Generation and Propagation of Multiple Pure Tones Inside Turbofans at Transonic Regime

Abstract

Whereas the sound radiated from the inlet of turbofans is mainly due to rotor–stator interactions in approach flight, the shock waves (or N-waves) emitted by the rotor at transonic rotation speeds can be a dominant noise source during takeoff and climb. The study of N-waves needs to take account of two main processes: 1) the generation of N-waves for a perfect rotor (in which all blades are identical) and for a real rotor (considering small geometrical blade dispersion), and 2) The N-wave propagation through the inlet duct producing the blade passing harmonics for a perfect rotor, and the multiple pure tones (harmonics of the rotation frequency) for a real rotor. Several approaches have been investigated for the past 40 years. This paper intends to cross-check the main methods by applying them to two modern turbofan demonstrators. Moreover, a prospective way of N-wave generation based on geometrical considerations is investigated thanks to test data related to near-field pressure signatures and blade stagger angle measurements during the engine operation. This prediction overcomes some drawbacks of other methods. Moreover, it appears to be very efficient to build rotor blade arrangements that reduce the sound pressure level of the multiple pure tones.