Analysis of vorticity flux during laminar-turbulence transition in channel flow*

In channel flow, the wall-normal flux of spanwise vorticity is directly related to drag and energy dissipation. This flux starts from the wall and persists to the channel centerline, sustained by diffusive and advective transport. We investigate the contributions to this vorticity flux during transition from laminar flow, where the flux is entirely viscous, to turbulent flow, where viscous, advective, and stretching/tilting effects all contribute. Ensemble-averaged statistics of K-type and H-type transition are computed, and the analysis focuses on the nonlinear contribution (advective and stretching/tilting term) to the vorticity flux profile during the transition process. How these terms change is explained with reference to the changes in the base flow. In terms of flow structures, the lambda-shaped vortices that are observed prior to reaching the fully turbulent state are also examined, and they feature both upgradient and downgradient flux of vorticity. Conditionally sampled structures are also computed to distinguish the contributions to the upgradient and downgradient transport of spanwise vorticity.