Abstract: A rotary mechanism including a rotor assembly having a plurality of piston members distributed about its periphery, and a rotary member in peripheral engagement with the rotor assembly. The rotary member (7) includes a plurality of piston receiving formations in its periphery for receiving the piston members, whereby during rotation of the rotor assembly the piston members move into and out of the piston receiving formations, and a plurality of conduits arranged to connect peripheral portions of the rotary member to an interior portion thereof. The rotary mechanism includes a flow separation device having an inlet port for receiving fluid into the rotary mechanism and an outlet port for enabling fluid to exit the rotary mechanism, a first passageway for connecting the inlet port with the interior side of at least one of the conduits formed in the rotary member and a second passageway for connecting the outlet port with the interior side of at least one of the conduits formed in the rotary member.