Effect of Aspect Ratio on Swept Wing Dynamic Stall

View Video Presentation: https://doi.org/10.2514/6.2021-2948.vid The role of aspect ratio on the dynamic stall process of swept finite wings is investigated using high-fidelity, implicit large-eddy simulations. Two aspect ratios (4 and 8) are explored for a 30 degree swept wing with a NACA 0012 profile and rounded wing-tips. Following previous work, the swept wing at chord-Reynolds number of 200,000 and freestream Mach number of 0.1 pitches sinusoidally from an initial incidence of 4 degrees to a maximum angle of attack of 22 degrees with reduced frequency of pi/16 over one pitching cycle. The flow evolution at the higher aspect ratio maintained some consistencies with that of the lower aspect ratio. Both flow structures showed the emergence of lambda-vortices near the wing-tips on either side of the wing following laminar separation bubble bursting. These structures evolved into arch-like structures and shed into the wake as ring-like vortices. Further, the development of trailing edge vortices was limited to the outboard sections of the wing for both aspect ratios. The higher span did, however, show features that contrasted with the lower span. For example, the bending of the vortex core caused a secondary stall region that merged with the tip stall zone as the entire core lifted away from the surface, producing stall over the majority of the leading edge. The aspect ratio of 8 wing also showed a stronger trailing edge vortex than that of the aspect ratio of 4 wing. Motion-induced effects promoted earlier formation and propagation of the unsteady flow features that include, but are not limited to, the passage of transition, formation/evolution of the leading edge vortices, lambda-vortices, and arch-like vortices, and shedding as ring-like vortices. The time-histories of the loads showed an increase in mean and peak loads, consistent with the previously studied unswept configuration, and an earlier onset of stall with increase in aspect ratio. The susceptibility of the wing to torsional instability was assessed by integrating the local quarter-chord pitching moment to give the aerodynamic pitch damping coefficient. The damping coefficient was positive (i.e., suppressive to flutter onset) at the higher aspect ratio and negative (i.e, promotive of flutter onset) for the lower aspect ratio.