Large eddy simulation of the hydrodynamics around rectangular vegetation patch arrays

Arrays of bank-attached rectangular vegetation patches, common in river ecosystems, exhibit complex hydrodynamics influenced by patch density (ϕ) and aspect ratio (l/B). Understanding these effects is crucial for river management but remains incomplete, particularly regarding the coupled influence of ϕ and l/B. This study employs numerical simulation, validated by physical experiments, to investigate flow structure, momentum exchange, and drag characteristics around such arrays along one channel bank. Results show the arrays induce an inner–outer double shear layer structure. Cavity vortex formation and position are significantly modulated by the coupled effects of ϕ and l/B. Lateral outflow proportion (Qm/Qc) increases markedly with ϕ. Notably, turbulent kinetic energy at the cavity–mainstream interface and mean streamwise drag per patch display non-monotonic dependence on ϕ, peaking at an intermediate density (ϕ = 0.098), suggesting an optimal density for maximizing turbulence and resistance. Quadrant analysis reveals momentum exchange is dominated by sweep (Q4) events, while ejection (Q2) contributions show complex trends with ϕ. The lateral-to-total drag ratio generally increases with ϕ but decreases for high-density, large-aspect-ratio cases. Drag fluctuations intensify at medium-to-high densities and large aspect ratios. This research quantitatively elucidates the complex, coupled mechanisms through which vegetation density and geometric morphology influence flow structure, turbulence characteristics, and hydraulic resistance within laterally arranged rectangular vegetation patch arrays. It enhances the understanding of hydrodynamics associated with spatially heterogeneous vegetation and provides valuable insights for ecological channel design, river restoration efforts, and aquatic habitat management strategies.