Aerodynamic damping of vibrations in rotor blades

An axial gas flow machine comprising:

• a duct wall bounding a duct, a rotor having blades mounted thereon for rotation within said duct, said blades being subject to a flutter at a known flutter frequency, said flutter giving rise to non-propagating pressure waves of less frequency than pressure waves of far field noise frequencies in gas flow past said blades, said blades during rotation having radially outer tips which sweep out a path adjacent said duct wall as said rotor rotates, said path having a leading edge and a trailing edge defined with respect to said gas flow and corresponding respectively to leading edges and trailing edges of said blades, said duct wall having openings therein which openings are circumferentially spaced around said duct at least radially adjacent to said path of said radially outer tips of said blades, said duct wall further incorporating resonator cavities tuned for resonance to said known flutter frequency, each one of said cavities being tube-shaped and in communication with said duct by a corresponding one of said openings at an extremity thereof, said tube-shaped cavities being disposed with their major axes extending substantially parallel to the extent of said duct wall and substantially in an axial direction of the axial gas flow machine, each of said tube-shaped cavities further having lengths effectively one quarter of a wave length of said known flutter frequency whereby when said cavities resonate at said known flutter frequency, said non-propagating resonant pressure waves at said openings of said tube-shaped cavities are one-half of one wave length out of phase with incoming non-propagating pressure waves caused by said flutter in said blades thereby causing aerodynamic dampening of said flutter to be achieved by partial cancellation of said incoming non-propagating pressure waves.