LES study of turbulent flows with submerged vegetation

Fully developed turbulent flows with a submerged vegetation are investigated using Large Eddy Simulation (LES), with a focus on understanding the role of the coherent structures on the momentum transfer across the water-plant interface. The LES model results compare reasonably well with laboratory measurements reported in the literature. As with Reynolds-Averaged Navier-Stokes models, LES models effectively simulate the effects of submerged vegetation on the mean flow field, but they also account for the anisotropy of the Reynolds stresses due to the vegetation layer, and resolve coherent structures observed in the instantaneous flow field. Comparisons with fully developed flows in unobstructed (non-vegetated) channels are made to show how the vegetation significantly changes the mean flow, Reynolds shear stress, turbulence intensities, turbulence event frequencies and the energy budget within and above the vegetation layer. LES provides direct visual evidence that coherent structures, namely spanwise vortices (rolls) and streamwise vortices (ribs), develop at the water-plant inter-face at the top of the vegetation due to the well-known Kelvin-Helmholtz instability.