Numerical investigation of aeroacoustic sound generation by wave packets in turbulent jets

降低喷气噪声是现代航空的关键挑战之一。在商用喷气式飞机的起飞阶段，由喷气发动机产生的湍流自由射流是噪声的主要来源。存在实验以及数值证据表明，相关的声发射源于声近场中的长尺度相干结构。到目前为止，导致这种结构或波包产生的物理机制还没有得到很好的理解。然而，要有效地制订噪音预防策略，必须深入了解有关的物理过程，这是拟议研究计划的出发点。长尺度波包的起源和时空演变将在技术上相关的马赫数和雷诺数制度进行分析。为此目的，湍流自由射流计算的大涡模拟。必要的湍流入流边界条件是基于局部线性稳定性问题的解的叠加。 相干结构是通过光谱和正交分解方法从数据中提取的。 数值装置的流动参数和几何形状取自已发表的实验研究，并由主办机构提供。实验数据也用于验证模拟。待研究的波包是紊流自由射流的固有特征。为了理解生成过程，尽管事实上，两种新的方法被引入到提取自然和人工触发的波结构。在这两种情况下，自然和人工强迫，相干结构提取参考和扰动计算之间的差异，并测试相关的声音生成。该研究的目的是建立一个物理的了解，基本的航空声学的声音产生过程，可用于减少湍流喷气噪声排放，即有效实现主动或被动的控制手段。

The reduction of jet-noise is one of the key challenges of modern aviation. During the takeoff phase of commercial jet aircraft, the turbulent free jet generated by the jet engines is the dominant source of noise. There exists experimental as well as numerical evidence that the relevant acoustic emissions stem from long-scale coherent structures in the acoustic near-field. The physical mechanisms that lead to the generation of such structures or wave packets are not well understood up to now. However, an in-depth understanding of the underlying physical processes is required for efficient development of noise prevention strategies.This is the starting point of the proposed research project. The origin and the spatio-temporal evolution of long-scale wave packets is to be analyzed in a technically relevant Mach and Reynolds number regime. For this purpose, a turbulent free jet is computed by means of large eddy simulation. The necessary turbulent inflow boundary condition is based on the superposition of solutions to the local linear stability problem. Coherent structures are extracted from the data via spectral and orthogonal decomposition methods. The flow parameters and the geometry of the numerical setup are taken from a published experimental study, and are provided by the host institution. The experimental data also serves for the validation of the simulation. The wave packets to be investigated are an inherent feature of the turbulent free jet. In order to understand the generation process despite that fact, two novel approaches are introduced to extract natural and artificially triggered wave structures.In both cases, natural and artificially forced, coherent structures are extracted from the difference between a reference and the perturbed calculation, and are tested for relevance for sound generation. The aim of the study is to establish a physical understanding of the underlying aeroacoustic sound generation processes that can be utilized to reduce turbulent jet noise emissions, i.e. for the efficient realization of active or passive means of control.