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522 PATEL ET AL. Fig. 11 Lift coeffcient versus angle of attack and drag polar for the NACA 0015 airfoil at 21 m=s with the plasma actuator off (squares), steady on (circles), and unsteady on (triangles) operation [2]. 2500 2000 1500 1000 500 0 characteristic provides a method to both sense incipient separation to turn the actuator on, and sense when the actuator no longer needs to be on, saving actuator power. Based on this explanation of the APSC, a control procedure shown in Fig. 9 is implemented. First, at any given of the airfoil, the unsteady plasma actuator is turned on, the pressure sensor time series is sampled, and the frequency spectrum is computed. If a spectral peak is found at the actuator unsteady forcing frequency, the flow is sensed to be close to separation, or at a large at which if the actuator were turned off, the flow would separate. Thus, the unsteady plasma actuator stays on. If the peak at the unsteady actuator frequency does not appear in the pressure spectra, the airfoil is low enough for the flow to be far from separating. In this case, the plasma actuator is turned off. This control loop is exercised every time is changed in the laboratory experiment. In a flight scenario, it would be operating autonomously in the control loop to always sense and control incipient separation. A demonstration of the control procedure is presented in Fig. 10. This shows the lift coefficient versus , and drag polar for the baseline airfoil condition (actuator off) and for the feedback control of the actuator using the APSC approach. The results are indistinguishable from the open loop forcing for the same airfoil conditions by Corke et al. [2] with the same increase in stall and L=D; see Fig. 11. However, in this case (Fig. 10) the actuator is only operating when necessary, where in flight scenarios will ultimately use less energy compared to open-loop flow control. When the vortex generated by a plasma actuator goes through the separation bubble, it collects the energy that resides in small eddies in the separated turbulent boundary layer and becomes strong enough to be sensed by the pressure sensor. That causes the dominant frequency to appear in the power spectrum when the actuator is on. The stronger vortex brings high momentum fluid to the surface and keeps the separation bubble from bursting when the trailing-edge boundary-layer separation moves forward and emerges with the separation bubble as increases. After the stall, the lift coefficient continues to increase to CLmax and then drops very gradually instead of an abrupt decrease (see Fig. 10). Pressure Amplitude Sense-and-Control (PASC) Method Previous work by Patel et al.[8] has shown that a single high- bandwidth pressure sensor placed optimally on the surface of a wing can be used to detect incipient flow separation. In this earlier work, a standard deviation (STDEV) formulation based on the time- and frequency-domain analysis of the pressure data was used to predict flow separation and incipient stall. Using this feedback rule, active stall control of a slow-pitching 30-deg sweep NACA 0020 airfoil was demonstrated using a system of dynamic pressure sensors, leading- edge deployable vortex generators, and a closed-loop controller. The present PASC approach uses a more refined method of predicting separation/stall signature using the power spectrum density (PSD) Threshold 0 5 10 15 20 a Fig. 12 A representative plot showing amplitude fluctuations of a single frequency bin from the pressure signal during the NACA 0015 airfoil pitch-up experiment. Fig. 13 function of angle of attack and time during the NACA 0015 pitch-up experiment with the actuator off. A rise in the amplitude level of discrete frequency bins is observed at 12 deg before stall , which occurs at 14 deg in this case with no control. Spectral power distribution plot showing frequency bins as a Single Frequency ValuePDF Image | Autonomous Sensing and Control of Wing Stall Using a Smart Plasma Slat
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