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Accueil > Actualités > Séminaires > Séminaires 2015

Jeudi 25 juin 2015 à 13h30 en salle K118

Justin Finn, Postdoc à l’Université de Liverpool

Titre/Title : Particle based modeling and simulation of natural sand dynamics in the wave bottom boundary layer

Contact : David Hurter (équipe MEIGE)

Résumé/Abstract :
Sand transport and morphological change occur in the wave bottom boundary layer due to particle interactions with an oscillatory flow and granular interactions between particles. Although these interactions depend strongly on characteristics of the particle population, i.e. size and shape, little is known about how natural sand particles behave under oscillatory flow conditions, and how variations in particle size influence transport behavior. To enable this to be studied numerically, a particle based ``Euler-Lagrange’’ model has been developed at the University of Liverpool. In the first part of this talk, I will discuss how the model has been designed specifically to capture the individual and collective dynamics of sub-aqueous natural sand grains. The rest of the talk will focus on two simulations of sand particle dynamics in asymmetric oscillatory flow conditions, corresponding to the vortex ripple experiments of Van der Werf et al. (J. Geophysical Research, vol 112, 2007) and the sheet flow experiments of O’Donoghue and Wright (Coastal Eng., vol 50, 2004). The particle-based datasets from the simulations have allowed us to investigate the spatio-temporal dynamics of the particle size distribution and the influence of three dimensional vortical features on particle entrainment and suspension processes. Even for the relatively well sorted sands considered, the characteristics of the local grain size population exhibit significant space-time variation during the flow cycle. Both conditions demonstrate a distinct coarse-over-fine armoring at the bed surface during low velocity phases, which restricts the vertical mobility of finer fractions in the bed, and also results in the strong pickup events involving disproportionately coarse fractions. The near bed layer composition is seen to be very dynamic in the sheet flow condition, while it remains coarse through most of the cycle in the vortex ripple condition. Particles in suspension spend more time sampling the upward directed parts of both flows, especially the smaller fractions, which delays particle settling and enhances the vertical size sorting of grains in suspension.

Figure Caption : Three dimensional visualizations of oscillatory boundary layer flows simulated using an Euler-Lagrange model, illustrating particle interactions with vortical structures. Individual particles are colored and rendered according to their size (red=large, blue=small), and instantaneous vortex cores are visualized as iso-surfaces of the swirling strength criteria.