Blade Tonal Noise Reduction Using Stator Trailing-Edge Articulation

The biologically-inspired method of trailing-edge articulation is investigated as a means of reducing tonal noise due to the stator wake / rotor blade interaction in underwater vehicles. This work is experimental in nature and conducted in the closed channel water tunnel at Naval Undersea Warfare Center in Newport, Rhode Island. Tail articulation is carried out with a life scale stator model with hinged flapping tail designed to (i) operate in freestream velocities corresponding to Reynolds number in the range 75,000 < Re < 300,000 and (ii) operate at frequencies up to 30 Hz in order to investigate the range of Strouhal number 0.0 < St < 0.35. Velocity measurements in the active stator wake are carried out by Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) in order to investigate the effects of tail articulation on the stator wake. Time averaged measurements of the stator wake by LDV show that Strouhal number of the tail articulation has a dominant effect on the time mean stator drag. Instantaneous phase-averaged measurements of the stator wake by PIV show three regimes of the stator wake as Strouhal number is increased; quasi-steady wake spreading, vortex roll up, and strong vortex wake. Ongoing experiments with an instrumented propeller will demonstrate the efficacy of stator trailing-edge articulation on reducing unsteady blade forces.