ACTIVE JET ACOUSTIC CONTROL OF LOW FREQUENCY, IN-PLANE HELICOPTER HARMONIC NOISE
Sargent, Daniel Caleb
Schmitz, Fredric H.
Baeder, James D.
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A new approach to reducing low frequency, in-plane harmonic noise of helicopter rotors is explored theoretically and experimentally in this dissertation. The active jet acoustic control methodology employs on-blade, tip located unsteady air blowing to produce an acoustic anti-noise waveform that reduces or cancels the observed noise at targeted positions in the acoustic far-field of the rotor system. This effectively reduces the distance at which the helicopter rotor can be aurally detected. An extended theoretical model of the subsonic air jet, which is modeled as both a source of mass and momentum, is presented. The model is applied to a baseline, full-scale, medium weight helicopter rotor for both steady and unsteady blowing. Significant reductions in low frequency, in-plane harmonic noise are shown to be possible for the theoretical rotor system by using physically reasonable unsteady jet velocities. A new model-scale active jet acoustic control experimental test rotor system is described in detail. Experimental measurements conducted in the University of Maryland Acoustic Chamber for the ~1/7th rotor, operated at a full-scale hover tip Mach number of 0.661, indicate that active jet acoustic control is a viable option for reducing low frequency, in-plane harmonic noise. Good correlation between theoretical predictions and measured data for four valve control cases are observed in both the time and frequency domains. Model-scale limitations of the tip-jet blowing experiment limited the peak noise level reductions to 30%. However, theory suggests that if the limitations of the model-scale controller are mitigated, much larger noise reductions are possible.