Abstract
Accurate prediction of sediment transport in the presence of bedforms such as sand ripples requires an advanced understanding of how dynamic sediment beds interact with turbulent oscillatory flows. In this paper we propose a new approach for simulating these interactions, based on a fixed grid multiphase Euler-Lagrange simulation, that fully couples dynamic bed evolution to the motion of a sub-grid scale Lagrangian sediment phase. The sediment phase is evolved by computing hydrodynamic and inter-particle forces and torques acting on individual particles, and is coupled to the fluid phase through the volume-filtered Navier-Stokes equations. We validate the approach for sediment transport applications using hindered settling velocity tests, and show very good agreement with the experimental data of Baldock et al. (2004). We then apply the approach to simulate sediment transport and ripple bed morphology in oscillatory flow conditions corresponding to the experimental studies of Van der Werf et al. (2007). During the simulation, particles near ripple surface are isolated from immobile ones below allowing the computation to devote resources only to particles that may be become mobilized. Although preliminary in nature, the simulation results demonstrate that that the model can correctly capture the near bed velocities, suspended sediment concentrations, and pick-up of sediment by key vortical structures.
Highlights
In a coastal environment, sediment particles on the seabed surface will start to move once the shear force exerted by fluid exceeds the critical values
Motion of the sediment phase is computed by evaluating the sum of hydrodynamic and inter-particle forces and torques acting on individual Lagrangian particles
We have demonstrated the capabilities of a coupled Euler-Lagrange model for simulating the dynamics of full scale sand ripples in oscillatory flow
Summary
Sediment particles on the seabed surface will start to move once the shear force exerted by fluid exceeds the critical values Such motion often leads to complex fluid-particle interactions and results in various bed formations, such as vortex ripples, that are frequently found on the near-shore beach (Van Rijn et al, 1993). Typical approaches to simulate the bedform dynamics and sediment suspension are based on the shear force concept which was initially derived for sphere particle sitting on a plane bed under steady current Both experimental and theoretical studies have revealed that within the wave boundary layer over a rippled bed, strong vortex shedding dominants the particle entrainment processes, which is very different from that in a plane bed conditions We present a novel approach to isolate and simulate only these grains as the ripple evolves, reducing the computational cost by roughly an order of magnitude for a full scale ripple simulation
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