Surface Fluctuations and Air Entrainment in a Turbulent Free-Surface Boundary Layer*

Air entrainment due to turbulence in a free surface boundary layer flow created by a horizontally moving vertical surface-piercing wall is studied through experiments and Direct Numerical Simulations (DNS). In the experiments, the moving wall is created by a stainless steel belt loop that is driven by two vertical rollers; one length of the belt between the rollers acts as the moving wall. The belt is accelerated suddenly from rest until reaching constant speed, thus creating a temporally-evolving boundary layer. Experimental observations and measurements of air-water interface deformation patterns and bubbles were performed with time-resolved photographic techniques.  In the DNS, a temporally evolving boundary layer problem was studied with the computational domain consisting of a streamwise section of the belt flow field with  periodic boundary conditions. The simulation uses a single-fluid, two-phase (air and water) formulation with an exact projection method on a staggered grid to resolve the air-water interface.  From the DNS results, the physics and prevalence of several classes of air entrainment events  is assessed.