Theoretical methods to predict near-field fuselage installation effects due to inlet fan tones

Abstract

There are many analytical models to predict turbofan noise radiation in a free field.

However, these models explicitly ignore the effect of the aeroplane components on the engine noise. The effects of the aeroplane on the radiated noise must be included because it is the installed engine that affects people in the cabin and community. The original contribution of this thesis is to present a theoretical model for the near-field fuselage installation effects on tonal noise radiating from a turbofan engine inlet.

Historically, the fuselage installation effects have been modelled using theoretical methods for open-rotor type sources. Installation effects include the scattering effect from the fuselage, and the refraction effect of the boundary layer running down the fuselage. In this thesis the established techniques are extended to include a new sophisticated analytical source for spinning modes radiating from a circular duct. The source model includes the diffraction effect of the duct lip. The model applies Fourier methods and implements the Wiener-Hopf method for diffraction. Owing to the physics of the problem, simplifications in geometry and flow do not curtail the validity of the predictions.

The refraction effect was quantified by evaluating the difference between sound pressure levels with and without the boundary layer. Upstream of the source the refraction leads to a paucity of surface acoustic pressure, yet downstream the boundary-layer effect was minimal. Further investigations led to an alternative method of simulating refraction by altering wavenumbers in Fourier space. The installation method was optimised for a turbulent boundary-layer profile by replacing a power-law with a scaled step-change boundary-layer profile.

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Identifiers

ORCID for Alan Mcalpine: orcid.org/0000-0003-4189-2167

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