A Eulerian Two-Phase Modeling of Ripple Dynamics

Ali Salimi Tarazouj, University of Delaware, Department of Civil and Environmental Engineering, Newark, United States, Zhen Cheng, Convergent Science, Madison, United States, Tian-Jian Hsu, University of Delaware, Civil, Construction, and Environmental Engineering, Newark, United States, Peter Traykovski, Woods Hole Oceanograph Inst, Falmouth, MA, United States and Julien Chauchat, University of Grenoble Alpes, LEGI, Grenoble, France
Abstract:
Oscillatory flow motions of surface wave near the seabed can generate various bedforms with different sizes and geometries. The importance of bedforms arises from their effect on the seabed roughness and wave-current driven onshore/offshore sediment transport. A key finding from field and laboratory observation on onshore/offshore sediment transport in orbital ripples suggests that under onshore velocity skewed waves, suspended load well above the ripple crest is offshore directed while onshore transport takes place via bedload and near-bed suspended load forced ripple migration. These complex processes associated with suspended load and ripple migration pose a challenge to conventional single-phased sediment transport models as they typically adopt major assumptions in modeling suspended load and bedload. This is study, we present a novel numerical model that is able to resolve sediment transport over ripple and ripple migration altogether in a Eulerian two-phase modeling framework, called SedFoam. With a two-equation k-epsilon closure on flow turbulence in two-phase flow, interphase drag and particle stresses, particularly regarding the fluid-like to solid-like behavior of the sediment bed, the model can resolve full profiles of sediment concentration and particle/fluid velocities. The model is validated with oscillating tunnel experiment of orbital ripple driven by a Stokes 2nd order wave motion by van der Werf et al. (2007, J. Geophys. Res.). The model is able to capture measured sediment concentration and velocity fields and a net offshore-directed suspended load transport flux due to the asymmetric intensity of vortices. More importantly, the model can simulate an onshore ripple migration speed that is similar to measured value. The model is then used to study the effect of wave orbital velocity amplitude and wave period (having the same wave orbital length) in determining the resulting ripple geometry and sediment transport due to varying degree of suspended load flux versus the bedload flux due to ripple migration.
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