Oblique breakdown to turbulence was initiated by low amplitude forcing in a laminar flat-plate boundary layer at Mach three. The growth of the instability waves was investigated using spatial Direct Numerical Simulations (DNS). Excellent agreement with theory was obtained in the linear stage corroborating that the entire transition process from the linear regime to the final breakdown was captured. A fully turbulent flow was reached demonstrating that this transition scenario is a viable path to turbulence. Key events in the late nonlinear stage of breakdown are studied in detail.

A comparative investigation of shear-layer instability mechanisms in wall-bounded flows is performed by employing both spatial and temporal direct numerical simulations. In this study, two complex flows are considered, the transitional backward-facing step flow and the turbulent wall-jet over plane and curved surfaces (Coanda flow). For the temporal simulations, (mean) profiles that are obtained from the spatial simulations are used as base flows. Temporal simulations neglect any spatial evolution of both the base flow and the disturbances and can therefore be used to investigate the local stability behavior. First the primary Kelvin-Helmholtz type instability of the shear layer is addressed. For secondary instabilities (subharmonic and fundamental), the investigation aims to determine the dominant instability mechanisms responsible for the occurrence of the typical flow structures that are observed in the spatial simulations. The complimentary nature of the temporal simulations provides crucial information for the interpretation of the spatial simulation results.