Buzz-saw noise : propagation of shock waves in aero-engine inlet ducts

Abstract

For supersonic flows relative to turbo‐engine fan blades, measured acoustic spectra near the inlet present tones at fan blade passing frequency (BPF), engine shaft rotation frequency, or Engine Order (EO), and their respective harmonics. The latter are responsible for the Buzz‐saw noise and are thus referred to as “Buzz‐saw” or “multiple pure” tones.This work first attempts to reformulate McAlpine and Fisher's frequency domain model (2001) for the propagation of a unidimensional sawtooth waveform spiralling inside a hard‐walled cylindrical duct in the presence of a uniform flow. The non‐dissipative Burgers equation is solved using a shock fitting method, and modal attenuation and dispersion are added using a split‐step computational method.In practice, shocks do not only occur at blade tips but on a significant portion of the blade span. The plane wave hypothesis being no longer valid, a new three dimensional model is required. This model is based on the computation of the axially varying amplitudes of the modal solutions, in order to take into account the nonlinear modal interactions.