Dynamic Stall of a Finite-Aspect-Ratio Wing

An investigation of dynamic stall on a pitching NACA 0012 aspect ratio 4 wing is performed by means of high-fidelity wall-resolved large-eddy simulations. The flow parameters are freestream Mach number M infinity=0.1 and chord Reynolds number Rec=2x105. Three cycles of a sinusoidal pitching motion are considered with reduced frequency k=pi fc/U infinity=pi/16 and minimum and maximum angles of attack of 4 and 22 deg, respectively. Details of the unsteady flow structure are elucidated to provide a model of three-dimensional dynamic stall at higher Reynolds number than previously studied using large-eddy simulation. The initial process involves the upstream movement of transition, the formation of a short laminar separation bubble (LSB), and the emergence of a dynamic stall vortex (DSV) following LSB bursting. The DSV is initially fairly regular in the spanwise direction; however, as it propagates downstream, it is distorted into a Lambda-type shape with legs pinned at the wing front corners. A marked kink appears in the DSV core outboard and near the wing surface. This induces a circulatory motion that self-intensifies, leading to the formation of an arch-type vortex. The legs of the arch vortex move downstream and inboard, leaving a distinct imprint on the instantaneous surface pressure. Eventually, through a reconnection process, the arch vortex transforms into a ringlike structure that is shed into the wake. The effect of a simulated endwall is also investigated because this arrangement is used in experiments. It is found that, during the early stages of the DSV, the endwall has no significant impact on the flow structure and loading; however, following the formation of the arch vortex, the presence of the wall has a pronounced effect.