Investigation and Control of Unsteady Flow Conditions on Airfoils

Abstract

In this thesis, efficiency enhancement approaches for two unsteady aerodynamic scenarios are considered to provide the basis for future improvements. The first scenario addresses transient inflow conditions on a stationary airfoil, representative of gust loads on wind turbine blades or bridge decks. It aims at the experimental validation of aerodynamic transfer functions, known as the Sears and Atassi formulations, which allow for a prediction of unsteady loads. Both functions are found to be capable of load prediction if inflow assumptions are carefully reproduced. It is also shown that no fundamental difference between both functions exists, if they are normalized appropriately. In order to simplify the generation of periodic inflow conditions in a wind tunnel, a gust generation approach utilizing a single pitching and plunging airfoil is derived and experimentally validated. It is demonstrated that high frequency and amplitude gusts can be generated with the aid of optimized airfoil kinematics, derived from the Theodorsen theory, using a single airfoil. The second scenario considers steady inflow conditions on a pitching and plunging airfoil in the deep dynamic stall regime where a leading edge vortex (LEV) occurs, representative for beating wings of Micro Air Vehicles (MAVs). The detachment process of the vortex is investigated to provide the basis for consecutive flow control efforts. A model that allows for the prediction of the occurrence of secondary structures, which can initiate the vortex detachment, is derived and validated using facilities with complementary parameter spaces due to different working media. In order to prolong the LEV growth phase on the airfoil and attain higher overall lift, a DBD plasma actuator is used to manipulate the flow field at topologically critical locations on different airfoils. The growth phase of the vortex is prolonged, which indicates an enhancement of the induced lift. Considerations regarding the control authority of the actuator are used to derive and test the minimum effective actuation period, which is sought to demonstrate the potential for future enhancements of the control concept.