(547a) Simulations of Granular Bed Erosion Due to Laminar Shear Flow

Simulations of granular bed erosion due to laminar shear
flow

Sediment
transport and erosion of granular beds occurs in natural (river beds, dune
formation) as well as in engineered (dredging, sand issues in oil production)
systems. The incipient erosion, i.e. the minimal flow conditions that bring
about motion of solids in the upper layer of the granular bed is characterized
by a critical value of the Shields number (the dimensionless Shields number is
the ratio of viscous forces at the bed surface and net gravity on the grains
forming the bed). The critical shields number depends on the granular bed
characteristics (topology, particle size (distribution) etc.) and on the flow
above the bed. We report direct simulations on the initial stages of granular
bed erosion where the bed is made of uniformly sized spherical particles, and
the flow above the bed is a laminar simple shear flow. The simulations make use
of the lattice-Boltzmann method (LBM) for simulating the flow above the bed and
in between the solid particles. We largely resolve the solid-liquid interfaces
and the solid-liquid interaction by means of an immersed boundary method. The
spheres undergo hard collisions with one another. Lubrication forces are
explicitly added when solid surfaces are in such close proximity that the LBM
cannot resolve hydrodynamic interactions anymore. The results provide detailed
insight into the granular bed erosion process, including information regarding
hydrodynamic forces acting on the particles (their averages and fluctuation
levels), and solid and liquid velocity profiles in and closely above the bed. The
figure shows sample results for three values of the Shields number (0.1, 0.2, 0.4 from left to right) that are around the critical value.
In the figure the spheres are coloured by their absolute velocity.