Beyond bending: multi-directional vegetation sway as a driver of turbulence and momentum transport in submerged canopies

Conventional studies of vegetated shear layers have typically modelled aquatic vegetation as rigid, upright structures to simplify submerged canopy hydrodynamics. While this approach has advanced the understanding of basic flow-canopy interactions, it overlooks vegetation bending and swaying, which are critical but challenging to characterize in flexible systems. As a result, comparative analyses of turbulent flow dynamics between flexible and rigid canopies remain limited. This study employs a coupled fluid-structure interaction framework to investigate turbulent open-channel flows across four vegetation configurations: (1) rigid upright canopy, (2) rigid curved canopy, (3) flexible canopy with constrained motion, and (4) flexible canopy with unconstrained sway. The hydrodynamics are resolved using Large Eddy Simulation, while vegetation motion is computed with the Vector-Form Intrinsic Finite Element method. This integrated approach enables systematic evaluation of how vegetation compliance modifies momentum transport. Results show that dynamic sway generates additional flow resistance beyond bending reconfiguration, amplifying shear-layer turbulence relative to rigid canopies. Under strong reconfiguration, unconstrained flexible canopies exhibit mean sway amplitudes approaching half the stem diameter, producing up to 60% enhancement in Reynolds stresses and 20% increase in turbulent kinetic energy within the canopy. Streamwise bending and sway also enhance mechanical dispersion, with dispersive stresses 10–50% greater than Reynolds stresses, and near-bed stress ratios peaking at 400%. Flow visualization reveals that streamlined bending suppresses flow-around effects and promotes symmetric secondary circulation, whereas lateral sway expands turbulence propagation and intensifies mixing. Momentum exchange is dominated by coherent Kelvin-Helmholtz vortices at the canopy-water interface, which transfer momentum 5–6 times more efficiently than other turbulent mechanisms. Streamlined reconfiguration reinforces turbulence organization, while multi-directional sway shifts transport deeper into the canopy, enhancing diffusion. These findings highlight the critical role of vegetation flexibility in shaping canopy hydrodynamics and provide new insights into turbulence-vegetation interactions in aquatic environments.