Large Eddy Simulation of Turbulent Flow Through Submerged Vegetation

Abstract

Large Eddy Simulations (LES) are performed for an open channel flow through idealized submerged vegetation with a water depth (h) to plant height (h p ) ratio of h/ h p = 1.5 according to the experimental configuration of Liu et al. (J Geophys Res Earth Sci, 2008).They used a 1D laser Doppler velocimeter (LDV) to measure longitudinal and vertical velocities as well as turbulence intensities along several verticals in the flow and the data are used for the validation of the present simulations. The code MGLET is used to solve the filtered Navier-Stokes equations on a Cartesian non-uniform grid. In order to represent solid objects in the flow, the immersed boundary method is employed. The computational domain is idealized with a box containing 16 submerged circular cylinders and periodic boundary conditions are applied in both longitudinal and transverse directions. The predicted streamwise as well as vertical mean velocities are in good agreement with the LDV measurements. Furthermore, fairly good agreement is found between calculated and measured streamwise and vertical turbulence intensities. Large-scale flow structures of different shapes are present in the form of vortex rolls above the vegetation tops as well as locally generated trailing and vonKarman-type vortices due to flow separation at the free end and the sides of the cylinders. In this paper, the flow field is analyzed statistically and evidence is provided for the existence of these structures based on the LES.

High-resolution numerical modelling of flow—vegetation interactions

“…This constraint on model resolution has meant that to date, most stem-scale models have focussed on high resolution analysis of smaller-scale canopy properties and have not fully considered large or highly submerged canopies. Stoesser et al (2006) performed Large Eddy Simulation (LES) experiments on an array of submerged cylinders using a spatially variable very fine grid resolution in order to fully capture the stem-scale turbulence. Their results agreed well with previous experimental results, as well as replicating the classical vortex regimes known to be present (e.g.…”

Flow structure along the cross-section of an open-channel caused by patch of submerged vegetation

“…The discharge values were decreased in vegetated part when increased at the nonvegetated part. The percentages of the discharge reduction over the cross-section were calculated as 3% and 12% for vegetation intensities 1 …”

“…The flow area is quite influenced by submerged and/or emergent vegetation in the natural rivers [1]. Aquatic plants affect the flow characteristics such as the velocity distributions, turbulence and coherent structure, as well as mass and momentum exchanges between the vegetated and non-vegetated zones [2].…”

“…These flow phenomenon causes shear layer formation between the vegetated layer and overlying water [1]. The vegetation effect on the flow phenomenon has become very important on river and coastal restoration, the establishment of flood control and coastal preservation projects [1].…”

A fluid-mechanics based classification scheme for surface transient storage in riverine environments: quantitatively separating surface from hyporheic transient storage

“…In sparse submerged canopies, bed drag is larger than canopy drag and the mean velocity profile follows a turbulent logarithmic velocity profile, whereas in dense submerged canopies, bed drag is smaller than canopy drag and the mean velocity profile has an inflection point near the top of the canopy (Ghisalberti and Nepf, 2002;Wilson et al, 2003;Sukhodolov and Sukhodolova, 2006;Nepf and Ghisalberti, 2008). At the top of the canopy, the drag discontinuity increases velocity shear, causing flow separation and the formation of a mixing layer (Gambi et al, 1990;Stoesser et al, 2009). The inflection point in the velocity profile causes vortical structures in the mixing layer to billow and grow downstream due to Kelvin-Helmholtz instabilities (Ikeda and Kanazawa, 1996;Ghisalberti and Nepf, 2002;Nepf and Ghisalberti, 2008;Stoesser et al, 2009).…”