The effect of aspect ratio on the near-wake flow of rectangular wings

Abstract

The near-wake flow at mid-span of low-aspect-ratio rectangular wings (A = 1–2.5) at a chord-based Reynolds number of Re = 8 × 10 is investigated experimentally. The angle of attack is ranged from zero to stall. The reattachment of the leading-edge shear layer for sufficiently small aspect ratios is explained by the corresponding tip vortices rolling up trailing-edge spanwise vorticity and converting it to streamwise vorticity, thereby increasing their strength and effectiveness to provide additional momentum (e.g. downwash) to reattach the flow; this process is also aided by an increase in angle of attack. These concepts are evinced in the current data through the time-averaged vorticity fields and streamlines for A = 1, which show that the trailing-edge shear layer actually decreases in strength and the wake narrows with increasing incidence above 15◦, all while maintaining reattached flow. Complimentary to this idea is an increase in the turbulent kinetic energy of the trailing-edge shear layer, which increases as fully separated, stalled flow is approached for largerAs. Instantaneous vorticity fields show that near-wake vortex shedding is a complex process with shear-layer instabilities, mutual shear layer interaction, and indistinct shear layer segments being shed irregularly into the near-wake.