Rear mixer ejector for a turbomachine

A rear mixer ejector of a turbomachine for a supersonic aircraft has an ejector casing of rectangular cross-section fitted with movable flaps defining a variable geometry exhaust nozzle, and cold air mixing devices mounted on the casing side walls upstream of the nozzle. The mixing devices comprise a plurality of U-section chutes extending into the casing from apertures in the casing side walls, each chute being formed by a pair of lateral walls and a transverse wall disposed between them. The chutes are adjustable between a deployed position and an inoperative position, either by pivoting the chutes about axes at the downstream edges of the apertures or by pivoting only the transverse walls about the upstream edges of the apertures. In each case the lateral walls of the chutes remain in the gas flow path through the casing, parallel to the casing axis, in both the deployed and the inoperative positions, and the transverse walls are parallel to the casing axis in the inoperative position.