Large-eddy simulations of ducts with a free surface

The goal of this work is to provide further understanding of the turbulent energy transport mechanism near a free surface, and, in particular, on the corner between a free surface and a solid wall. We perform temporal large-eddy simulation of incompressible, fully developed turbulent flow in a square duct bounded by a top free-slip wall, for Reynolds number based on the mean friction velocity and the duct width equal to 360 and 600 (5800 and 10000 based on bulk velocity, respectively). Various statistical comparisons and instantaneous flow visualizations are presented. The numerical results partially confirm experimental observations. As the free surface is approached, an increase in the surface-parallel Reynolds stress components is observed. The budgets of the Reynolds stresses show a strong reduction of all terms of the dissipation tensor in both the inner and outer near-corner region. The turbulent-kinetic energy budget exhibits a reduction in both turbulent kinetic energy production and dissipation toward the free surface. Very close to the solid boundary, within 15-20 viscous lengths from the wall, the turbulent kinetic production and the surface-parallel fluctuations appear to rebound in the thin layer adjacent to the free surface.