Interactions between the submerged flexible vegetation array and two-dimensional laminar boundary layer flow

Submerged flexible vegetation exists widely in nature and provides multiple ecological and engineering values. Interactions between the vegetation array dynamics and large-scale vortices in a laminar boundary layer flow are systematically investigated using two-dimensional numerical simulation at dimensionless bending rigidity γ = 0.04–0.34, incoming boundary layer thickness δ = 0.2–1.5, Reynolds number Re=400, mass ratio β=1.0, and gap distance d=0.5. Five distinct Monami modes triggered by Kelvin–Helmholtz (KH) instability, as well as sectional waving and upright oscillation modes initially excited by a shear layer instability related to vortex shedding, are identified and characterized. The spatial and temporal features of the KH instability onset position and the low-frequency initial response position are key to distinguishing different dynamic modes. In Monami modes, the frequency of KH instability decreases to approach the first-order natural frequency of the vegetation element until the KH vortex is excited. After that, the oscillations of vegetation elements exhibit a lock-in phenomenon, which can shift to a purely fluid-driven response due to complex vortex merging. Influences of γ and δ on instability and drag are also discussed, including the effects on dynamic modes, instability onset position, low frequency, and the array's drag. In particular, the maximum at γ=0.08 and decreasing trend with increase δ of the mean drag of the array result from evolutionary properties of vortices and a reduced effective flow velocity, respectively.