Inverse modal-decomposition method for complete determination of freestream disturbancesin supersonic flows

风洞是研究高超声速飞行器壁面边界层层流-湍流转捩所必需的，因为这些流动表现出许多物理流动现象，而这些现象在相关雷诺数下无法通过数值模拟充分准确地确定。然而，风洞试验在试验段流中受到不可避免的干扰，通常会出现声波、熵波动和涡度波动。这些必须在不稳定边界层的相关频率范围内量化。在过去的60年里，扰动的识别是高超声速技术中一个未解决的问题，因为一直缺乏时间分辨的测量技术，而且风洞扰动的简化物理模型不能得到验证。通过利用测量压力、速度和流体密度的时间分辨仪器的最新进展，并通过使用代表世界先进水平的中型流动设施的高分辨率流动模拟，高超声速空气动力学的这一重要问题将在本研究项目中得到解决。RWTH亚琛大学空气动力学研究所将模拟TU布伦瑞克的高超声速Ludwieg管从储存管到试验段的风洞流动，包括试验段中的所有自由振荡扰动源。计算结果被用来提取和量化相关的干扰模式在测试部分。将使用试验段中驻点探针上方气流的高分辨率模拟来确定用于识别波速方向的适当探针几何形状。流体力学研究所将进行粒子图像测速测量，以测量速度波动，并进行聚焦激光差分干涉测量，以测量密度波动，扩展和校准到300 kHz的分辨率，并确定测量不确定性。结合新的驻点探针的结果，结果将产生声学模式，熵模式，涡模式的贝叶斯不确定性量化。结果定义了未来在高速风洞中边界层行为的声感受性分析。亚琛和布伦瑞克之间长达一年的数值实验合作是成功解决高超音速流中模态分解这一具有挑战性问题的基础。

Wind tunnels are necessary to investigate laminar-turbulent transition of wall boundary layers for hypersonic vehicles, since these flows exhibit numerous physical flow phenomena that cannot be sufficiently accurate determined by numerical simulations at relevant Reynolds numbers. Wind tunnel tests, however, suffer from unavoidable disturbances in the test section flow, where generally acoustic waves, fluctuations of entropy, and vorticity fluctuations occur. These must be quantified over the relevant frequency range of the unstable boundary layers. The identification of the disturbances is an unsolved problem in hypersonics over the last 60 years, since there has been a lack of time-resolving measurement techniques, and the reduced physical models of wind tunnel disturbances could not be validated. By taking advantage of recent progress in time-resolving instruments for measuring pressure, velocity, and fluid density, and by using high-resolution flow simulations of a mid-size flow facility that represents world-wide state of the art, this important problem of hypersonic aerodynamics will be solved in this research project. The Institute of Aerodynamics of RWTH Aachen University will simulate the wind tunnel flow of the Hypersonic Ludwieg tube of the TU Braunschweig from the storage tube to the test section with all sources of freestream disturbances in the test section included. The computational results are used to extract and quantify the relevant disturbance modes in the test section. Highly resolved simulations of the flow over stagnation point probes in the test section will be used to determine appropriate probe geometries for identifying wave-speed directions. The Institute of Fluid Mechanics will perform Particle-Image Velocimetry to measure velocity fluctuations and Focussed Laser Differential Interferometry for density fluctuations with extensions and calibrations toward a resolution of 300kHz and to determine the measurement uncertainties. Combined with the findings of the new stagnation point probes the results will yield the acoustic modes, entropy modes, and vorticity modes by Bayesian Uncertainty Quantification. The results define future sound receptivity analyses of boundary layer behaviour in high-speed wind tunnels. The year-long numerical-experimental collaboration between Aachen and Braunschweig is the basis to successfully work on the challenging problem of modal decomposition in hypersonic flows.