Abstract: A rotary device for interacting with a flow of liquid. The rotary device comprises a casing provided with an inlet opening and an outlet opening. The casing contains a first rotary member having a first axis of rotation (YY') and external teeth, and a second rotary member surrounding the external teeth of the first rotary member and guided to rotate about a second axis (XX') parallel to the first axis. The second rotary member includes two circular rings mounted inside the casing to occupy two respective planes perpendicular to the first axis of rotation (YY'), and a plurality of longitudinal components mounted between the rings to lie parallel with the second axis of rotation (XX'). Each of the longitudinal components has a cross-section in the shape of a gear tooth for meshing with the teeth of the first member. The rings have regularly spaced-apart notches around their peripheries in order to receive the ends of the longitudinal components, the longitudinal components not being fixed to the rings.