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

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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.