Parametric Study of the Noise Emitted by Uncorrelated, Distributed, Electric Fans

Kurzfassung

The development of new technologies does not prevent from considering the natural effects such as the hearing sensitivity and the sound absorption by the atmosphere as possible solutions to reduce aircraft noise and its perception. In this study, the noise emitted by distributed, electric, shrouded fans is predicted by means of semi-analytical models. The impact due to a variation of the propulsor number from 2 to 30 is quantified for a short-to-medium range airplane similar to an Airbus A320. The propulsors are assumed not to interact with each other. The investigation is conducted for nine different fans having a pressure ratio ranging from 1.47 to 1.28 at the design point. The propulsive architecture is not modelled in detail but the impact of engine size and weight on drag are considered. The acoustic assessment is based on two noise metrics standing for loudness and annoyance. The results indicate that multiplying the number of propulsors--each fan becomes smaller and rotates faster--can help reduce the noise perceived by a far-field observer. Two conditions regarding the peak of noise are then required: i) it should occur at sufficiently high frequency to benefit of the absorption mechanism by the atmosphere and ii) it should be outside the range of maximal hearing sensitivity. One remarkable result shows that a design modification, which is effective in reducing the noise emitted by the fans in a conventional configuration with two engines, loses its benefit when the engines are downscaled for the purpose of distribution.