Transient development of the leading-edge vortex and its role in unsteady aerodynamic load variations

Large eddy simulations are conducted for a pitching National Advisory Committee for Aeronautics 63(3)418 airfoil section at a reduced frequency of K=0.4 and a chord-based Reynolds number, Rec=100 000. The goal of this work is to understand how transient stall flutter dynamics are affected by initial flow field conditions associated with a high incidence angle. Perturbation starting times are varied parametrically to investigate the differences in the transient dynamic stalling processes over a single pitching cycle. It is found that Kármán vortex shedding associated with an initially static airfoil interferes with the leading-edge vortex detachment and dynamic lift stall as per large variations in the post-stall unsteady aerodynamic loading. Monitored leading-edge vortex trajectories and computed Lagrangian averaged vorticity deviation fields both confirm this notion and are utilized to link this phenomenon to the status of a trailing-edge vortex. It is shown that the leading-edge vortex trajectory standard deviation leading up to dynamic lift stall contracts at a singular point in space. This event appears to be weakly connected to the leading-edge vortex circulation during its maturation stage. The leading-edge vortex morphology is also investigated near its detachment location through several morphometric quantifiers such as the area, aspect ratio, and eccentricity. It is found that the leading-edge vortex morphology undergoes a similar contraction in the aspect ratio and eccentricity standard deviations. Finally, near-surface measurements of the u and v signals grant an indirect quantification of the leading-edge vortex strength and position as it advects over the airfoil chord.