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

Jinyuan Ni,Zhimeng Zhang,Chunning Ji

Published 2025 in The Physics of Fluids

ABSTRACT

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.

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