Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA, 02115, USA.
Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA.
Sci Rep. 2023 Mar 8;13(1):3910. doi: 10.1038/s41598-023-30401-9.
Monami is the synchronous waving of a submerged seagrass bed in response to unidirectional fluid flow. Here we develop a multiphase model for the dynamical instabilities and flow-driven collective motions of buoyant, deformable seagrass. 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. Our simplified model, configured for unidirectional flow in a channel, provides a better understanding of the interaction between these vortices and the seagrass bed. 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. This causes the grass to oscillate periodically even in the absence of water waves. Crucially, the maximal grass deflection is out of phase with the vortices. A phase diagram for the onset of instability shows its dependence on the fluid Reynolds number and an effective buoyancy parameter. 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. While higher Reynolds number leads to stronger vortices and larger waving amplitudes of the seagrass, waving amplitude is maximized at intermediate grass buoyancy. All together, our theory and computations develop an updated schematic of the instability mechanism consistent with experimental observations.
Monami 是指水下海草床对单向流体流动的同步波动。在这里，我们为浮力、可变形海草的动力不稳定性和流动驱动的集体运动开发了一个多相模型。我们表明，由于海草对流动的阻抗导致冠层界面处不稳定的速度剪切层，从而导致下游传播的周期性涡旋阵列。我们的简化模型配置用于通道中的单向流动，为更好地理解这些涡旋与海草床之间的相互作用提供了依据。每个通过的涡旋局部减弱冠层顶部的沿流速度，从而降低阻力，并允许变形的草在其下方伸直。这导致草即使在没有水波的情况下也能周期性地摆动。至关重要的是，草的最大偏转角与涡旋不同步。不稳定性起始的相图显示了它对流体雷诺数和有效浮力参数的依赖性。浮力较小的草更容易被流动变形，形成较弱的剪切层，具有较小的涡旋和穿过冠层顶部的物质交换较少。虽然较高的雷诺数会导致更强的涡旋和更大的海草摆动幅度，但海草的摆动幅度在中等浮力时达到最大值。总的来说，我们的理论和计算为与实验观察一致的不稳定性机制发展了一个更新的示意图。
