Dynamic stall characteristics of unswept pitching wing

Abstract

An experimental investigation of dynamic stall on upswept pitching wing was conducted using Particle Image Velocimetry (PIV) and Pressure Sensitive Paint (PSP) techniques. The wing had an aspect ratio AR = 4 and a NACA 0012 airfoil section profile assembled with a round-tip. The wing motion of interest in this study is pitching at a certain angle of attack range and the wing was robustly designed and machined out of Aluminum to minimize elasticity and tip flapping motions. The freestream Mach number, M∞ = 0.1, and the Reynolds number based on chord length, Rec = 2 × 105, were considered as flow parameters for observing dynamic stall phenomena and for comparison with available numerical simulations from literature. The pitching motion of the wing is sinusoidal with a reduced frequency k = πfc/U∞ = 0.1, and the minimum and maximum angle of attack of the pitching motion were 4° and 22°, respectively. Details of flow structures during pitching at various phase angles were captured using planar two-dimensional (2D) PIV at three spans, x/c = 0.5, 0.75, and 1.0 and the pressure imprint on the surface of the wing was captured by a single shot lifetime method using fast porous pressure-sensitive paint. The initial results show the formation of a separation bubble at a small incident angle and the emergence of a dynamic stall vortex (DSV) after the bursting of the separation bubble. The DSV propagates towards the trailing edge of the wing with its integral scale varying across the three spans of the wing revealing three-dimensionality. The PSP results show a uniform build of suction pressure at low angle of attack which later intensifies in a non-uniform manner due to a circulatory motion of the flow during pitching. After the end of the upstroke phase, the flow starts to reattach to the surface of the wing while the DSV grows and moves away from the surface as it travels downstream along the chord until sheds beyond the trailing edge. The PSP results also capture the tip unloading period during the pitching motion.