Flow structures around a butterfly-shaped low-aspect-ratio wing

Abstract

In the present study, we numerically investigate three-dimensional flow structures around a butterfly-shaped low-aspect-ratio wing and their effect on the aerodynamic force at the Reynolds number of 1000 based on the wing chord length and free-stream velocity. When the angle of attack is less than 10°, the flow is steady and fully attached to the upper-wing surface, by which the lift force increases almost linearly with the angle of attack. As the angle of attack further increases, the flow around the wing becomes unsteady and contains the leading-edge, trailing-edge, wing-tip, and hairpin vortices. At these angles of attack, the drag force increases rapidly with increasing angle of attack due to massive separation at the leading edge, but the lift force increases gradually without abrupt fall-off. This is because the wing-tip vortices induce a strong downward flow interacting with the flow separated from the leading edge and delay subsequent vortex roll-up in the downstream. The wing-tip vortices themselves also produce low-pressure regions on the upper-wing surface and thus provide an additional lift force. The flows separated from the leading and trailing edges are eventually combined into pairs of hairpin vortices which travel downstream in the wake. The formation of the hairpin vortices above the upper-wing surface also generates lowpressure regions, and they are another source of the lift force.