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PATEL ET AL. 519 experimental use of plasma actuators using feedback control before this work. To this end, this paper presents the first look into the use of smart plasma actuators for autonomous sense and control of separated flows. The two main outcomes of the present work are 1) a smart skin that can operate continuously in an autonomous mode to maintain the aerodynamic efficiency at optimum settings, and 2) reduction of power requirements for the plasma actuators by turning them off when they are either not necessary or would be ineffective. In the present work, the plasma actuators used were made from two 0.0254 mm thick copper electrodes separated by two 0.1 mm (4- mil) thick Kapton film layers. The Kapton has a breakdown voltage of approximately 7 kV per 103 in: thickness and a dielectric constant of 3.3, which provide good electrical properties. The electrodes were arranged in the asymmetric arrangement illustrated in Fig. 2. They were overlapped by a small amount (approximately 1 mm) to ensure a uniform plasma in the spanwise direction. The plasma actuator was bonded directly to the surface of the model. At the leading edge, where the flow is sensitive to the nose radius, a 0.1 mm recess was molded into the model to secure the actuator flush to the surface. The electrodes were positioned so that the junction between the exposed and covered electrodes was precisely at the leading edge. The actuator induced an accelerating velocity component in the mean freestream direction over the suction surface of the model. The leading-edge plasma actuator, located at x=c0:0, was operated in an unsteady manner. The a.c. carrier frequency supplied to the electrodes was 5 kHz and the a.c. voltage supplied to the electrodes was on the order of 3–12 kVp-p. The power used by the actuator was approximately 2–4 W per linear foot of actuator span. In the unsteady mode, very short duty cycles are possible, which reduces the actuator power requirements significantly. For example, a 10% duty cycle provided results better than the “steady” operation which used 100% duty cycle. The unsteady actuator frequency f was determined based on a Strouhal number scaling of a dimensionless frequency, F fLsep=U1 1. For all cases presented here, the unsteady modulation frequency of the actuator was 166 Hz and the actuator was operated at 10% duty cycle. Experimental Setup The airfoil used for this study was a 2-D NACA 0015 (hereafter 0015) with a 12.7 cm (5-in.) chord and a 25.4 cm (10-in.) span. Photographs of the 0015 are shown in Fig. 3. The 0015 was chosen for study because its characteristics are well documented in the literature and the airfoil was also the subject of an experiment on dynamic stall control using plasma actuators, which provided flow visualization records (see Fig. 4) [22]. The size of the airfoil was a compromise between minimizing blockage effects, especially at high and maintaining a large enough chord Reynolds number, Rechord.Atthelargestangleofattacktested,thatis,23 deg,the blockage was 8.5%, which still ultimately required correction in the measured lift and drag coefficients. The airfoil was cast using an epoxy-based polymer in a two-piece mold. The mold was precisely machined using a numerical-controlled milling machine. Experiments were conducted at Rechord 1:8 105 (U1 21 m=s) in a subsonic wind tunnel located in the Center for Flow Physics and Control (FlowPAC) in the Hessert Laboratory at the University of Notre Dame. The facility is an open-return draw-down wind tunnel with a 0:421 m 0:421 m 1:8 m (long) test section. The tunnel consists of a removable inlet with a series of 12 screens followed by a 24:1 contraction that attaches to the test section. The test section is equipped with a clear Plexiglas sidewall that allows optical access to view the model. The back wall of the test section has removable panels to allow access into the test section. The 0015 used in the study was mounted vertically to the support sting of a lift-drag force balance on top of the test section. The airfoil was suspended between endplates that were attached parallel to the ceiling and floor of the test section. The endplates were designed to produce a two-dimensional flow around the airfoil. A hole in the ceiling endplate accommodated the sting supporting the airfoil. A Fig. 3 Exposed Electrode Electrode covered with Kapton Film Suction side Photographs of the 2-D NACA 0015 airfoil model. hole in the floor endplate allowed access for the actuator wiring. This hole was aligned with the support sting so that it would not interfere with angular positioning of the airfoil when setting different angles of attack. A stepper motor on the force balance drove the angular Actuator OFF Actuator ON α = 16 deg α = 18 deg α = 20 deg α = 22 deg α = 24 deg Fig. 4 Flow visualization records of the 2-D 0015 airfoil model with the steady leading-edge plasma actuator off and on [22].PDF Image | Autonomous Sensing and Control of Wing Stall Using a Smart Plasma Slat
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