Abstract A detailed numerical study has been carried out to investigate the effects of leadingedge protuberance as a novel flow-separation control-technique for vertical-axis-windturbine (VAWT). The aerodynamic performance of a stationary protuberanced blade and an associated H-type Darrieus VAWT made of three protuberanced blades were investigated using unsteady Reynolds-Averaged-Navier-Stokes (URANS) and Implicit Large Eddy Simulation (ILES) methods. The current study involves a comprehensive set of five different types of sinusoidal leading-edge protuberances with three different wavelengths and amplitudes. It is found that this passive flow-control-method can favourably change lift and drag at stall and post-stall regions, which is of high importance for VAWTs. Static stall variation can be changed from leading-edge-stall type to trailing-edge-stall type. This is attributed to a “bi-periodic” phenomenon over the blade suction side and a reduced turbulence kinetic-energy production. Protuberance amplitude was found to be more important than the wavelength in improving aerodynamic performance. Of all the tested cases, the protuberance with amplitude of 1% chord-length (c) and wavelength of 2.5%c behaves the best. The leading-edge protuberance can significantly increase VAWT power-coefficient at low tip-speed-ratios, which are dominated by post-stall conditions. The dynamic stall was delayed and the coherent spanwise wavy flow-structures on the blade suction-side were observed.