| In the presence of free-stream disturbances, the boundary layer over a flat plate circumvents the process of transition through growth of Tollmien-Schlichting waves. Instead, transition occurs at lower Reynolds numbers, through processes that are yet to be fully explored. Recent studies have shed more light on the pre-transitional boundary layer, which contains streamwise velocity streaks that attain very large amplitudes. However, the actual transition process, and the relevance of the so-called Klebanoff modes, remain unexplained. Additional effects, such as that of a finite leading-edge, pressure gradients, and surface curvature have not received much attention.; Numerical simulations provide a unique complement to theory and experiments. However, large-eddy-simulation of compressible flows has remained a difficult task. In this work, a robust high-order method based on a staggered variable arrangement is developed with the aim of studying transitional and turbulent flows in a compressible medium. The present method is demonstrated to be more robust than those based on collocated grids. It has been extended to curvilinear coordinates and combined with multi-zonal implicit-explicit time marching to allow efficient simulations in the presence of solid walls.; The numerical method developed is applied to boundary layer transition on a flat plate with a blunt leading edge. Five simulations, conducted within a parameter space defined by the leading-edge geometry and free-stream intensity and length scales, confirm observed effects of various parameters. For the case of a sharp leading edge, with low levels of free-stream turbulence, transition occurs through instabilities on backward jets, as observed in earlier studies. The leading-edge enhances the transition process when it is blunt, or when the free-stream turbulence is intense. Transition is brought about by wave-packet-like spot precursors that are induced at the leading-edge by interaction with strong free-stream vortices. These wave-packets contain frequencies higher than those in the unstable Tollmien-Schlichting range and move at around 50% of the free stream speed. They grow in amplitude with downstream distance and eventually break down to form a turbulent spot. Qualitative and quantitative characterization of these precursors show that they inhabit the lower half of the boundary layer and contain frequencies higher than the unstable range of Tollmien-Schlichting waves. The wave-packets have also been traced back to the leading edge and shown to be induced by interaction with strong free-stream voriticity. |
