Aeroacoustic analysis of the tip-leakage flow of an ultrahigh bypass ratio fan stage

Abstract

A detailed aeroacoustic analysis of the flow induced by the clearance between the fan tip and the shroud is performed in a scale-model fan stage of an Ultra High Bypass Ratio turbofan engine. Wall-modeled Large Eddy Simulation has been performed at approach condition, which corresponds to a fully subsonic operating point. The contributions of the tip-gap noise to the total fan noise are investigated using the Ffowcs Williams and Hawkings analogy. The surface is split into two parts; the tip region and the rest of the blade in order to analyze the acoustic contributions of these two regions separately. It is shown that the tip-gap region generates a significant noise component above 2kHz, which corresponds to approximate 1.2 times the blade passing frequency. Two separate tip-leakage vortices are identified in the fan tip region. The dominant noise sources in the tip-gap region are observed at the trailing edge of the fan blade. The wall pressure spectra in the tip-gap region and the coherence of pressure fluctuations between monitor points at different positions in this region show an acoustic contribution of the tip-leakage flow at two different frequency ranges. The first range corresponds to medium frequencies between 2kHz and 9kHz, and the second range corresponds to high frequencies between 10kHz and 25kHz. The analysis of dynamic mode tracking, fluctuating pressure and velocity spectra, and instantaneous flow fields relates specific structures in the tip-gap flow to their spectral signature and paves the way for further analytical modeling of tip-gap noise sources.