Numerical Study of Turbulent Aquatic Canopy Flows in a Vegetated Channel with Flexible Stems

Under turbulent boundary-layer conditions with aquatic canopy vegetation, interactions between submerged flexible stems and the flow generate shear-layer turbulence that reorganizes the mean field and second-order statistics. Because measurements and modeling inside flexible canopies are difficult, stems are often idealized as rigid or statically bent. In this study, the developed fluid-solid coupling model is employed to reproduce submerged canopy flows with large stem deformation and dynamic oscillation. Large eddy simulation (LES) and the vector-form intrinsic finite-element (VFIFE) method are used to resolve turbulent behavior and canopy motion, respectively. Results indicate that bending deformation and swaying enhance spatial flow heterogeneity. Consequently, vertical profile characterization requires spatially averaged three-dimensional representations of the resolved flow around vegetation stems—rather than two-dimensional slices—especially for second-order turbulence statistics. Canopy deflection shifts the primary form drag region downward, and the streamlined geometry causes the local drag coefficient to decline progressively with distance from the bed. Reynolds shear stress at the canopy top is weakened by bending, whereas stem oscillations strengthen dispersive stress near the mean deflection height. As the Reynolds stress (turbulent diffusion effect) decays toward the canopy interior due to drag dissipation, its ratio to the dispersive stress (mechanical dispersion effect) approaches 1.3. Momentum exchange resembles obstructed shear-layer mixing: alternating sweep and ejection events dominate outside and inside the canopy, respectively; large-scale coherent structures remain quasi-periodic above the canopy, while stem oscillations superimpose counteracting perturbations within the interior.