High-frequency sound transmission in flow ducts

本提案中概述的工作是受涡扇飞机发动机应用的推动。在涡扇进气和旁路管道中传播的声音主要是由于风扇叶片。由于风扇的转速非常快，所以它能在高频下发出声音。利用声学衬垫进行被动噪声控制是降低涡扇发动机风扇噪声排放的主要策略之一。一般来说，吸声取决于声学衬垫的性质，但在高频时，吸声也会受到平均气流对声音的折射的影响。声音可以朝向或远离管壁(有声学衬里的地方)折射。为了设计在宽频率和发动机转速范围内提供最大效益的声学衬里，需要快速有效的分析和数值方法来预测衬里流道中的噪声传递损失。涡扇管道系统的声学建模是一个具有挑战性的问题，因为模型的关键元素通常包括：高频声音、剪切平均流和声学衬里管道。在高频下，使用计算流体力学或计算空气声学等其他方法是不切实际的。这项工作的目的是发展实用的管道声学方法，可用于高频下衬里流动管道中的音调和宽带噪声传输损失计算。然后，将研究在不同类型的普通涡扇管道中使噪声传输损失最大化的最优衬垫设计。这项研究将引起航空航天工业的兴趣，特别是航空发动机、机舱和声学衬里制造商。

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.