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 dur-ing 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.