Three-dimensional flow structures through submerged vegetation patches with uniform and non-uniform stem spacing in an open channel

This study uses large eddy simulation combined with the lattice Boltzmann method to analyze the three-dimensional flow structures through and around submerged finite vegetation patches in an open channel. Vegetation patches, modeled as arrays of circular stems arranged in alignment with varying streamwise and spanwise gaps, are distributed using three schemes: increasing gap, uniform gap, and decreasing gap along the channel. The findings reveal that increasing stem density results in smaller wake regions and a shift from large, organized coherent structures to smaller, more numerous and chaotic eddies. When both the streamwise and spanwise gaps are relatively small, the patch acts as a single stem with a complete horseshoe vortex and no detached vortices within the patch. On one hand, when the streamwise gap increases under a relatively small spanwise gap situation, the shear layers appear with reduced vortex shedding frequency and drag coefficient and allowing lateral fluid passage. On the other hand, as the spanwise gap also increases, it weakens the interactions between vortices generated from the adjacent stems, forming distinct horseshoe vortices and reducing the drag coefficient. Furthermore, for fixed vegetation patch dimensions, adjusting stem distribution along the channel alters flow structures by modifying wake length and vortex shedding patterns, with the drag coefficient decreasing from the increasing gap scheme (denser upstream, larger Cd), to uniform gap scheme (moderate Cd), and to the decreasing gap scheme (less dense upstream, smaller Cd), reflecting reduced flow resistance with upstream sparsity. Based on the results and findings of this study, lower upstream vegetation density is recommended for achieving reduced drag.