Measurements and Modeling of the Unsteady Flow around a Thin Wing

Abstract

Unsteady separated flows are encountered in many applications (e.g. dynamic stall in helicopters and wind turbines). Recent efforts to better understand the problem of unsteady separated aerodynamics have been prompted by growing interest in creating small-scale flight vehicles, termed micro air vehicles (MAVs). Because of their small size, all MAVs operate at low Reynolds numbers. In that regime, flow separation is a common occurrence either due to Reynolds number effects or aggressive motion. The dominant and most studied feature of these flows is the leading edge vortex (LEV). The LEV receives its circulation from a shear layer emanating from the leading edge of the wing, where the production of circulation occurs. In spite of its importance to the flow and the resulting forces, the production of circulation has received relatively little experimental attention. To fill this gap, water tank experiments on a surging flat plate wing at a high angle of attack have been performed at varying Reynolds number, acceleration, angle of attack, and aspect ratio. These experiments measured time resolved forces, LEV location, LEV circulation, and leading edge circulation production. These data were then used to explore how the LEV and the circulation production reacts to changes in kinematic parameters. This resulted in the proposal of a new relationship between the wake state and the leading edge circulation production, termed the boundary layer analogy (BLA). Additionally, existing potential flow modeling techniques were implemented and evaluated against the present experimental data. This analysis focused on evaluating the suitability of applying the Kutta condition at the leading edge. The Kutta condition was found to be a valid leading edge condition capable of predicting the LEV circulation seen in experiments. Representing the shed wake with multiple vortices was found to be necessary to capture the dynamics of vortex roll up and shedding. Other models struggle to account for these events, though simpler models may offer a better route to intuitive understanding of the fluid dynamic origin of the forces. The experimental data collected here, coupled with the novel analysis of the modeling techniques in the light of the leading edge circulation measurements, constitutes a significant step forward in the modeling and understanding of unsteady separated flows.