3D Effects of Synthetic Jet Flow Control on a Stalled Airfoil

Abstract

This thesis offers an experimental investigation conducted to understand the three-dimensional influence of active flow control on low Reynolds number airfoils. At a Reynolds number of Re_c=10^5 and an angle of attack of α=10°, a NACA 0025 airfoil was first studied in a baseline post-stall condition. The transition to turbulence was visualized, including the Kelvin-Helmholtz instability and shear layer roll-up, as well as the occurrence of large-scale vortex shedding in the wake. The research investigates both low- and high-frequency actuation strategies, utilizing a combination of novel smoke wire visualization techniques and quantitative measurements to evaluate the aerodynamic effects of an array of SJAs on flow reattachment and stability. By visualizing the controlled flow at different actuation frequencies (F+=1.18 and F+=11.76), the study reveals significant spanwise velocities and the formation of coherent structures that contribute to flow control. High-frequency actuation, in particular, demonstrates a greater ability to induce steady flow reattachment and more favorable aerodynamic characteristics compared to low-frequency control. However, the control efficacy diminishes with increasing distance from the midspan, limiting the spanwise control authority to approximately 40% of the SJA array's length. Additionally, this thesis explores energy-efficient flow control strategies by varying the duty cycle and blowing ratio, demonstrating that full flow reattachment can be achieved with duty cycles as low as 5%, leading to significant power savings. The research highlights the trade-offs between increasing the duty cycle or blowing ratio to meet the threshold momentum coefficient for reattachment. Furthermore, diminishing returns in control effectiveness are explained after the sharp initial improvements in the lift coefficient. Despite successful lift recovery, the challenge of extending the spanwise control length with a finite-span array persists, even with high-power control strategies. Lastly, a general three-dimensional model of the reattached mean flow over an airfoil controlled by a finite-span SJA array is developed. The SJA array's control effects create a controlled region where flow separation is suppressed, an uncontrolled region unaffected by the SJA array, and a transitional region bridging these two zones. By integrating multiple visualizations and measurements, a detailed understanding of the shear layer in three dimensions is achieved.