Toward the prediction of fan noise: From low-speed to high-speed turbomachineries.

Abstract

Environmental concern and comfort trigger more and more acoustic specifications and regulations on aircraft impact on airports and on ventilation systems in buildings or transportation systems (cars, trains, or airplanes), which in turn impose lower and lower maximum noise levels to such systems. For instance, turbofan engines have increased their bypass ratio in order to improve the aircraft performance while diminishing the nominal speed of rotation. The jet noise is then reduced, and the fan noise becomes a dominant source of noise, especially at approach. A quick calculation of the overall noise generated by a given fan geometry, either low speed or high speed, would be a valuable asset to any fan design designer and manufacturer prior to any installation in a building or an airplane, for instance. However, an accurate prediction of the sound by any full turbomachinery still remains a challenging goal and a daunting task to be achieved by a direct computation. In the present study, the noise predictions will then rely on a strip theory combined with an acoustic analogy based on the wall pressure fluctuations. For the low-speed fans, the model is an extension of the development by Schlinker & Amiet for helicopters. Turbofan engines induce two additional difficulties to noise modeling: the cascade effects and the duct configuration which are presently modeled.