Laser optical measurements of the acoustic particle velocity at bias flow liners with grazing flow

为了现代喷气发动机的稳定和低噪声操作，特别是在稀薄燃烧的情况下，需要具有偏流的高效消声器（偏流衬套），因为一方面，这种发动机的燃烧室倾向于遭受燃烧振荡，另一方面，较少的空气质量流可用于阻尼。然而，这些偏流衬里的阻尼效率的优化需要对物理阻尼机制有更深入的理解。这种衬垫的阻尼性能由入射声场与流场之间的相互作用的复杂现象决定。因此，研究项目的目标是更好地了解这些现象。为此，期望从声场到流场的传递的能量平衡。一方面，这需要声粒子速度和流速场的三维测量。为实现这一目标，将采用在第一个供资期内已证明合适的正弦激光调频多普勒全球测速仪。然而，目前的FM-DGV系统必须首先扩展到体积测量。在这样做时，计划集成高速相机以提高效率，这代表了迄今为止使用的检测器阵列的替代方案。此外，它旨在通过并行化来加速信号处理。另一方面，关于偏流衬管上方测量的速度场的评估的挑战是将声学引起的流体动力学振荡与主要的声音和流场分离。为此，必须测试和应用不同的分析和分离方法（例如亥姆霍兹霍奇分解、频谱分析和波数滤波）。在这里，目的是根据分离场的能量的量来开发从声学场到流场的能量传递。通过与传声器测量得到的衬垫的声能耗散进行比较，将证明这种转移。作为第二个资助期的最后一步，合格的测量技术和分析方法应用于专门建造的DUCT-R（管道声学试验台-矩形截面）的简化偏流衬里。使用所测量的速度场数据，可以平衡从声场到流场的能量传递，这使得能够获得关于偏流衬里处的物理阻尼机制的新见解。

For a stable and low-noise operation of a modern jet engine, especially in case of lean combustion, highly efficient sound attenuators with bias flow (bias flow liners) are needed, since on the one hand, the combustion chamber of such an engine tends to suffer from combustion oscillations, on the other hand less air mass flow is available for the damping. However, the optimization of the damping efficiency of these bias flow liners requires a deeper understanding of the physical damping mechanisms. The damping performance of such a liner is determined by complex phenomena of interaction between the incident sound field and the flow field. Therefore, the goal of the research project is to reach a better understanding of these phenomena. For this end, an energy balance of the transfer from the sound field to the flow field is aspired. On the one hand, this necessitates three-dimensional measurements of the acoustic particle velocity and the flow velocity field. To achieve this, the Doppler global velocimetry with sinusoidal laser frequency modulation (FM-DGV) will be applied, which has been proven suitable already during the first funding period. However, the current FM-DGV system has to be extended for volumetric measurements at first. In doing so, it is planned to integrate a high speed camera to increase the efficiency, which represents an alternative to the detector array used so far. Moreover, it is intended to accelerate the signal processing by parallelization. On the other hand, the challenge concerning the evaluation of the velocity fields measured above the bias flow liner is to separate the acoustically induced hydrodynamic oscillations from the primary sound and flow field. For this purpose, different analysis and separation methods (e.g. Helmholtz Hodge decomposition, spectral analysis and wavenumber filtering) have to be tested and to be applied. Here, the aim is to develop the energy transfer from the acoustic to the flow field based on the amount of energy of the separated fields. This transfer will be proven by a comparison with the sound energy dissipation of the liner obtained by microphone measurements. As a last step of this second funding period, the qualified measurement techniques and analysis methodology are applied at a simplified bias flow liner at the specifically built DUCT-R (duct acoustic test rig - rectangular cross section). Using the measured velocity field data, the energy transfer from the sound field to the flow field can be balanced which enables gaining new insights concerning the physical damping mechanisms at bias flow liners.