Counter-gradient vorticity transport mechanisms in boundary layer transition: Insights from conditional statistics

The mechanisms in the late stages of boundary layer transition caused by four vastly different disturbances are analyzed using direct numerical simulations.  The cases considered are (1) roughness-induced bypass transition behind a single discrete roughness element, (2) bypass transition downstream of a distributed roughness patch, (3) bypass transition triggered by strongly three-dimensional free stream forcing, and (4) a classical route involving the interaction of a large amplitude Tollmien–Schlichting wave with noise.  The present analyses follow the recent results (Goldstein et al. DFD2018, Suryanarayanan et al. IUTAM2019) that suggested universal features in the mechanisms of the amplification of near-wall streamwise vorticity, and focus on the connection between the streamwise vorticity and the increase in wall shear stress through the vorticity flux term wω_y.  Conditional statistics over regions with a specific range of instantaneous or mean shear stress, including the correlation between ω_x² and wω_y are presented.  Flow structures characteristic of different ranges of wall shear stress values are extracted and compared. The results point to common features across the widely different transition routes.  Implications for transition modeling and control will be discussed.