Acoustic characteristics of a Helmholtz-resonator liner with flexible structure elements

为了降低航空发动机的噪声排放，经常采用气缸套。它们通常由一个齐平安装的穿孔和这个面片下面的一系列空洞组成。为了在未来的降噪方面有所改进，线性技术需要在显著衰减的频率范围内进行改进，特别是在低频范围内。在拟议的项目过程中，将详细研究一种提高声学减振性能的创新方法，并通过适当的模型进行描述：使用经典的亥姆霍兹谐振器底座结构，部分胞壁结构将被具有本征阻尼的柔性材料所取代。它假设-通过细胞内波动的声压-柔性壁受到振动，从声激励中提取能量以保持振动，并额外消耗材料内部的能量。这一概念在之前的实验研究中得到了成功的证明。通过对一个基本单元(由一个谐振腔和一个附加的背腔组成)的实验研究和相应的数值模拟，需要一个合适的基于物理的模型来捕捉和描述后续的物理机理。因此，测量柔性壁挠度的空间分布，确定谐振腔和背腔内的压力场是至关重要的。因此，研究应该扩展到一个基本单元阵列，同时考虑相邻单元之间通过公共背腔的相互作用。这一扩展构成了这一概念的实际应用的基础。进一步的声学测量和相应的数值模拟将研究基本单元的不同放置组合。对于新的线性概念的未来应用，需要一个改进的模型来提供对有源单元和背腔完全布置的阻抗描述。这个模型将建立在基本元素模型的基础上，并考虑到相互作用。

Liners are frequently applied for the reduction of noise emissions of aero-engines. They usually consist of a flush-mounted perforation and an array of cavities underneath this face sheet. For future improvements in noise reduction, the liner technology needs to be improved with respect to the frequency range of significant attenuation and in particular also for the low-frequency range.In the course of the proposed project, an innovative approach for the improvement of the acoustic damping performance shall be investigated in detail and described by appropriate models: using a classical Helmholtz resonator base structure, part of the cell wall structures shall be replaced by a flexible material with intrinsic damping. It is assumed that - by the fluctuating acoustic pressure inside the cell - the flexible wall is subject to vibrations which extract energy from the acoustic excitation to maintain the vibration and additionally dissipates energy inside the material.The concept was proven successfully in a previous experimental study. However, the detailed mechanism, the quantitative share of individual effects, and the interaction of basic elements are not sufficiently understood.By an experimental investigation of a basic element (consisting of a resonator cell and an additional back cavity) and accompanying numerical simulations, the subsequent physical mechanisms shall be captured and described by an appropriate physics-based model.Therefore, the measurement of spatial distribution of deflection of the flexible wall and the determination of the pressure field in the resonator cell and the back cavity is of utmost importance.Subsequently, the investigation shall be extended to an array of basic elements taking into account also the interaction between neighboring elements via a common back cavity. This extension forms the basis for a practical application of the concept. Further acoustic measurements and corresponding numerical simulations will investigate the different combinations of placement of basic elements.For future applications of the new liner concept, an improved model is required providing an impedance description of the full arrangement of active cells and back cavities. This model will build on the basic element models and take the interactions into account.