Laser based tomographic measurement of the local acoustic impedance of overflowed liners (TOMLIM)

日益增长的空中交通需要高效的降噪措施，特别是解决航空发动机发出的噪声。噪声衰减的常见应用是沿发动机舱内流道放置的声学衬里表面，称为衬垫。衬里基本上由多孔壁组成，后面有一个中空结构。到目前为止，由于缺乏充分描述、充分分析的模型，这些衬里在流动条件下的最佳性能设计是基于启发式和经验性的方法。申请者甚至可以在以前的项目中表明，在某些情况下，带线条的表面可能会导致噪音产生，而不是衰减。这表明需要改进的线性模型描述，从而使线性设计优化具有稳健的预测能力。描述线性模型性能的一个关键量是声阻抗，即声场的压力波动与壁法向速度波动的比率。这种线性阻抗通常通过间接方法全局地、均匀地分布在线路表面上确定。然而，由于缺乏相应的测量技术，阻抗的局部分布及其与全局阻抗的相关性仍然未知，这将在本申请中解决。一种新的非侵入性阻抗测量技术，即层析声压测量和基于多普勒全球测速的声质点速度测量相结合，将被开发、验证并应用于实际放牧流动条件下的衬垫装置。因此，需要开发光学设置和后处理算法，以便于在流动管道试验台上对衬里部分进行有限光学访问的应用。

The increasing growth of air traffic demands highly efficient noise reduction measures especially addressing the noise emitted from aero-engines. Common applications for noise attenuation are acoustically lined surfaces, called liner, placed along the flow pass in the engine nacelle. Liners consist basically of perforated walls with a cavity structure behind. Up to now the design of these liners for an optimal performance under flow conditions is based on heuristic and empirical methods due to the lack of sufficiently describing, fully analytical models. The applicants could even show in previous projects that under certain circumstances lined surfaces can contribute to noise generation instead of attenuation. This demonstrates the need for improved liner model descriptions which allows a liner design optimization with robust prediction ability.A key quantity in describing the liner performance is the acoustic impedance, the ratio of the pressure fluctuations to the wall-normal velocity fluctuation of the sound field. This liner impedance is commonly determined globally, homogenously distributed over the line surface, by indirect methods. However, due to the lack of corresponding measurement techniques the local distribution of the impedance and its correlation with the global impedance is still unknown.This will be addressed in the present application. A novel non-intrusive impedance measurement technique, a combination of tomographic acoustic pressure measurements and Doppler Global Velocimetry based acoustic particle velocity measurements, will be developed, validated and applied on liner setups under realistic grazing flow conditions. Therefore, the optical setups as well as the post-processing algorithms need to be developed to facilitate the application with limited optical access to the liner section in a flow duct test rig.