Browsing by Subject "boundary layer transition"

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    Hypersonic Simulations and Analysis of Transition to Turbulence on BoLT-2
    (2022-11) Johnston, Zachary
    The study of laminar to turbulent boundary layer transition has been a flow phenomenonof research for many decades. Recently, there has been interest in understanding how transition occurs for hypersonic boundary layers of increasingly complex geometries. Therefore, the Air Force Research Laboratory/Air Force Office of Scientific Research (AFRL/AFOSR) introduced the Boundary Layer Turbulence (BoLT-2) flight experiment to help in the understanding and prediction of boundary layer transition to turbulence at high-speeds by collecting data in flight. The BoLT-2 research geometry allows for the existence of multiple instabilities to coexist and potentially interact thus leading to transition. This allows for the opportunity to assess current stability analysis tools and numerical methods to help improve prediction of thermal loading under flight conditions. In support of this task, the objective of this dissertation is to quantify transition mechanisms contributing to nonlinear breakdown using a forced DNS approach. Modal analysis techniques are applied to simulation datasets to extract pertinent information associated with dominant instabilities contributing to breakdown. This is meant to help in the understanding of the underlying flow physics contributing to breakdown on BoLT-2. Comparisons are made with experiments conducted in the Mach 6 Quiet Tunnel (M6QT) at Texas A & M University and show excellent agreement. Furthermore, flight conditions are investigated to identify instabilities that are potentially present at flight conditions. This is meant to help with the interpretation of flight data once it becomes available to the research community. The numerical methodology of the DNS approach presented in this dissertation is one that can be used to predict transition and help towards the development of multi-dimensional stability analysis methods for transition prediction.
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    Numerical Simulation Of Instabilities In Three-Dimensional Hypervelocity Boundary Layers
    (2020-03) Knutson, Anthony
    Direct numerical simulation has been used for decades to study the boundary layer transition process. The primary contributions of this dissertation are twofold. First, we identify barriers to performing accurate numerical simulation of instabilities using an existing finite-volume flow solver (US3D) and overcome these barriers by implementing improved numerical methods. In particular, we develop a new type of shock sensor that significantly reduces numerical noise and implement a time-accurate implicit method that significantly reduces numerical dissipation. Second, we perform numerical simulations of two different geometries - the boundary layer transition (BoLT) flight experiment geometry and a cone with a swept fin - to improve our understanding of instabilities in three-dimensional, high-speed boundary layers. We find a vortical mode and traveling crossflow are the dominant instabilities in the BoLT flowfield while a multi-modal instability in the horseshoe vortex leads to transition on the fin-cone geometry.