NUMERICAL STUDY ON SECONDARY FLOW AND AIR-WATER GAS TRANSFER IN OPEN-CHANNEL FLOW WITH FREE-SURFACE FLUCTUATION

We carried out a direct numerical simulation of open-channel flow with the free-surface fluctuation to investigate the effect of the fluctuation on the secondary flow and the air-water gas transfer. The DNS reproduces turbulent characteristics of the secondary flow such as the velocity-dip phenomena and counter-rotating vortex pairs, and they are almost similar to numerical results without the free-surface fluctuation. However, the maximum secondary flow velocity and the depth at which the streamwise velocity becomes maximum are changed by the influence of the free-surface fluctuation. The numerical results show the free-surface fluctuation enforces the secondary flow. The turbulent field near the sidewall that controls the air-water gas transfer is investigated on the basis of a statistical treatment such as the VITA technique. We can visualize the surface renewal eddies due to the secondary flow through the VITA technique.