Volumetric measurements and simulations of the vortex structures generated by low aspect ratio plunging wings

Volumetric three-component velocimetry measurements have been performed on low aspect ratio wings subjected to a small amplitude pure plunging motion. The paper focuses on the near surface vortex flows generated by rectangular and elliptical wings set to a fixed geometric angle of attack of α = 20°. The highly three-dimensional leading edge vortex structure is shown to be highly dependent on the oscillating frequency. A comparison with computational results shows good agreement, both demonstrating reorientation of the leading edge vortex filament. At low frequencies the vortex evolves quickly into an arch-type structure with an anchor point that progressively moves from the tip to the root of the wing. At larger frequencies, this anchor point is forced to remain closer to the tip. The study elucidates that the deformation of the leading edge vortex is significantly influenced by planform shape, creating large undulations along the span of the elliptical wing. At a sufficiently high Strouhal number a tip vortex ring may develop causing a spanwise expansion of the vortex system. The complex interaction of these 3D unsteady vortex structures is characteristic of oscillating low aspect ratio wings.