A flow transfer apparatus for transferring cooling flow from a primary gas flowpath (P) to a turbine rotor (128). The apparatus includes: a first supply plenum (92) communicating with the primary gas flowpath (P) and first inducers (148 A), the first inducers (148 A) configured to accelerate a first fluid flow from the first supply plenum (92) and discharge it toward the rotor with a tangential velocity; a second supply plenum (96) communicating with the primary gas flowpath (P) and second inducers (148B), the second inducers (148B) configured to accelerate a second fluid flow from the second supply plenum (96) towards the rotor with a tangential velocity; and a cooling modulation valve (98) operable to selectively permit or block the second fluid flow from the primary gas flowpath (P) to the second supply plenum (96). The valve includes a flow control structure (108, 110) disposed in the primary gas flowpath (P) and an actuation structure extending to a location radially outside of a casing (76) defining the primary gas flowpath (P).
