A gas turbine engine turbine comprises an annular array of nozzle guide vanes and an annular array of turbine blades mounted within its annular casing. An array of radially extending protrusions are positioned axially upstream of said array of said nozzle guide vanes and protruding inwardly from the inner casing wall so as to mix the tangential momentum component of the overtip leakage fluid flow before it reaches the array of nozzle guide vanes.