J. Fluid Mech. (2016), vol. 788, pp. 730–766. c  Cambridge University Press 2016 doi:10.1017/jfm.2015.611 730 Evolution of the streamwise vortices generated between leading edge tubercles Kristy L. Hansen 1, †, Nikan Rostamzadeh 2 , Richard M. Kelso 2 and Bassam B. Dally 2 1 School of Computer Science, Engineering and Mathematics, Flinders University, Tonsley Campus, Adelaide, South Australia 5042, Australia 2 School of Mechanical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia (Received 20 February 2015; revised 8 October 2015; accepted 19 October 2015) Sinusoidal modifications to the leading edge of a foil, or tubercles, have been shown to improve aerodynamic performance under certain flow conditions. One of the mechanisms of performance enhancement is believed to be the generation of streamwise vortices, which improve the momentum exchange in the boundary layer. This experimental and numerical study investigates the formation and evolution of these streamwise vortices at a low Reynolds number of Re = 2230, providing insight into both the averaged and time-dependent flow patterns. Furthermore, the strength of the vortices is quantified through calculation of the vorticity and circulation, and it is found that the circulation increases in the downstream direction. There is strong agreement between the experimental and numerical observations, and this allows close examination of the flow structure. The results demonstrate that the presence of strong pressure gradients near the leading edge gives rise to a significant surface flux of vorticity in this region. As soon as this vorticity is created, it is stretched, tilted and diffused in a highly three-dimensional manner. These processes lead to the generation of a pair of streamwise vortices between the tubercle peaks. A horseshoe-shaped separation zone is shown to initiate behind a tubercle trough, and this region of separation is bounded by a canopy of boundary-layer vorticity. Along the sides of this shear layer canopy, a continued influx of boundary-layer vorticity occurs, resulting in an increase in circulation of the primary streamwise vortices in the downstream direction. Flow visualisation and particle image velocimetry studies support these observations and demonstrate that the flow characteristics vary with time, particularly near the trailing edge and at a higher angle of attack. Numerical evaluation of the lift and drag coefficients reveals that, for this particular flow regime, the performance of a foil with tubercles is slightly better than that of an unmodified foil. Key words: aerodynamics, flow control, mixing enhancement 1. Introduction Tubercles are rounded leading edge protuberances that alter the flow field around a foil. It has been suggested that tubercles on the humpback whale flipper function † Email address for correspondence: kristy.hansen@flinders.edu.au