High Frequency Sound Transmission in Flow Ducts

The work outlined in this proposal is motivated by the turbofan aircraft engine application. The sound that propagates in turbofan intake and bypass ducts is primarily due to the fan blades. Owing to the fan's very fast rotation speed, it generates sound at high frequencies. Passive noise control by using acoustic liners is one of the main strategies to reduce fan noise emissions from a turbofan engine. In general, sound absorption depends on the properties of the acoustic liners, but at high frequencies the absorption can also be affected by refraction of sound by the mean flow. Sound can be refracted towards or away from the duct wall (where there is the acoustic lining). In order to design acoustic liners that provide the maximum benefit over a wide range of frequencies and engine speeds, fast and efficient analytical and numerical methods to predict noise transmission losses in lined flow ducts are required. Acoustic modelling of turbofan duct systems is a challenging problem because the key elements of the model will typically include: high-frequency sound; sheared mean flow; and acoustically-lined ducts. At high frequencies, it is not practical to use other methods such as computational fluid dynamics or computational aeroacoustics. The objective of this work is to develop practical duct acoustics methods that can be used for both tonal and broadband noise transmission loss calculations, at high frequencies, in lined flow ducts. Then, the optimum liner design that maximizes the noise transmission loss in different types of generic turbofan ducts will be examined. This research will be of interest to the aerospace industry, notably aero-engine, nacelle and acoustic liner manufacturers.