Abstract:

The main element is the main contributor to the lift generation of a multi-element airfoil. However， the boundary layer over the suction surface of the main element is inherently disturbed by the complex vortices in the slat wake. Therefore， the vortex/boundary-layer interactions over the main element are closely related to the aerodynamic performance of an aircraft. This study uses time-resolved particle image velocimetry to investigate the effects of angle of attack on the vortex/boundary-layer interactions over a 30P30N multi-element airfoil at a low Reynolds number， with the aid of Fourier mode decomposition and finite-time Lyapunov exponents. The results of both mean statistics and vortex dynamics are obtained. For the mean statistics， the attached boundary layer above the main element changes into a separated one as the angle of attack increases. Meanwhile， the mixing between the slat wake and the boundary layer is gradually limited with the increasing angle of attack. For the vortex dynamics， the disturbances of different frequencies， originating from the complex vortices in the slat wake， are found to penetrate the boundary layer and subsequently affect the vortex dynamics in the boundary layer. The “double-secondary vortices” topology， triggered by the wake disturbances of fundamental frequency and second harmonic， dominates the vortex dynamics in the boundary layer. These secondary vortices are vital to the vortex/boundary-layer interactions in that they inject the fluid in the slat wake into the boundary layer to promote the mixing between the slat wake and the boundary layer， while injecting the high-speed fluid around the boundary layer edge into the near-wall region and therefore enhancing the ability of the boundary layer to resist flow separation. As the angle of attack increases， the formation of secondary vortices moves downstream. As a result， the effects of secondary vortices on the wake/boundary-layer mixing and the flow separation suppression around the leading edge of the main element decrease， leading to the reduced wake/boundary-layer mixing with an increasing angle of attack and the flow separation at the largest angle of attack.

Key words: multi-element airfoil, vortex/boundary-layer interactions, low Reynolds number, water tunnel experiments, time-resolved particle image velocimetry