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Engineering

Research project: Sound radiation from jet exhausts

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Engineering models predicting how sound radiates from jet exhausts has been developed and used extensively for the acoustic design of liners on turbofan engines.

Project Overview

 Example of comparison of prediction (solid line) against experimental data (dashed line)
Prediction vs data

We have developed models to describe the radiation of sound from turbofan engines (both for the bypass duct and the core exhaust). Building on some earlier work by Munt (1977) the exhaust is modelled by a semi-infinite unflanged annular duct carrying a jet that issues into a uniform mean flow. The turbofan afterbody is represented by an infinite cylindrical centre body extending downstream from the duct exit. Noise propagates along the annular bypass duct, refracts through the external bypass stream and radiates to the far field. The instability wave of the vortex sheet and the refraction of sound waves by the jet shear layer are also accounted for.

Example of the pressure distribution in the near field (Note the presence of the instability wave)
Pressure distribution

Even though this model is for a simplified geometry, it has proved very valuable by providing rapid predictions of the sound radiation that can help during the early design of the acoustic treatments for nacelles. This model is now used in several aerospace companies (including Rolls-Royce and Airbus). With the possibility to calculate solutions in the near field, it has been used extensively as a benchmark solution to validate computational aero-acoustic methods.

Associated research themes

Acoustics

Fluid Dynamics

Related research groups

Acoustics Group

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