Drafting of intruders in dry and fluid-saturated granular beds

We discuss an experimental study of the drag experienced by two vertical rods as they follow each other around a circular track while being dragged across a granular bed. We systematically measure the drag experienced as a function of rod speed, separation distance, penetration depth, and the properties of the interstitial fluid. As in drafting in air and fluids, we find significant separation effects on the total drag experienced in comparison with that of a single rod [1]. The drag ratio is observed to decrease monotonically as the separation distance is decreased over a scale set by the penetration depth in a dry granular bed essentially independent of their speed. A complex rate-dependence is observed in a liquid-saturated granular bed at separation distances comparable to the penetration depth. A peak in drag is observed when the distance of separation is comparable to the penetration depth which is greater than the drag of experienced at large separation distance. We will discuss our results in light of resistive force theory, and nondimensional inertial and viscous numbers used to characterize granular mediums.

[1] Effective drag of a rod in fluid-saturated granular beds, Benjamin Allen and Arshad Kudrolli, Phys. Rev. E 100, 022901 (2019)