On the role of laminar/turbulent interface on energy transfer between scales in bypass transition

The role of laminar/turbulent interface on the interscale energy transfer in a boundary layer undergoing bypass transition is investigated with the aid of Karman-Howarth-Monin-Hill (KHMH) equation. A binary indicator is used to detect the interface and employed to define two-point intermittencies. These are used to decompose the interscale and interspace energy fluxes into conditionally-averaged components. We find that the inverse cascade in the streamwise direction arises due to events across the downstream or upstream interfaces (head or tail respectively) of a turbulent spot. The three-dimensional energy flux maps reveal significant differences between these two regions: in the downstream interface, inverse cascade is stronger and dominant over a larger range of streamwise and spanwise separations. This is explained by considering a propagating spot as it crosses a fixed streamwise location. The conditionally-averaged KHMH equation is also derived. We compare the three-dimensional maps of the conditionally-averaged production and total energy flux within turbulent spots against the maps of standard-averaged quantities within the fully turbulent region. The results indicate remarkable similarities in the two-point statistics between spots and the fully turbulent region.