A gas turbine rotor assembly (10) includes axially spaced rotor discs (18,20,22) carried on a shaft (12), together with first and second generally cylindrical actuators (40,42) mounted at each of their opposite ends to the shaft (12) and extending toward one another to overlap at their distal ends. The actuators (40,42) lie within the interior surface of the rotor discs (18-22) and have openings (48 and 50) in their overlapped portions. Upon a transient condition, the forward actuator (40) thermally expands in an axial direction to register at least in part its openings with the openings of the second actuator (42) to provide air flow through the partially aligned openings to opposite sides of an aft rotor disc (20,22). The second actuator (42) thermally expands in a forward axial direction to increase the registration of the openings and, hence, the flow-through area, affording increased air flow. When approaching steady state operation, the rotor assembly expands axially to displace the openings of the second actuator into misalignment with the openings of the first actuator to prevent the flow of air through the openings, whereby cooling losses during steady state operation are avoided.