Investigating sweep effects on the stability of leading-edge vortices over finite-aspect ratio pitch-up wings

Abstract

The flow field around a finite-span flat wing in pitch motion is modeled by means of large-eddy simulation. The effect of moderate sweep angles on the stability of the leading-edge vortex (LEV) is investigated. The relative stability of LEVs on flapping profiles can be improved by moderating LEV growth through spanwise vorticity convection and vortex stretching. The LEV growth over an unswept wing and two sweep angles, namely, Λ=15°; 30°, is studied by investigating the spanwise flow. The calculated results are in good agreement with experimental data, establishing confidence in the approach. Results show that sweeping the wing profile increases not only the scale of the secondary vortices but also expedites the initiation of the vortices at lower angles of attack. For the sweep angle of Λ=30°, increasing the angle of attack is associated with annihilation of vorticity and thereby limits the vortex growth as a necessary condition for LEV stability. Analysis shows that increasing the sweep angle results in a higher circulation intensity, especially in the inner region of the wing, and significant spanwise flow is observed through the vortex core. The pattern of vorticity remains stable and attached to the surface as the angle of attack continues to grow for the swept wing, while the patterns of vorticity depart the wing surface for the unswept wing. It is suspected that sweeping the wing can control the scale of the vortex by introducing a substantial vortex stretching.