Fluid–structure interaction of a vegetation canopy in the mixing layer

The waving motion of a submerged vegetation canopy is associated with the generation of coherent vortices on the top of the canopy, which arise as the manifestation of the mixing layer instability. The interaction between this unsteady flow above the mixing layer, as well as its influence on the vegetation canopy itself, remain relatively underexplored, with most prior studies employing significant modeling assumptions to simplify the system. In this paper a fully coupled fluid–structure modeling approach is used to investigate the interaction mechanism between the vegetation canopy and the mixing layer that arises above it. The dependence of vegetation behavior on the bending rigidity and canopy density is explored and five representative behavior states of vegetation are observed: static, flapping, dual, regular waving and irregular waving. The vortical structures observed in the mixing layer associated with each state are compared, and the influence of canopy density on the mixing layer development is investigated, with a particular focus on flow features relevant to vegetation. In addition, the lock-in effect is observed in the crossover region between the frequency of the mixing layer instability and the natural frequency of the vegetation, which indicates an attraction effect of the mixing layer on the waving frequency of the vegetation.