Influence of aspect ratio on dynamic stall of a finite wing

Dynamic Stall;Aspect Ratio;Finite Wing;Unsteady Flow;Three Dimensional Flow;Transitional Reynolds Numbers;Experimental Performance Data;Stereo PIV;Velocity Isosurface;Vorticity Contours;Spanwise Velocity Component;Arch Vortex, ,

Andreu Angulo, Ignacio ; Ansell ; Phillip J.

关键词: Dynamic Stall;    Aspect Ratio;    Finite Wing;    Unsteady Flow;    Three Dimensional Flow;    Transitional Reynolds Numbers;    Experimental Performance Data;    Stereo PIV;    Velocity Isosurface;    Vorticity Contours;    Spanwise Velocity Component;    Arch Vortex, ,;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101056/ANDREUANGULO-THESIS-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The influence of aspect ratio, AR, on the unsteady aerodynamics of pitching wings was examined by comparing experimental performance, flow measurements, and computational results across a range of different AR wings. Performance measurements were acquired for three wings of AR = 3, 4, 5, and an airfoil undergoing dynamic stall inside of a low-speed wind tunnel, at Rec = 4 × 105, 5 × 105 and respective reduced frequencies k = 0.1, 0.05. In addition, the influence of reduced frequency was analyzed by comparing performance measurements at k = 0.1, 0.15, 0.2 for a constant Rec = 2 × 105. A NACA 0012 airfoil was used for all wing models and the sinusoidal motion profile consisted of an angle of attack range between 4 and 22 degrees. After acquiring the experimental performance data, it was found that decreasing the AR leads to a decrease in the unsteady loading of the wing and postpones the dynamic stall process in a fashion similar to that observed for static stall of finite wings. Consequently, the pitch damping characteristics were also observed to significantly vary with changing AR. The differences in unsteady forces and moments across the unsteady pitch oscillation were attributed to variations in the influence of the trailing vortex system on the dynamic stall behavior of the finite wings. Moreover, the computational results revealed that, for wings pitching at low angles of attack, a decrease in aspect ratio corresponds to a decrease in phase lag of the unsteady circulatory loads with respect to the motion.The interaction of the trailing vortex system with the dynamic stall vortex was characterized through a series of three-component velocity measurements across different spanwise locations. The results revealed that the dynamic stall vortex originates with flow separation at the leading edge of the root section of the wing, displacing the shear-layer vorticity from the wing surface. The newly formed structure convects downstream across the chord, while the unsteady separation is spread across the span towards the wing tips with increasing angle of attack. The vortex then detaches from the surface of the root leading edge, while remaining connected outboard, forming a Ω-shaped vortex that has been observed in similar studies at much lower Reynolds numbers. With the emergence of this three-dimensional vortex structure, spanwise pressure gradients act to induce an inboard velocity near the wing trailing edge and an outboard velocity close to the wing leading edge. The evolution of all these vortical structures is directly tied to the variations in the unsteady wing loading. As a result, it can then be concluded that, along with the motion profile and the Reynolds number, the aspect ratio plays a critical role on the appearance of the dynamic stall behavior and the stability of pitch.