Instability and Transition of Swept-Wing Flow: Mechanisms of interaction between crossflow instabilities and forward-facing steps

This thesis presents a theoretical and numerical investigation of how a surface forward-facing step alters the stability and transition mechanisms of a laminar incompressible swept-wing boundary layer dominated by stationary crossflow instabilities (CFI). The results elucidate the processes that cause significant transition advancement due to the step and, in turn, challenge the classic paradigm in fluid mechanics that rapid, localised surface-geometry variations are universally detrimental to laminar flow. Through Direct Numerical Simulations (DNS) and new theoretical modelling, this thesis shows that, under specific conditions, a forwardfacing step stabilises pre-existing stationary crossflow vortices and thereby delays laminar-turbulent transition...