Abstract
Direct numerical simulations of the secondary flow and the gas transfer in an open-channel flow with the sidewalls are carried out to investigate the effects of Prandtl's second kind of secondary flow on the turbulence dynamics and the gas transfer at the water surface. Turbulent characteristics on the secondary flow such as the velocity-dip phenomena and counter-rotating vortex pairs near the upper and lower corners are verified to be reproducible in comparisons with existing experimental and numerical results. Turbulent structures visualized based on the second invariant of the velocity gradient tensor make it clear that swirling vortex tubes generate near not only the bed but also the sidewalls of the channel. The turbulence dynamics due to such vortex structures may play important roles in driving the surface velocity divergence. The numerical results don't agree quantitatively with the gas transfer velocity by a surface divergence model. It is concluded from the present results that the secondary flow in open channel influences significantly the gas transfer at the water surface.
