Vortex Dynamics over an Airfoil Immersed in a Harmonic Longitudinal Gust

Understanding unsteady aerodynamic forces during gust encounters is critical for the design of aircraft, rotorcraft, and wind turbines, particularly in regimes characterized by extensive boundary-layer separation. While pitching and plunging airfoil vortex dynamics are well-studied, characterization of airfoils in periodic longitudinal gusts under massively separated flow remains limited. This study experimentally investigates vortex shedding from an SD7003 airfoil at a poststall angle of attack ([Formula: see text]) encountering harmonic longitudinal gusts, focusing on a subharmonic lock-in case ([Formula: see text], [Formula: see text], [Formula: see text]). Using phase-averaged particle image velocimetry, the relationship between vorticity generation, leading-edge vortex (LEV) roll-up, and longitudinal gust parameters is explored. Results show that LEV detachment is triggered not by the rear reattachment point reaching the trailing edge, but by the outer flow’s ability to maintain curvature around the LEV, dictated by local pressure gradients and momentum. Turbulent shear stress measurements reveal that outer-flow reattachment anchors the LEV, reducing cycle-to-cycle positional variability. Furthermore, the phase-averaged lift coefficient correlates more closely with circulation within a chord-bounded control volume than with LEV circulation alone. These findings inform the development of future low-order models and flow-control strategies for gust responses in the case of separated flow.