Predicting unsteady flow separation in response to a flow disturbance

We present here a summary of the activities and results of dynamically stalling airfoils from the UNFoLD lab at EPFL that have been presented at the meetings of the NATO AVT-282 discussion group on Unsteady aerodynamic response of rigid wings in gust encounters during the past three years. The results are based on experimental data for a sinusoidally pitching OA209 airfoil in a wind tunnel at $\Rey=\num{9.2e5}$, a sinusoidally pitching NACA0015 airfoil profile with a trailing edge flap in a wind tunnel at $\Rey= \num{5.5e5}$, and a sharp-edged flat plate undergoing a ramp-up motion in a recirculating water channel at $\Rey= \num{77 500}$. The first two data sets are used to study the onset of dynamic stall and the third one is used to analyse post-stall load fluctuations. Based on the experimental data, we derived a new model of the leading edge suction parameter that accurately predicts the value and the timing of the maximum leading edge suction and dynamic stall onset on a pitching airfoil. The model is based on thin airfoil theory and links the evolution of the leading edge suction and the shear layer during stall development. By including an oscillating trailing edge flap, the lift polars can be significantly altered but the dynamic stall time delay is only marginally affected by the kinematics of the flap. The maximum lift coefficient is strongly affected by both the main wing and the trailing edge flap kinematics. For a flat plate without flap, the maximum lift coefficient and the subsequent post stall peak values increase with increasing pitch rate up to a critical pitch rate beyond which the entire lift response become independent of the pitch rate. The convective time delay to reach the primary lift peak decreases with increasing pitch rate up to the critical pitch rate and the time delay between subsequent peaks slightly decreases until the limit cycle oscillation period is reached.