Numerical Investigation of ‘Monami’ in Flexible Submerged Aquatic Vegetation

Abstract:

Submerged aquatic vegetation (SAV) plays important roles in riverine and estuarine processes. By attenuating flow and modulating flow turbulence, SAV promotes sediment stability, provides food and shelter for ecologically and economically important species and improves water quality in these environments. Understanding the physical-biological-ecological coupling between SAV structure/function and flow regime is critically important for natural resources management. For flexible SAV, vegetation blades reconfigure their postures, interact with each other, and form various patterns of motion under different flow conditions. In this study, we developed a novel structural model that integrates the soft-body dynamics used in computer graphics community to study the collective behavior of SAV blades under different flow conditions. The Absolute Nodal Coordinate Formulation (ANCF) finite element method was employed to model large deflection of vegetation blades. A soft-body dynamic model was incorporated to simulate blade-to-blade interactions and prevent unrealistic physical contact. The structural solver was validated with experimental results of oscillatory flow over a single vegetation blade. Idealized flows representing shear layer instabilities were applied to the vegetation canopy to generate coherent waving motion (monami), and parametric studies were conducted to investigate vegetation canopy properties (flexibility, canopy density) by varying the length scale, center position, and velocity of the vortex. From simulation results, vortex characteristics and vegetation flexibility had strong effects on the deflected height of SAV, which determines the magnitude of flow attenuation by vegetation. Canopy density was an important factor in blade-to-blade interactions and caused significant differences in the dynamic motion of vegetation blades. In summary, the numerical framework successfully simulated interactive, waving motion of a flexible SAV canopy, which can be utilized to obtain a better understanding of the hydrodynamic responses of SAV to different flow regimes.