Drag and interactions of rigid bodies moving through submerged granular beds

We discuss experimental investigation of the forces encountered by a rod moving through a fluid-saturated granular medium to understand the dynamics of intruders and organisms in sedimentary beds at the bottom of lakes and oceans. By dragging vertically oriented rods through a granular bed of glass spheres, immersed in a fluid, we probe the observed transition from a quasi-static granular-like response to a viscous fluid-like behavior of the medium with speed. The relative importance of inertia, gravitational, and viscous forces is probed in terms of the dimensionless Stokes number, inertial number, and viscous number by varying the rod speed, rod depth, rod diameter, and the viscosity of the fluid. We find that the measured drag is best scaled with the integrated hydrostatic pressure along the rod and the Stokes number at low drag speeds corresponding to the quasi-static region. The transition between the quasi-static and fluid behavior scales with the viscous number, which is the ratio of viscous stress and gravity. We further discuss the interaction of two intruders as a function of the distance of separation compared with their diameter and length.