Group stability of bed particles near the critical threshold of motion
Abstract
The unsteady flow above a rough bed and its interaction with a group of mobile spherical particles is investigated with Direct Numerical Simulations. The velocity and pressure are resolved at sub-particle scales using a new Cartesian grid method based on a discontinuous extension of the pressure Poisson equation across particle boundaries. The hydrodynamics is fully resolved everywhere except in the gap between colliding particles when the latter becomes smaller than the grid step. The particle hydrodynamic forces are determined as a combination of the numerically resolved pressure/shear outside the gap and an analytical contribution for the unresolved gap dynamics. Theoretical Stokes flow models are used to estimate the unresolved lubrication pressure/shear force in the subgrid gap. For the mechanical contact, we use a soft-sphere approach where the normal and tangential forces are modeled using a linear elastic-plastic law and a history dependent friction law, respectively. The proposed collision model is validated against experimental data for normal and oblique immersed collisions of spherical particles. We find that the lubrication corrections for the unresolved gap flow are essential to correctly predict the observed decrease in the coefficient of restitution with decreasing collisional Stokes number including the value of the critical Stokes number where collisions cease to rebound. The low collisional Stokes number effects are important for dissipating the momentum of flow-induced vibrations of surface particles. The results from our numerical simulations for the initiation of motion are compared with existing laboratory data.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP53G..03S
- Keywords:
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- 1862 HYDROLOGY / Sediment transport;
- 4217 OCEANOGRAPHY: GENERAL / Coastal processes;
- 4558 OCEANOGRAPHY: PHYSICAL / Sediment transport