Method and injector for controlling gas flow in a duct. The gas flow in the duct is energised by injecting an auxiliary gas into the duct at predetermined locations at the surface of the gas flow in the duct in the direction of the gas flow at an injection angle of 0 to 3° with respect to the axis of the duct and at a skew angle of 0 to 30° with respect to the axial and radial plane of the duct  to prevent and/or reconstitute flow separation and to ensure smooth gas flow in the duct. The injector (1) comprises  an annular member (2) having an inlet end (5) and an outlet end (6) and being disposed between and fixed to two duct elements (3, 4) forming a duct in a leak tight manner. One duct element (3) is at the inlet end of the annular member and the other duct element (4) is at the outlet end of the annular member. The inlet end of the annular member describes an inner diameter marginally smaller than the inner diameter of the duct element (3) at the inlet end thereof and the outlet end of the annular member describes an inner diameter matching with the inner diameter of the other duct element (4) at the outlet end thereof. The annular member further comprises a plurality of orifices (8) formed therein in a predetermined pattern. Each of the orifices defines a wide outer end (9) and a narrow inner end (10). The narrow ends of the orifices open into the duct in the direction of the gas flow in the duct at an injection angle of 0 to 3° with respect to the axis (A) of the duct and at a skew angle of 0 to 3° with respect to the axial and radial plane of the duct. A plenum chamber (11) is mounted to the annular member in a leak tight manner and communicates with the wide outer ends of the orifices.