A New Experimental Approach to Study Helicopter Blade-Vortex Interaction Noise
A New Experimental Approach to Study Helicopter Blade-Vortex Interaction Noise
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Date
2007-10-15
Authors
Koushik, Sudarshan
Advisor
Schmitz, Fredric H
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Abstract
A unique and novel experimental approach has been developed to study
the aerodynamics and acoustics of the helicopter Blade-Vortex
Interaction in a controlled hover environment. This is achieved by
having a non-lifting single-bladed rotor with a rigid hub interact
with a carefully controlled gust disturbance that replicates the
essential characteristics of the vortex velocity. This experimental
approach termed the Blade-Controlled Disturbance-Interaction or the
BCDI, decouples the rotor parameters from the charactersitics of the
incident disturbance velocity, thus providing an ideal setup for
studying the blade's aerodynamics and acoustic response in
detail. Moreover, the angle of interaction between the disturbance
field and the rotor blade can be controlled by orienting the gust,
providing the ability to study both parallel and oblique
interactions. The noise data was recorded at thirty different
microphone locations.
A series of experiments at various rotor tip Mach numbers and
interaction angles, replicating many of the conditions of helicopter BVI,
were performed. The results show
that the the directionality of the BVI noise is strongly determined by
the interaction angle. A small change in interaction angle results in
the radiation of noise over a larger azimuthal area compared to the
parallel interaction. Moreover, as the
interaction becomes more oblique, the peak noise elevation angle
approaches closer to the rotor plane.
A linear unsteady lifting-line aerodynamic theory (corrected for
chord-wise non-compactness )was used to estimate the blade
aerodynamics during the interaction and hence the radiated
noise. Although the theory under-predicted the noise levels for most of
the cases, and did not replicate exactly the general pulse shape, the
general directionality trends were predicted reasonably well. The
theory was used to separate the contribution to the acoustics, from
different spanwise blade sections, providing significant insights into
the phasing mechanism of BVI noise.