On the unsteady aerodynamics of a surging airfoil at \(90{^\circ }\) incidence

Abstract

Wind turbine blades in standstill or parked conditions often experience large angles of attack (AoA), where vortex-induced vibrations (VIV) may occur that increase the risk of structural damage. To better understand the VIV of airfoils at high AoA from an aerodynamic perspective, we conducted experimental investigations into the vortex dynamics of a surging airfoil at a \(90^\circ\) incidence undergoing forced vibrations. Experiments were conducted at two reduced frequencies (k) to demonstrate the lock-in effect, where the vortex shedding frequency aligns with the motion frequency. Results indicate distinct vortex shedding behaviors: at higher k value of 0.38, downstream wake vortices form when the airfoil is moving upwind, while upstream vortices emerge during the downwind motion, interacting with the downstream vortices and leading to an outward flow. At lower k value of 0.19, the wake remains directed to the downwind side, regardless of the airfoil’s motion direction. Lock-in is evident in both cases, with one vortex pair shed per cycle at lower k and two pairs at higher k. Furthermore, the study examines the influence of vortex dynamics on unsteady aerodynamic loads. The results show that drag peaks when the airfoil moves upwind near the center position of its trajectory; at higher k, negative drag occurs as the airfoil moves downwind near the center, driven by the interactions among convection, turbulent momentum, pressure, and viscous forces. A reduced-order load estimation model for a flat plate is applied to the experimental data, showing good agreement during the upwind motion of the airfoil, which is the design condition for the original flat plate model. However, during the downwind motion, as the flow condition does not match the original flat plate design condition, the circulatory part of the model is modified to account for the presence of two pairs of vortices in the flow field, yielding improved agreement with the drag values determined from the measured flow field. The findings highlight distinct flow patterns and vortex interactions for the two motion cases, offering insights into their impact on aerodynamic loads.