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

为了研究高超声速飞行器壁面边界层的层流-湍流转变，风洞是必要的，因为这些流动表现出大量的物理流动现象，而在相应的雷诺数下，数值模拟无法足够准确地确定这些现象。然而，风洞试验在试验段流动中不可避免地受到干扰，在试验段流动中通常会出现声波、熵波动和涡度波动。必须在不稳定边界层的相关频率范围内对它们进行量化。在过去的60年里，扰动的识别一直是高超声速领域中一个尚未解决的问题，因为缺乏时间分辨的测量技术，而且风洞扰动的简化物理模型也无法得到验证。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，以及通过使用代表世界最先进水平的中型流动装置的高分辨率流动模拟，这一重要的高超声速空气动力学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 Braunschweig的高超声速路德维格管从存储管到测试段的风洞流动，包括测试段中的所有自由流扰动源。计算结果被用来提取和量化试验段中的相关扰动模式。对测试段中驻点探头上方的流动进行高分辨率模拟将被用来确定合适的探头几何形状，以确定波速方向。流体力学研究所将使用粒子图像测速仪测量速度波动，使用聚焦激光差动干涉仪测量密度波动，并扩展和校准分辨率至300 kHz，并确定测量不确定度。结合新的驻点探头的结果，结果将通过贝叶斯不确定性量化得到声学模式、熵模式和涡度模式。这些结果定义了未来对高速风洞中边界层行为的声音感受性分析。Aachen和Braunschweig之间长达一年的数值实验合作是成功解决高超声速流动中具有挑战性的模式分解问题的基础。

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.