Patent ID: 7645121

Claim:
A rotor arrangement comprising a rotor and plurality of circumferentially spaced blades extending radially outwardly from the rotor, each blade comprising an aerofoil portion, each aerofoil portion having a leading edge, a trailing edge and a tip remote from the rotor, each aerofoil having a concave pressure surface extending from the leading edge to the trailing edge and a convex suction surface extending from the leading edge to the trailing edge, a casing surrounding the rotor and blades, the casing having an inner surface facing the tips of the blades, the inner surface of the casing having a circumferentially extending groove and the circumferentially extending groove being arranged axially between the leading edges and the trailing edges of the blades, the circumferentially extending groove extending axially by a distance such that at least half a wavelength of an unsteady pressure wave fits within the groove to provide a geometrically tuned cavity to suppress the upstream propagating unsteady pressure waves.