A boundary layer energizer (20) for energizing a boundary layer flow over a surface (22), the boundary layer energizer (20) comprising a plurality of passages (24), each passage terminating in a respective hole (26) provided on the surface (22), the holes being arranged in a cluster (23) on the surface, wherein the plurality of passages are angled with respect to one another at the surface such that, when in use, a vortex (24) is formed by a fluid flowing through the plurality of passages.