INSTALLATION EFFECTS ON THE PRODUCTION OF BLADE-VORTEX INTERACTION NOISE BY A DUCTED ROTOR

An analysis is made of the sound produced when discrete vortices and mean flow turbulence interact at low Mach number with a blade of a ducted rotor situated near the open end of the duct. The duct is rigid and of circular cross-section, and the blade chord is assumed to be small relative to the diameter of the duct. The variation of the amplitude and frequency dependence of the sound with the distance of the blade from the duct exit is examined. For short ducts operating at Mach numbers characteristic of those in underwater applications, it is shown that the acoustic power is increased over a range of frequencies when the rotor plane is close to the exit. For longer ducts, in which the turbulence is fully developed, the influence of the exit is predicted to be negligible when the distance of the rotor plane from the open end exceeds about one-tenth of the duct diameter. © 1992.