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

为了使现代喷气发动机稳定、低噪声地运行，特别是在稀薄燃烧的情况下，需要高效的偏流消声器(偏流衬套)，因为这种发动机的燃烧室一方面容易受到燃烧振荡的影响，另一方面可用于减振的空气质量流量较少。然而，要优化这些偏流衬板的减振效率，需要对其物理减振机理有更深入的了解。这种衬垫的减振性能是由入射声场和流场相互作用的复杂现象决定的。因此，研究项目的目标是更好地了解这些现象。为此，希望在从声场到流场的传递过程中达到能量平衡。一方面，这就需要对声学粒子速度和流动速度场进行三维测量。为了实现这一目标，将采用正弦激光频率调制的多普勒全球测速技术(FM-DGV)，这种测速技术已经在第一个资助期被证明是合适的。然而，目前的FM-DGV系统首先必须扩展到体积测量。在这样做的过程中，计划集成一个高速摄像机以提高效率，这是迄今为止使用的探测器阵列的替代方案。此外，通过并行化来加速信号处理。另一方面，测量偏流衬垫上方的速度场所面临的挑战是将声学诱发的流体动力振荡与主声场和流场分开。为此，必须测试和应用不同的分析和分离方法(例如Helmholtz Hodge分解、频谱分析和波数滤波)。这里的目的是根据分离的场的能量量来发展从声场到流场的能量传递。这一转移将通过与通过麦克风测量获得的衬垫的声能耗散进行比较来证明。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被应用于专门建造的DUDET-R(管道声学试验台-矩形截面)的简化偏流衬垫。利用测量的速度场数据，可以平衡从声场到流场的能量传递，这使得对偏流衬垫的物理减振机理有了新的认识。

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.