Stall cells organisation on an aerofoil with leading-edge tubercles

Abstract The three-dimensional flow structure of an aerofoil with leading-edge tubercles at angles of attack (AoA) up to the post-stall regime is studied in this work. The three-dimensional distribution of instantaneous and time-averaged velocity and vorticity at the suction side is characterised by large-scale high-speed particle tracking, using helium-filled soap bubbles as tracers. The tubercles generate a periodic system of counter-rotating vortices, strongly altering the boundary layer from its inception and developing along the aerofoil. At mild incidence (AoA ≤ 5°), counter-rotating streamwise vortices adhere to the aerofoil profile, modulating the shear layer by creating regions of downwash and upwash motions. In the stall regime, the flow organisation is profoundly altered, with large-scale separation cells (stall-cell like structures, SCLS) forming on the aerofoil with no apparent the spanwise periodicity. Furthermore, the flow pattern becomes highly unsteady. Before stall (AoA= 10°), SCLS appear intermittently and the separated region is not sustained. In contrast, at AoA≳ 25°, the unsteadiness is ascribed to the competing effects of separation bubbles growth and SCLS. These observations clarify the role of tubercles in the process of aerofoil stall, which introduces a more gradual decrease of aerodynamic lift in the separated regime.