Numerical analysis of the drag on a rigid body in an immersed granular flow

Abstract

The drag exerted on an object moving in a granular medium is subject to many studies: in dry grains, it depends on the stress -often due to gravity -inside the grains. For Froude numbers less than 10, the drag is independent of the velocity. In immersed grains, it is proportional to the apparent weight of the grains and it depends on both velocity and fluid viscosity. In this paper, the drag on a cylinder in dry and immersed granular flow is simulated with a multi-scale FEM-DEM model. The Janssen stress saturation effect before the flow and velocity independence of the drag are both reproduced. The semi-circular orientation of the stress field supports the hypothesis of a spherical zone of influence of the cylinder. This orientation does not significantly change upon flowing. The results show the velocity independence of the drag is due to particle friction. In the immersed case, the fluid contribution is negligible on its own. A dimensional analysis suggests that shear thickening should be taken into account. The transition between the quasi-static and the fluidized regime is accompanied by a decrease of the stress field downstream of the cylinder and a change in the shape of the shear zone. KeywordsGranular drag • Janssen effect • Nonsmooth contact dynamics • Unresolved model • MigFlow B Nathan Coppin

Force on a sphere suspended in flowing granulate

“…Experiments measuring drag force on objects (spheres, cylinders, circular plates or cones) suspended in an emptying silo [46][47][48][49][50], that were confirmed in DEM simulations [49][50][51], found a velocity-independent force. In these cases, the grain-grain contacts are mobilized everywhere in the flowing material, unlike the above mentioned cases with intruders moving in a static granular bed.…”

Collapse dynamics of two-dimensional dry and immersed granular columns of elongated grains

“…Since the resolution of one contact can influence other contacts in which any of the two grains are implied, the FEM-DEM framework uses a queue for iterating over the contacts until a convergence criterion is met. This criterion corresponds to the change in the velocity correction from the contact resolution between two iterations of the contact solver, multiplied by the time step [15]. In the present work, its value was set to 10 −6 .…”