Characteristics of turbulent/turbulent interface in bypass transitional boundary layer over an axisymmetric body of revolution

The present study investigates the turbulent/turbulent interface (TTI) in a transitional boundary layer developing on a SUBOFF model subjected to free-stream turbulence (FST), using two-dimensional time-resolved particle image velocimetry. Three key issues are analyzed in detail, including the spatial evolution of TTI geometry, its correlation with transitional flow structures, and the associated entrainment process. Geometrically, the mean interface height and its fluctuations relative to the boundary-layer thickness decrease with transition, while the fractal dimension increases. Additionally, an increase in FST intensity drives the interface closer to the wall and leads to enhanced interface fluctuation. Linear coherence spectra and conditional averaging reveal that the geometry and kinematics of the TTI are governed by distinct energy-containing structures at different transitional stages. In the laminar and early transitional stages, TTI dynamics are dominated by low-frequency, large-scale intermittent streaks linked to the Klebanoff mode. Further downstream, as higher-frequency disturbances grow, small-scale ejection and sweep events near the interface become the dominant influence. The most significant finding is the shift in the entrainment process from detrainment to entrainment as the transition process. At the initial transitional stage, the detrainment is primarily associated with the lift-up of low-speed streaky structures, which transport fluid away from both sides of the TTI interface. By contrast, in the nonlinear transition stage, entrainment process associated with high-speed structures that bend the TTI convexly toward the turbulent region, causing fluid from both sides to converge and produce a compression effect.