Surface roughness is experimental1y used to control the flat-plate boundary layer bypass transition induced by an upstream convected two-dimensional (2D) circular cylinder wake.It is shown that the later stage of this bypass transition is successfully postponed,at the price of accelerating the earlier transition stage.A regularisation process of the hairpin vortices,which are the dominant coherent structures to promote the transition process,are observed under the influence of roughness elements.Significant scale reduction and localisation of these hairpins are achieved,which enhances the possibility of a hairpin self-annihilation process.Therefore,the cascade to large-scale structures in the later transition stage might be impeded/weakened.