Abstract: A regenerative fluid dynamic machine, typically a blower or exhauster, includes a rotor (21) having a set of blades (24) which are curved, and preferably of aerofoil cross-section, in a manner which causes fluid flow passing through the set of blades to be turned with respect to the blades through an angle more than 90.degree., preferably 100.degree. to 140.degree.. The rotor is enclosed in stator structure (20) defining a toroidal passage (25) between inlet and outlet ports (26, 27) fluid flow in said passage following a path which forms a spiral centered within the cross section of the passages so that it passes through the set of blades more than once between the inlet and outlet.