The unsteady character of the wingtip vortex formed on a low-amplitude/high-frequency heaving wing is examined using high-fidelity numerical simulation. The wing of aspect ratio AR = 6 has a NACA0012 airfoil section and a rounded tip, and it operates at a Reynolds number of Re = 2.0 × 10~5, free-stream Mach number of M_∞ = 0.1, and fixed incidence of α = 8°. Three plunging motions are considered with amplitudes of A/c = 0.02 or 0.03 and reduced frequencies of k = πfc/U_∞ = 1.05 or 2.09. The chosen kinematics provide cases of mild, moderate, and extreme excursions in effective angle of attack that induce significant unsteadiness in the tip vortex but also avoid wing stall. The overall flow structure is characterized by a similar separation, transition, and reattachment process, albeit with varying levels of intensity and time scales. The tip vortex formation is distinguished by an initially laminar feeding sheet that separates from the underside of the wingtip and reattaches on the upper surface. As it continues to roll up towards the tip, a secondary separation ensues at advanced chordwise stations. As the wing plunges downward, this separation progresses forward towards the leading edge, which also leads to the inception of spiraling sub-structures in the feeding sheet that are entrained into the tip vortex periphery and are stretched longitudinally. Higher frequency and amplitude promote a more compact tip vortex core as the structure separates from a more upstream position. The location of separation balanced with the variation in adverse streamwise pressure gradient magnified within the tip vortex core, precipitate alternating moments of axial velocity wake- and jet-like profiles that persist well into the near-wake, where axial velocity fluctuations on the order of 60% of the free-stream speed are observed. Additionally, the near-field evolution of the tip vortex is augmented by a motion-induced wandering that becomes increasingly pronounced with more aggressive kinematics. Significant and growing vertical excursion are found with downstream location that far exceeded the initial offsets of the imposed wing motion. All cases develop spanwise excursions in the vortex trajectory leading to an induced orbital motion. The higher frequencies and amplitudes also promote dramatic tilting of the trajectory relative to its initial vertical motion. This unintended wandering may have serious implications for reliable vortex tracking based on wing position alone.
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