Analysis of Unsteady Airfoils at Low Speeds

In this study exemplary results on the influence of two fundamental unsteady parameters, the reduced frequency k and the non-dimensional pitching rate α+, on the transition from a steady to an unsteady flow field around the reference airfoil BAC3-11/RES/30/21 are presented using experimental and numerical techniques. The tools used are described and their results verified with each other. On the experimental side, water tunnel experiments are conducted using particle-image-velocimetry (PIV) and strain-gauge-balance (SGB) measurement techniques. The experimental results are enhanced by two-dimensional laminar unsteady numerical Navier-Stokes simulations of the experiments with a state of the art code from the Featflow package. The numerical simulations are found to agree well with experimental results, despite three-dimensional effects and a relatively high Reynolds number of Rec = 16000. An influence of the reduced frequency on the lift coefficient at a local non-dimensional pitching rate value of α+ = 0 for the relevant range is not detected, while for the drag and the pitching moment a decrease and an increase respectively is observed. Nevertheless a phase difference for the lift with increasing reduced frequency was calculated, which appears at least above k = 0.14 and indicates unsteady effects. The same was observed for the pitching moment. The effects of the airfoil camber are clearly seen during the phase of negative angle of attack, inducing leading and trailing edge separation. The flow separation regions are discussed and the formation of a triple vortex system at the trailing edge in the pitching up motion of the airfoil is identified in the experimental and numerical data.