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PATEL ET AL. 521 start a) Schematic of separation bubble position at < 13 deg; change angle of attack actuator on acquire data FFT analysis spike at f No actuator off actuator on Yes Fig. 8 b) schematic of interaction between separation bubble and vortex generated by the plasma actuator. separation bubble that merges with the trailing-edge turbulent boundary-layer separation plays a key role in the low-frequency oscillation. When the plasma actuator is turned on, this low frequency vanishes and the spectral peak at 166 Hz again appears. A similar peak in the measured pressure spectrum is observed all the way up to 23 deg, which was the highest angle of attack investigated. Atalowangleofattack(<13 deg),justbeforetheleading-edge stall, a small separation region (bubble) begins to form downstream of the sensing port at x=c 0:05. This would be point S1 in Fig. 1. It is speculated that this location (S1) is just past the maximum thickness point of the 0015 (x=c 0:3), as illustrated in Fig. 8. The plasma actuator design and frequency operation were meant to force spanwise vortices that efficiently mix outer high-momentum fluid with the low-momentum fluid near the surface, causing the flow to reattach. At lower angles of attack, the flow at the leading edge is attached. When the plasma actuator is on, the strong favorable pressure gradient around the leading-edge nose damps the unsteady input from the actuator so that even close to the leading edge, just downstream of the actuator (x=c 0:05), the pressure fluctuations due to the actuator are not sensed. As increases, the small separation bubble gradually moves forwarduntilinthecaseofthepresentexperiment,at13 deg,S1 moves forward of the pressure sensor location. The small separation bubble is very receptive to the unsteady condition produced by the plasma actuator. As a result, the pressure sensor now shows a spectral peak at the actuator unsteady frequency. Note that this small separation bubble is a precursor of the full leading-edge separation Fig. 9 feedback control method. on this airfoil at low Reynolds number. Therefore, having it appear in the spectrum at prestall angles of attack provides a feedback signal to keep the actuator on. The evidence of a separation bubble that occurs near the leading edge comes from two bits of information. The first is the observation that when the unsteady pressure was measured at the 10% c location, a larger (1.5–2 deg) precursor of stall was observed compared to the 1 deg precursor obtained with the unsteady pressure measurement measured at the 5% location. This indicates that the separation bubble was moving forward as the increased. The second indication of our interpretation was the prediction of a separation point between 5–10% c at a 13 deg obtained using the X-Foil program. A more remarkable feature of this method of separation detection is that even after the flow has been reattached by the unsteady plasma actuator, the pressure sensor near the leading edge still senses a peak in the spectrum at the unsteady frequency as long as the flow will not attach naturally. However, even with the actuator on, if is low enough for the flow at the leading edge to be naturally attached, the spectral peak at the actuator frequency is not visible. Thus, this Flowchart for the amplitude peak sense-and-control (APSC) Fig. 10 Lift coefficient versus angle of attack and drag polar for the NACA 0015 airfoil at 21 m=s with APSC feedback control.PDF Image | Autonomous Sensing and Control of Wing Stall Using a Smart Plasma Slat
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