Knutson, Anthony2020-08-252020-08-252020-03https://hdl.handle.net/11299/215199University of Minnesota Ph.D. dissertation.March 2020. Major: Aerospace Engineering and Mechanics. Advisor: Graham Candler. 1 computer file (PDF); v, 90 pages.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.enboundary layer instabilitiesboundary layer transitioncomputational fluid dynamicshypersonicsThesis or Dissertation