Large eddy simulation of 3-D flow structure and mass transport in open-channel flows with submerged vegetations

In natural rivers, a lot of aquatic plants are often observed and they have significance effects on hydrodynamic properties. In such vegetated open-channel flows, velocity profiles are largely changed in the vertical and spanwise directions. The vegetated canopies generate large-scale coherent structures near the vegetation top and these coherent motions promote the mass and momentum transport between the within- and over-canopies. However, it is fairly difficult to investigate the relation between coherent structure and mass diffusion property only by experimental methods, such as PIV and laser Doppler anemometer (LDA), because the submerged vegetated flows have 3-D complex patterns. Therefore, in the present study, 3-D large eddy simulation (LES) was conducted in vegetated canopy open-channel flows, in which six kinds of the submergence depth H/h were changed systematically including emergent vegetation. The vegetation elements were calculated exactly by the non-slip condition by using fine grids without introducing any additional drag force term. The effects of submergence depth on turbulence structure and scalar transport were examined numerically and compared with the measured values. The former part of the present study focused on the mean-flow and turbulence structures. The latter part examined an interaction between 3-D coherent structure and mass transport.