Spectral Broadening in Aeroacoustics

尽管商业航空运输迅速发展，但喷气发动机的噪音和排放（二氧化碳和氮氧化物）是一个主要问题，公众希望天空更安静、更清洁。在英国，飞机噪音的研究对许多利益相关者来说都是非常重要的。伦敦希思罗机场的噪音管制是世界主要城市机场中最严格的。劳斯莱斯是英国首屈一指的工程公司之一，目前在民用发动机市场占有30%的份额，是世界第二大民用飞机发动机供应商。除了经济效益外，减少飞机噪音和排放物也有利于社会，改善在机场附近生活和工作的人们的生活质量，在某些情况下改善他们的健康。欧洲航空研究咨询委员会（ACARE） 2020年愿景的主要目标之一是将感知到的平均噪音水平降低50%。尽管在过去的二十年里，欧洲和美国在飞机噪音研究方面投入了大量资金，但要实现这一愿景，仍需要相当大的技术进步才能使飞机更安静。大多数航空声学研究的关键应用是飞机噪声。谱展宽是指音调声场在湍流作用下的散射，即声音与随机时变湍流的相互作用导致音调的功率损失，并分布到音调频率周围的宽带场中。当散射功率的比例相对于保留在音调中的功率很小时，这被称为“弱散射”。然而，光谱增宽会导致音调本身的消失，取而代之的是一个宽带驼峰：这被称为“强散射”。高涵道比涡扇发动机的出现导致了喷气发动机噪音的显著递减，这主要是由于喷气发动机噪音水平的降低。射流噪声减少的结果是，相对于其他噪声源，风扇、核心和涡轮噪声成为更重要的噪声源。在涡扇发动机中，由于尾部辐射声通过排气射流剪切层传播而发生频谱展宽。这影响涡轮音调的辐射，在较小程度上影响风扇音调。为了进一步降低飞机发动机的噪音，很可能需要对发动机的设计进行彻底的改变。目前，先进的开式转子对转螺旋桨概念正被重新评估，因为它们可以提供显著的燃油效率节约。然而，开放式转子产生大量的音调，历史上，他们一直被认为是嘈杂的相比，涡扇发动机。在自由射流风洞中进行的开式转子噪声测试可能会受到风洞射流剪切层的影响，因为开式转子产生高度突出的音调声场。剪切层使音调的光谱变宽。开发稳健、有效的预测方法（理论和计算）将是本研究的关键成果。预测强散射的能力是关键目标；目前还没有有效的预测方法来预测涡扇和开式旋翼飞机发动机的强散射。在实验室中获得的光谱展宽测量的模型尺度实验数据库的获取将是本研究的另一个关键成果。目前还没有这样的数据库；这些数据将用于验证目的，以及提高我们对散射现象的理解。总而言之，该研究项目将是航空声学中光谱展宽的第一个综合研究，其关键应用与涡扇和开式旋翼飞机发动机的噪声排放直接相关。

Noise and emissions (carbon dioxide and nitrogen oxides) from jet engines are a major issue, with public expectations of quieter and cleaner skies, despite the rapid growth in commercial air transportation. Research on aircraft noise is of major importance to many stakeholders in the UK. London Heathrow enforces some of the most stringent noise regulations of any of the world's major city airports. Also Rolls-Royce, one of the UK's premier engineering companies, currently has a 30% share of the civil-engine market, making it the world's second largest supplier of civil aircraft engines. In addition to the economic benefits, reducing aircraft noise and emissions also benefits society, improving the quality of life, and in some instances the health, of people living and working near airports. One of the principal aims in the ACARE (Advisory Council for Aeronautics Research in Europe) 2020 vision is a 50% reduction in perceived average noise levels. Notwithstanding the significant investment in aircraft noise research in Europe and the U.S. during the last two decades, this vision will still require considerable technological advances to make airplanes substantially quieter. The key application of the majority of research in aeroacoustics is aircraft noise. Spectral broadening refers to the scattering of tonal sound fields by turbulence, whereby the interaction of the sound with a random, time-varying, turbulent flow results in power lost from the tone and distributed into a broadband field around the tone frequency. When the proportion of scattered power is small relative to the power that remains in the tone, this is termed "weak scattering". However, spectral broadening can lead to the disappearance of the tone itself, replaced by a broadband hump: this is termed "strong scattering". The advent of the high-bypass-ratio turbofan engine led to a significant step-change reduction in noise from jet engines, principally due to lower levels of jet noise. A consequence of this reduction in jet noise was that, relative to other sources, fan, core and turbine noise became more important noise sources. In turbofan engines, spectral broadening occurs due to the aft radiated sound propagating through the exhaust jet shear layers. This affects the radiation of turbine tones, and to a lesser extent fan tones. It is likely that in order to generate another step-change reduction in aircraft engine noise, radical changes to the engine's design will be required. Currently advanced open-rotor contra-rotating propeller concepts are being reappraised due to the significant fuel efficiency savings they can provide. However open-rotors generate a multitude of tones, and historically they have been perceived as being noisier compared to turbofan engines. Open-rotor noise testing conducted in free-jet wind-tunnels can be affected by the presence of the wind-tunnel jet shear layers through which the sound propagates because open-rotors generate highly protrusive tonal sound fields. The shear layers cause spectral broadening of the tones. The development of robust, validated prediction methods (theoretical and computational) will be a key output from this research. The capability to predict strong scattering is the key aim; currently there are no prediction methods available to predict strong scattering of tones from turbofan and open-rotor aircraft engines. The acquisition of a model-scale experimental database of measurements of spectral broadening obtained in the laboratory will be the other key output from this research. There is currently no such database available; the data will be used for validation purposes, as well as to improve our understanding of the scattering phenomenon. In summary, the research project will be the first comprehensive study on spectral broadening in aeroacoustics, with key applications directly linked to noise emissions from both turbofan and open-rotor aircraft engines.