Rotary device

A device includes a ring having a tubular interior surface centred about an axis. The surface includes a plurality of axially extending, inwardly-projecting ridges. On opposite sides of each ridge is a first port and a second port. A rotor rotates in the ring about the axis. A plurality of vanes is mounted to the rotor body for rotation therewith and for radial extension and retraction relative thereto such that the surface can be swept by the vanes. The rotor and the ring are sealed to permit fluid communication into and out of the device only via the ports. The vanes retract and extend as the body rotates such that chambers are created which decrease in volume when in communication with the first ports and chambers are created which increase in volume when in communication with the second ports. A fluid pressure mechanism can cause vane retraction.