trailing-edge stall, where there is a gradual loss of lift at high CL as the Fig. 1 Shape of the pressure distribution and typical airfoil stall patterns for single-element airfoil [11]. turbulent separation point moves forward from the trailing edge. Figure 1, adopted from [11], shows a typical pressure distribution for a single-component airfoil exhibiting either laminar stall (short and long bubble stall) or turbulent stall (trailing-edge stall) on the left, and the typical lift curves for the airfoils exhibiting laminar short bubble stall, laminar long bubble stall, and turbulent or trailing-edge stall for single-element airfoils on the right [11]. In Fig. 1, P1 pressure distribution for incipient laminar stall (short bubble and long bubble); P2 pressure distribution for incipient turbulent stall (trailing-edge stall); S1 point of incipient laminar separation and reattachment or laminar separation only; and S2 point of incipient turbulent separation. Figure 1a shows the laminar short bubble at S1PDF Image | Autonomous Sensing and Control of Wing Stall Using a Smart Plasma Slat
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