Numerical optimization of porous surfaces to reduce trailing-edge noise

在着陆进场时，机身噪声是影响飞机总体噪声水平的主要因素之一。由于机身尾缘噪声是机身噪声的重要组成部分，因此有必要降低它，以减少飞机噪声排放对机场附近人群的环境影响。本课题采用大涡模拟/计算气动声学(LES/CAA)相结合的方法，研究了一种用于后缘噪声的被动降噪技术，即应用于一般后缘的多孔表面。对不同后缘形状和多孔结构的计算表明，降低了湍流涡旋的相关性，使总声压级降低了11分贝。通过新开发的LES/CAA伴随公式优化了孔隙率，音调噪声降低了4db。在第二阶段，研究将扩展到宽带降噪。这种分析的新颖性在于多孔面形状的横向变化，即考虑了锯齿形状，以及多孔结构在流向和跨向上的变化。基于高保真的代理优化模型，对多孔表面的形状和结构进行了优化。此外，还考虑了攻角对降噪增益的影响，研究了较高马赫数对尾缘声学增益的影响，即对声学扰动的影响及其对整体空气动力学的影响。

At landing approach, airframe noise is one of the major contributors to the overall noise level. Since trailing-edge noise defines an essential part of airframe noise, it is necessary to lower it, to reduce the environmental impact of aircraft noise emissions on the population near airports. In this project, a passive noise reduction technology for trailing-edge noise, i.e., porous surfaces applied to generic trailing edges, is investigated by a hybrid large-eddy simulation/computational aeroacoustics (LES/CAA) approach.In the first phase, the impact of porous surfaces to reduce tonal trailing-edge noise has been analyzed. The computations for various trailing-edge shapes and porous structures have shown to reduce the correlation of the turbulent eddies and to lower the overall sound pressure level by 11dB. The tonal noise has been lowered by an additional 4dB for a porosity having been optimized by a newly developed adjoint formulation of the LES/CAA aproach.In the second phase, the investigation will be extended to broadband noise reduction. The novelty of this analysis is defined by a spanwise variation of the shape of the porous surface, i.e., a sawtooth shape is considered, and an alteration of the porous structure in the streamwise and spanwise direction. Based on a high-fidelity surrogate optimization model, the shape and the structure of the porous surface are optimized. Furthermore, the impact of the angle-of-attack on the noise reduction gain is taken into account and the influence of higher Mach numbers on the acoustic gain at the trailing edge, i.e., on the acoustic perturbations and their effect on the overall aerodynamics, is investigated.In brief, it is the overall objective of this project to analyze the impact of porosity on broadband trailing-edge noise and to reduce it.