The Structure of Boundary Layer Along a Vertical, Surface-Piercing Flat Plate

摘要

The present work reports on Direct Numerical Simulations of a temporally developing, zero pressure gradient, turbulent boundary along a surface piercing flat plate and its interaction with the free surface. The simulations were driven by experiments of the same flow regime. Three separate Froude numbers were considered in increasing order. Consequently the interface progresses from a rigid and undisturbed surface to one with violent eruptions, breaking waves and air entrainment. At the lowest Froude number where the surface stays flat, the simulations agreed well with prior studies and captured the recirculation regions in the cross-stream plane which are shown to be due to Reynolds stress anisotropy. At intermediate Froude numbers it was found that the main source of vorticity beneath the surface is not the Reynolds stress anisotropy but rather the vorticity generated at the interface. This vorticity was found to affect turbulent statistics including distribution of friction velocity and the slope of the log-law layer. Moreover, the present work shows that the surface generated vorticity interacts mainly with eddies of small and intermediate wave numbers and the smaller scales with high wave numbers remain intact.;Air entrainment due to turbulence was also investigated. With the aid of a prototypical problem the parameters that play a role in entrainment are established. A novel approach to quantify the turbulent structures was defined. Using this method, turbulent structures were categorized into entraining and non-entraining vortices. A Linear Logistic Regression model was trained and validated to help predict future entrainment events. The model performs well and can accurately predict entrainment events for both the turbulent regime and the prototypical problem.