Intermittent behavior of a bypass transition of boundary layers over an axisymmetric body of revolution

A transitional boundary layer developing over the forebody of a SUBOFF model subject to high free-stream turbulence (FST) was experimentally measured using both laser induced fluorescence and time-resolved two-dimensional particle image velocimetry. Typical coherent structures in the bypass transition, including low-speed streaks, hairpin-like vortices and vortex packets, are clearly visualized. Continue wavelet transform analysis on the time-frequency characteristics of instantaneous Reynolds shear stress (RSS) indicates that the intermittent behavior is mainly confined in mild-to-high-frequency domain, whereas low-frequency component is attributed to the Klebanoff mode. A wavelet-based approach was proposed to detect multi-scale turbulent events from wavelet coefficients of RSS, and the spatial evolution of the multi-scale intermittency factor was then calculated. Furthermore, the conditional sampling of laminar and turbulence events provides a new perspective on the route of the disturbance growth in the bypass transition. It is clearly shown that FST-introduced disturbances with moderate-to-high frequency are unable to penetrate the near-wall region; meanwhile, multi-scale characteristics inside the boundary layer evolve spontaneously from the Klebanoff mode.