A turbomachine for a gas turbine engine, comprising: a rotor disc arranged to rotate about a fixed central axis. A plurality of rotor blades connected to the rotor disc, wherein the rotor blades extend within a gas flow path. A rotating annulus is arranged to rotate in a fixed rotational alignment relative to the rotor disc. An outer surface of the rotating annulus is arranged to define part of the boundary of the gas flow path. A gas turbine engine for an aircraft and a method of defining the boundary of a gas flow path within a turbomachine is also disclosed.