Laminar boundary layer-free-stream turbulence interface, turbulent spot-free-stream turbulence interface, and boundary layer turbulence-free-stream turbulence interface

The BTFTI concept refers to the interface between boundary layer turbulence and free-stream turbulence (Wu et al, PNAS, 114, 2017) . Here, we further introduce the TSFTI and the LBFTI to denote the interfaces separating, respectively, boundary layer transitional-turbulent spots and the laminar boundary layer from free-stream turbulence. Previous work has focused almost exclusively on the traditional concept of the turbulent/non-turbulent interface (TNTI), often ignoring the possibility that the boundary layer’s free-stream can be occupied by finite amplitude vortical turbulent flow. In this work, we study the BTFTI, LBFTI and TSFTI using DNS of  a spatially-developing, smooth, zero-pressure-gradient, flat-plate boundary layer beneath a decaying free-stream of homogeneous isotropic turbulence at momentum-thickness Reynolds numbers from 80 to 3000. These interfaces are quantified using the iso-surface of temperature at a threshold value of 1 percent of the wall temperature. Geometric characteristics of the interfaces were measured. Conditional sampling of velocity, scalar, swirling strength and vorticity across the interfaces was performed. Connections between the interfaces with boundary layer vortices have been established through high-quality flow visualizations.