Numerical study of flow and turbulence through submerged vegetation.

Hyung suk Kim, Moonhyeong Park, Mohamed Nabi, Ichiro Kimura

Monday 29 june 2015

14:50 - 15:05h at Europe 1 & 2 (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 2G. Environment – Ecohydraulic

Vegetation elements obstruct the flow and considerably decrease the mean flow velocities compared to non-vegetation zone. The additional form drag exerted by vegetation significantly affect the velocity distributions, turbulence structures and Reynolds stress profiles, and thus influence sediment transport and water pollutant. Especially, unlike the flow through emergent vegetation, the flow through submerged vegetation creates relatively high velocity gradient and induces a strong shear layer near the top of the vegetation element. An understanding of flow structures and exchange of mass and momentum through submerged vegetation is essential to be successful river restoration. In this paper, we perform LES for open channel flows through submerged matrix cylinders which are regarded as rigid-submerged vegetation. The computational model solves the filtered Navier-Stokes equations on a Cartesian grid with local refinement and employs the ghost-cell immersed boundary method to deal with solid boundaries. The cylinders are explicitly handled by computational grids. First, to validate the present computational model, the computation is conducted on experimental configuration of Liu et al. (2008) who measured streamwise and lateral velocity profiles and turbulence intensities using lase Doppler velocimetry (LDV). A good agreement between computation and measurement is found. The effects on submergence ratio (water depth to vegetation height) and vegetation density are investigated. The presence of the streamwise velocity inflection results in a strong shear layer region near the top of the cylinders and creates high turbulence. The coherent structures are produced above and behind the cylinders and those intensities increase with increasing vegetation density. The large scale vortices, which are a main mechanism of momentum exchange between the vegetation layer and the out of vegetation, are generated above the vegetation and these penetration depths decrease with an increase in the submergence ratio and vegetation density. It is demonstrated that LES can capture large scale vortices originating at the top of vegetation and account for detailed instantaneous flow fields through submerged vegetation.