Near wake of emergent vegetation patches in shallow flow

Abstract

Vegetation patches are particularly diffcult to quantify in terms of flow resistance due to their complex geometry and topological behaviour under hydrodynamic loading. They not only influence the water level and mean velocities due to the drag they exert, but they also affect the turbulence and hence all transfer processes such as the sediment transport dynamics in the surrounding area. Existing studies dealing with the interaction of flow and vegetation concern mostly measurements of the drag of single plants followed by analyses of the flow through and above homogeneous canopies. However, studies of the flow around single patches are uncommon and are mostly restricted to arrays of cylindrical elements. For leafy plants there is very limited information and understanding of how the flow evolves through and around the plants. This work aims at filling these gaps via complementary physical lab-scale and numerical experiments of the flow through and around an artificial vegetation patch. The experimental work focuses on PIV measurements in the wake of the patches whereas the method of large-eddy simulation is employed to provide additional insights of the flow inside the patch. Here we focus on results based on the PIV measurements.

LES of Flow Through and Around a Finite Patch of Thin Plates

“…The LESs are designed to complement a series of laboratory experiments conducted at the University of Hull (Wunder et al, ). A fully developed channel flow approaches a porous artificial vegetation patch consisting of thin plates of length L P arranged in a staggered pattern (Figure ).…”

“…Hence, their flow resistance is believed to be mainly due to viscous skin friction drag (Miler et al, ). What follows is that instead of rigid cylinders, thin plates have been proposed as better geometric models to mimic blades/leaves of above mentioned aquatic vegetation (Wunder et al, ).…”

“…This study should be considered complementary to the experimental study carried out by Wunder et al (2018) who carried out Particle Image Velocity measurements just upstream and downstream of an artificial vegetation patch (consisting of a collection of thin glass plates), which met the allometric characteristics of a real leafy plant. Their study revealed new information about the flow and turbulence structure downstream of the patch.…”

“…Their study revealed new information about the flow and turbulence structure downstream of the patch. In the study reported here, LES, a numerical method that allows the calculation of the most energetic part of a turbulent flow (Stoesser, ), is employed to simulate the instantaneous flow through a patch of thin glass plates arranged in a very similar geometrical configurations to the Wunder et al () study. The three principal objectives of the study are as follows: (1) to describe the detailed hydrodynamics of flow through idealized leaf/blade dominated vegetation, (2) to quantify and present plate‐scale boundary layers and to evaluate their structure and quantities in comparison with empirical solutions for flat plate laminar boundary layers, and (3) to quantify the drag force at element scale and at patch scale for various patch geometries.…”