Large eddy simulation of free surface turbulent flow in partly vegetated open channels

A large eddy simulation (LES) model has been developed to simulate the hydrodynamic behaviour of turbulent flow in an open channel with a domain of vegetation. Vegetation is considered as an internal source of resistant force and turbulence energy. The model is modified from the LES model of Li and Wang (International Journal for Numerical Methods in Fluids 2000; 34), and is distinctive in that the subgrid scale turbulence is parameterized by a k-l model. The length scale of turbulence l is proportional to the grid size and the turbulence energy k is obtained from the solution of the turbulence energy transport equation. An operator splitting method, which splits the solution procedure into advection, diffusion and pressure propagation steps, is employed so that different numerical schemes can be used for the solution of different physical processes. The model has been applied to simulate open channel flow with transverse shear produced by vegetation drag. Some organized large eddies were found in the interface between the vegetated and non-vegetated regions and the organized structure clearly has a life cycle. At the interface the transverse velocity profile exhibits a steep gradient, which induces significant mass and momentum exchange, acts as a source of vorticity, and generates high Reynolds stresses. The logarithmic vertical velocity variation becomes uniform in the vegetated domain. The agreement between the numerical results and the experimental data (Tsujimoto and Kitamura, KHL Progressive Report '92, Hydrology Laboratory, Kanazawa University, Japan, 1992; 21) is satisfactory. The present k-l LES model is proven to be a useful tool for engineering applications, as it can simulate the dynamic development of large eddies and the associated intermittent turbulence.