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Flexible Vegetation And Its Implementation In The Swash Ocean Model

This paper reports on the implementation of a flexible vegetation solver for the SWASH nearshore ocean model intent on resolving motion of highly flexible vegetative stems and the resultant influences on the flow field. Currently, vegetation in SWASH and most ocean models is represented as rigid cylinders, which can account for some of the features of vegetation, but does not realistically capture the ability of vegetation to adapt to flow, or flex, thereby affecting the resultant calculations of turbulence, drag, and inertia in the water column. Existing flexible vegetation solvers use finite elements schemes, which can be prohibitively expensive to calculate for large vegetative fields. We implement a finite difference scheme to solve the fourth order differential equation for highly flexible rods in a Lagrangian grid attached to the stems. This scheme is included in new subroutines created for SWASH, calculating deflection on a moving subgrid, and returning the force exerted by the vegetative stem to the existing Navier-Stokes k-epsilon model at the appropriate computational grid points. To validate the accuracy of the scheme and the behavior of the flexible vegetation, this scheme is modeled in MATLAB. It is validated against physical beam bending experiments, and against other physical and computational experiments of vegetation in flow fields. Implementation in SWASH shows the subgrid tracking through the computational domain of SWASH, and the behavior of the blade matches expected behavior of vegetation in a flow.