Rotating blades and aerodynamic sound

Noise shielding for aircraft

“Buzz-saw” noise: Prediction of the rotor-alone pressure field

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 stag...

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

The nonlinear propagation of finite amplitude acoustic waves in hard-walled guides is treated in the following paper. A convected nonlinear wave equation is derived, and a quasianalytical solution based on axially varying modal amplitudes is presented for a no-flow case. Plane and non-planar finite amplitude wave propagation is investigated, and nonlinear interactions of higher order modes are studied in two-dimensional and three-dimensional cylindrical configurations.

A Nonlinear Computational Method for the Propagation of Shock Waves in Aero-engine Inlets Towards a new Model for Buzz-saw Noise Prediction

The buzz-saw noise made by a two-stage transonic research compressor has been investigated experimentally over a range of tip relative Mach numbers up to 1.56. The results show that the phenomenon is due to the propagation at supersonic relative tip speeds of the steady rotating pressure field associated with the first-stage rotor blades. The flow entering the tip section of the rotor has been analyzed theoretically and the circumferential pressure fluctuations computed, with good agreement with near-field measurements. The analysis leads to a clearer understanding of the dependence of the noise on inlet Mach number and three-dimensional effects and indicates the types of rotor irregularity which will most influence the harmonic content.

M. G. Philpot

Papers

The Buzz-Saw Noise Generated by a High Duty Transonic Compressor