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

空中交通的不断增长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声学衬里表面，称为衬里，沿着发动机短舱中的流动通道放置。衬层基本上由多孔壁和后面的空腔结构组成。到目前为止，由于缺乏足够的描述，充分的分析模型，这些衬里的设计在流动条件下的最佳性能是基于启发式和经验的方法。申请人甚至可以在以前的项目中表明，在某些情况下，有衬里的表面会产生噪音，而不是衰减噪音。这表明需要改进的衬垫模型描述，使衬垫设计优化具有鲁棒的预测能力。描述衬垫性能的关键量是声阻抗，即声场的压力波动与壁面法向速度波动的比值。该线性阻抗通常通过间接方法在线路表面上均匀分布地全局确定。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是未知的，这将在本申请中解决。一种新的非侵入式阻抗测量技术，结合层析声压测量和多普勒全球测速声粒子速度测量，将开发，验证和应用于现实的掠流条件下的班轮设置。因此，需要开发光学设置以及后处理算法，以便于在流管道试验台中对衬里部分进行有限的光学访问的应用。

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.