Influence of aspect ratio on dynamic stall of a finite wing

The influence of aspect ratio, AR, on dynamic stall was examined by comparing experimental performance measurements across a range of different AR pitching wings. Measurements were acquired for three wings of AR = 3, 4, 5, and an airfoil during sinusoidal pitch oscillations inside of a low-speed wind tunnel, at Re_c = 400,000, 500,000 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 Re_c = 200,000. 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 aerodynamic loads, 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. This interaction was characterized through a series of three-component velocity measurements across different spanwise locations in the dynamic stall flow field. The results revealed that the dynamic stall vortex originates with flow separation at the leading edge of the root section of the wing and 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.