ABSTRACT
Flow-vegetation interaction has been one of the key research topics for river management. The purpose of the present work is: (1) the applicability of a Large-Eddy Simulation (LES) method with a drag-force model to turbulent open-channel flows through emergent vegetation is carefully examined against recent experimental data, and (2) effect of the presence of gaps between vegetation patches aligned in the streamwise direction is studied on flow features such as the formation of large-scale horizontal vortices. Four LES runs are performed with different types of vegetation configurations: (i) continuous in the streamwise direction along the centerline of the flume; (ii) discrete along the centerline; (iii) continuous along a side wall; and (iv) discrete along a side wall. The effect of vegetation on flow is described by a classical drag-force model, and the Smagorinsky model is used to represent the subgrid scale stress. A great agreement is confirmed between the present LES results and the corresponding experimental data in the case where vegetation stands continuously in the streamwise direction, in terms of prediction accuracy of the instantaneous flow features as well as the mean flow statistics. When applied to the flows with patched vegetation zones, the present LES is found to reproduce the flow characteristics qualitatively but the predicted flow field requires slightly longer relaxation time to fit with continual appearance and disappearance of the vegetation patches. This failure is primarily because a uniform distribution of the drag coefficient involved in the drag-force model is assumed on the inside of the vegetation zone in the present study. A single predominant characteristic length scale of the large-scale horizontal vortices is found in the flow with a continuous vegetation belt. In an alternate arrangement, another length scale related to the periodic appearance and disappearance of the vegetation patches is also involved to the flow.
