Effect of flexibility of submerged aquatic vegetation on drag force

Three-dimensional fluid-structure interaction (FSI) simulations were conducted to investigate the coupled dynamics of water flow and the deformation of flexible aquatic vegetation. In these simulations, the vegetation exhibited streamwise bending due to hydrodynamic forces exerted by the oncoming current, while also undergoing lateral oscillations caused by vortex shedding. These lateral oscillations were not merely passive responses but actively interacted with the flow, further enhancing vortex shedding. As a result, the wake became more pronounced, and the momentum deficit behind the vegetation increased. This led to a greater pressure difference between the upstream and downstream sides of the vegetation compared to rigid vegetation, which lacked such oscillatory behavior.

In water, which has relatively low viscosity, the drag experienced by submerged bodies is governed primarily by form drag associated with pressure differences, rather than by skin friction drag resulting from viscous shear stress. For flexible vegetation, the enhanced vortex shedding and associated wake intensification significantly increased this pressure difference. Consequently, the total drag acting on flexible vegetation became larger than that on rigid vegetation. These results highlight the important role of vegetation flexibility in modulating hydrodynamic forces in aquatic environments and demonstrate how flow-induced deformation and unsteady motion can lead to increased drag through pressure-based mechanisms.