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

降低喷气机噪声是现代航空面临的主要挑战之一。在商用喷气式飞机的起飞阶段，喷气发动机产生的湍流自由射流是主要的噪声源。有实验和数值证据表明，相关的声发射源于声近场的长尺度相干结构。导致这种结构或波包产生的物理机制到目前为止还没有得到很好的理解。然而，深入了解潜在的物理过程是有效开发防噪策略所必需的。这是拟议研究项目的起点。长尺度波包的起源和时空演化将在技术上相关的马赫数和雷诺数范围内进行分析。为此，采用大涡模拟的方法对湍流自由射流进行了计算。必要的湍流流入边界条件是基于局部线性稳定性问题解的叠加。通过光谱分解和正交分解方法提取数据的相干结构。流动参数和数值装置的几何形状取自已发表的实验研究，并由主办机构提供。实验数据也可用于验证仿真结果。待研究的波包是湍流自由射流的固有特征。尽管如此，为了理解产生过程，引入了两种新方法来提取自然和人工触发的波浪结构。在这两种情况下，从参考和扰动计算之间的差异中提取自然和人为强迫的连贯结构，并测试其与声音产生的相关性。这项研究的目的是建立对潜在的空气声学声音产生过程的物理理解，可以用来减少湍流射流噪声的排放，即有效地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.