Dynamic stall is observed in numerous applications, including sharply maneuvering fixed-wing aircraft, biomimetics, wind turbines, and most notably, rotorcraft. The associated unsteady loading can lead to aerodynamic flutter and mechanical failure in the system. The present work explores the ability of nanosecond pulse-driven dielectric barrier discharge plasma actuators to control dynamic stall over a NACA 0015 airfoil. The Reynolds number, reduced frequency, and excitation Strouhal number were varied over large ranges: Re 167;000-500;000, k 0.025-0.075, and St e 0-10, respectively. Surface pressure measurements were taken for each combination of Reynolds number, reduced frequency, and excitation Strouhal number. Phaselocked particle image velocimetry measurements were acquired for select cases. It was observed that the trends of effect of St e were similar for all combinations of Reynolds number and reduced frequency, and three major conclusions were drawn. First, it was observed that low Strouhal number excitation (St e < 0.5) results in oscillatory aerodynamic loading in the stalled stage of dynamic stall. This oscillatory behavior was gradually reduced as St e increased and was not observed beyond St e > 2. Second, all excitation resulted in earlier flow reattachment. Last, it was shown that excitation, especially at high St e , resulted in reduced aerodynamic hysteresis and dynamic stall vortex strength. The decrease in the strength of the dynamic stall vortex is achieved by the formation of large-scale structures induced by the excitation that bleed the leading-edge vorticity before the ejection of the dynamic stall vortex. At sufficiently high excitation Strouhal numbers (St e ≈ 10), the dynamic stall vortex was completely suppressed.

Flow control experiments were performed using nanosecond dielectric-barrier-discharge plasma actuators on a NACA 0015 airfoil with flow approaching from the geometric trailing-edge side, which is a condition anticipated to occur on the retreating blade side of advanced helicopters such as slowed-rotor compound rotorcraft. This symmetric airfoil, which is not typical of those used in rotorcraft blades, was used for simplification of an otherwise very complex problem. The Reynolds number based on the chord length was fixed at 0.50 · 10 6 , corresponding to a freestream flow of approximately 38 m∕s. An angle of attack of 15 deg was used. Fully separated flow on the suction side extended well beyond the airfoil with naturally shed vortices occurring at a Strouhal number of 0.19. Plasma actuation was evaluated at both the aerodynamic leading and trailing edges of the airfoil. Excitation at very low (impulse excitation) to moderate (∼0.4) Strouhal numbers at the aerodynamic leading edge generated organized coherent structures in the shear layer over the separated region with a shedding Strouhal number corresponding to that of the excitation, which caused changes in the size of the wake, the separation area, lift, and drag. Excitation at higher Strouhal numbers resulted in weaker naturally shed vortices (rather than generating new vortices) that diffused quickly in the wake. The excitation caused the wake to elongate slightly and skew toward the aerodynamic trailing edge, but it still reduced the separation area and significantly reduced drag. The primary mechanism of control at the aerodynamic leading edge is excitation of instabilities associated with the leading-edge vortices; the excitation generates coherent large-scale structures over a range of excitation frequencies, increasing their entrainment abilities to bring high-momentum fluid into the separation region to reduce the separation size and increase the lift. On the other hand, excitation over a broad range of frequencies at the aerodynamic trailing edge was found to significantly reduce organization of the naturally shed large-scale wake structures.

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Flow field surrounding a moving body is often unsteady. This motion can be linear or rotary, but the latter will be the primary focus of this thesis. Unsteady flows are found in numerous applications, including sharp maneuvers of fixed wing aircraft, biomimetics, wind turbines, and most notably, rotorcraft. Unsteady flows cause unsteady loads on the immersed bodies. This can lead to aerodynamic flutter and mechanical failure in the body. Flow control is hypothesized to reduce the load hysteresis, and is achieved in the present work via nanosecond pulse driven dielectric barrier discharge (NS-DBD) plasma actuators. To better understand the physics of unsteady flow over an airfoil a new facility was constructed, and new processing codes were developed and implemented. A NACA 0015 airfoil was mounted to oscillating mechanism, and the angle of attack was varied sinusoidally. The Reynolds number was varied from 0.17 • 10 6 − 0.50 • 10 6 , and the reduced frequency of oscillation was varied from 0.025 − 0.075 to gain a better understanding of these parameters on the unsteady flow dynamics. The plasma actuator was mounted at / = 0.01, just downstream of the airfoil leading edge. It was noted that the construction of the actuator influenced baseline behavior. Validation of the facility was achieved via qualitative comparisons of the baseline results to the results in a similar experimental setup in literature. After validation, As a first year student who wanted to explore the frontiers of technology, Dr. Samimy gave me with the opportunity to learn so much about flow control. These facilities, along with the wonderful group of students provided an engaging environment that I am truly grateful for. I am also thankful to Dr. Datta Gaitonde and Dr. James Gregory for making the graduation process an exciting one! Many thanks go to Dr. Igor Adamovich and Dr. Munetake Nishihara of the Non-Equilibrium Thermodynamics Laboratory for their work on nanosecond pulse driven dielectric barrier discharge plasma actuators. Their expertise was only surpassed by their willingness to provide assistance when needed. I cannot thank the many students of the Gas Dynamics and Turbulence Laboratory enough. Their encouragement, patience, and willingness, made even the long nights on the tunnel fun ones! My first day at the laboratory, I was greeted by Cameron DuBois who taught me the basics of the laboratory, and guided my curiosity. Dr. Chris Clifford challenged me to continually improve the facility, and I am grateful for the skills I gained in doing so. Special thanks to Dr. Michael Crawley, who among many other things, made my time here an entertaining one and to Dr. Nathan Webb for his support. David Castañeda helped acquire and analyze some of the results presented herein, and his help is much appreciated! I would also like to thank

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