High-frequency sound transmission in flow ducts

本提案中概述的工作是由涡扇飞机发动机应用驱动的。在涡轮风扇进气和旁路管道中传播的声音主要是由于风扇叶片。由于风扇的旋转速度非常快，它会产生高频率的声音。采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，声吸收取决于声学衬垫的特性，但在高频时，吸收也会受到平均流对声音折射的影响。声音可以折射到或远离管道壁（那里有声学衬里）。为了设计出能够在宽频率和发动机转速范围内提供最大效益的声学衬管，需要快速有效的分析和数值方法来预测衬管流道中的噪声传输损失。涡轮风扇管道系统的声学建模是一个具有挑战性的问题，因为该模型的关键要素通常包括：高频声音；剪切平均流量；还有隔音管道。在高频率下，使用其他方法如计算流体动力学或计算空气声学是不实际的。这项工作的目的是开发实用的管道声学方法，可用于在高频率下，在内衬流管道中计算音调和宽带噪声传输损失。然后，在不同类型的通用涡扇风道中，将研究使噪声传输损失最大化的最佳衬管设计。这项研究将引起航空航天工业的兴趣，特别是航空发动机、机舱和声学衬垫制造商。

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.