Further studies on the statistics and structures of flat-plate boundary layer with passing wakes

We present a numerical study of a flat-plate boundary layer with free-stream periodically passing planar wakes without external pressure gradients from Re θ = 80 to fully turbulent. Two direct numerical simulations were performed on the same flow with slightly different simulations parameters. In the constrained simulation design, the wakes are restricted to six boundary layer thicknesses above the wall at the inlet, while in the other design the wakes are un-constrained. Boundary layer statistics before, during and after transition are thoroughly evaluated through comparison with theoretical and experimental data of the zero-pressure gradient flat-plate boundary layer. Three-dimensional flow visualizations reveal the instant and location of breakdown events. Negative streaks are not of the shape of a rectangular box, rather, their wedged shape narrows towards the downstream passing wake and their strength diminishes towards the wall and towards the upstream direction. Matured turbulent spots have arrowheads directed upstream. Despite the intermittency of the passing wake disturbances, statistics in the early transitional region exhibit characteristics in agreement with previous studies on boundary layer. Statistics in the turbulent boundary layers differ from the canonical case only mildly near the outer edge.