Abstract: Apparatus for increasing the flow speed of a medium such as air and for recovering its kinetic energy, comprising a series of concentrical, substantially symmetrically arranged annular members separated by annular slots, said members in the direction towards length axis of the apparatus, which coincides with the flow direction of the medium, having successively and substantially symmetrically decreasing dimensions in the direction of said length axis to the formation of an inlet opening wherein said annular members have curved leading edges with intermediary annular slots for removal of turbulent medium, and to the formation of an outlet opening acting as a diffuser for the medium, wherein said annular members are successively thinner to the formation of sharp edges directed outwards from the length axis of the apparatus; the kinetic energy of the medium being recovered by conventional means. Separate accelerator means are also provided.