Most previous simplified models fall into one of two categories: models of flow over a specified set of rigid obstacles 16, 24, or models where grass deformation can occur, but does not impact the flow profile 32. These challenges have demanded sophisticated studies, both experimental 19, 23, 24, 25, 26 and numerical 27, 28, 29, 30, 31. Thus, in general, the fluid flow must be solved simultaneously with the configuration of each structure. In a two-way coupled dynamic model, the fluid applies a load on each vegetative structure, which causes a resultant deformation that, in turn, affects the flow 22. There are numerous modeling challenges in capturing the properties of this system, primarily related to the feedback mechanism between flow and vegetation. Our numerical simulations provide a complementary and comprehensive picture of the fluid instability, vortex-seagrass interaction, and tracer exchange between the seagrass bed and the overflow in terms of its dependence on seagrass buoyancy and Reynolds number. Transport of material across the canopy has also been studied experimentally 20, 21. These perturbations to the mean flow have been observed experimentally and feature “sweeps” and “ejections” that occur at the leading and trailing edges of vortices, respectively 17, 18, 19. These vortices perturb the flow, which locally changes the deformation of grass blades and leads to synchronous oscillations of the grass bed. Through a mechanism similar to the Kelvin–Helmholtz instability 16, the enhanced velocity shear near the grass top creates a sheet of vorticity that destabilizes into vortices over time. Current explanations of monami 13, 14, 15 rely on the existence of a shear layer at the top of the grass bed due to vegetation drag. Instabilities of flow through submerged canopies yield a phenomenon known as monami-the progressive, synchronous oscillation of aquatic vegetation 8, 12. Seagrass meadows also influence sediment deposition and resuspension 9, as vegetation can trap suspended materials 10 and reduce sediment movement 11. Hydrodynamic processes resulting from these interactions influence environmental processes such as sedimentation, transport of dissolved oxygen 4 and nutrients, plant growth, and biomass production 5, 6, 7, 8. Seagrass beds exhibit a particularly rich set of dynamic behaviors due to their collective interaction with the flow. In order to photosynthesize, submerged canopies typically occupy shallow coastal environments 3, and in some cases this results in a significant portion of the flow being obstructed by the canopy. While emergent canopies-those that are in the inter-tidal zone and emerge above the water surface-need stiffness for the stems to stand up out of the water, fully submerged seagrass species (such as Halodule wrightii, Syringodium filiforme and Zostera marina) tend to stand up by buoyancy 2. ![]() Seagrass is typically deformable, which allows the grass blades to reconfigure according to the fluid load 1. All together, our theory and computations develop an updated schematic of the instability mechanism consistent with experimental observations. While higher Reynolds number leads to stronger vortices and larger waving amplitudes of the seagrass, waving amplitude is maximized at intermediate grass buoyancy. ![]() Less buoyant grass is more easily deformed by the flow and forms a weaker shear layer, with smaller vortices and less material exchange across the canopy top. A phase diagram for the onset of instability shows its dependence on the fluid Reynolds number and an effective buoyancy parameter. Crucially, the maximal grass deflection is out of phase with the vortices. This causes the grass to oscillate periodically even in the absence of water waves. Each passing vortex locally weakens the along-stream velocity at the canopy top, reducing the drag and allowing the deformed grass to straighten up just beneath it. Our simplified model, configured for unidirectional flow in a channel, provides a better understanding of the interaction between these vortices and the seagrass bed. We show that the impedance to flow due to the seagrass results in an unstable velocity shear layer at the canopy interface, leading to a periodic array of vortices that propagate downstream. ![]() Here we develop a multiphase model for the dynamical instabilities and flow-driven collective motions of buoyant, deformable seagrass. Monami is the synchronous waving of a submerged seagrass bed in response to unidirectional fluid flow.
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