Abstract
Spatial direct numerical simulations are performed to study the formation and development of three-dimensional structures in a compressible flat plate boundary layer, where the freestream Mach number is 0.5 and the Reynolds number at the inlet based on the displacement thickness 1000. A pair of neutral/stable oblique modes and a TS wave are superimposed on the laminar profile at the inlet plane of the boundary layer computational box. The magnitudes of the TS and oblique waves are chosen to be 1% of the freestream velocity, respectively. After saturation of the T-S wave, the growth of the oblique modes produces peak-valley splitting downstream and hairpin vortices (hairpin packets) on the low speed streak are observed further downstream. The oblique subharmonic mode leads to sooner transition to turbulence downstream in the boundary layer. Numerical results show the evolution of hairpin vortices into a large-scale structure, in which fine-scale vortical structures with low-/high-speed streaks develop near the wall.
