Abstract:
Collective locomotion in nature is ubiquitous. Extensive studies in fluids have found that
organized motion of multiple objects yield a reduction in drag force, and consequently, an
increased efficiency. This characteristic is often utilized in engineering applications. It has been
observed in nature that reptiles move in a formation, in sand. Hence, the current research
investigates whether a similar phenomenon is present in granular media. Granular drag force for a
single intruder has been thoroughly studied. However, for horizontal movement of multiple
intruders, nearly no experimentation or simulations have been carried out. In this study, granular
drag force on two spherical intruders is investigated using Discrete Element Method (DEM), for
two different configurations, i.e. in-line and side-by-side. DEM is a particle-scale numerical
method for modeling the bulk behavior of granular materials. The inter-intruder distance and
Froude number ( = / ), where and are intruder velocity and radius, respectively and
is gravitational acceleration) were varied and corresponding drag forces were evaluated and
compared to the drag force encountered by a single intruder. The horizontal drag force on the
trailing intruder for in-line configuration decreased by 41.07 to 60.64 percent. The phenomenon
contributing to this decrease was, decrease in packing fraction in upstream region of the trailing
intruder. Similarly, the average horizontal drag force on side-by-side intruders also decreased by
7.73 to 13.23 percent. This decrease was associated to the cooperative dynamics of the intruders.
There was an increase in average kinetic energy of particles, upstream of the intruders which
confirmed the collaborative nature of side-by-side motion. Additionally, the compression of
particles between the side-by-side intruders produced a net outwards lateral force on the intruders.
Furthermore, for a partial two-way coupled system in which the intruder was allowed to rotate
under the influence of particles. In this case, a consistent 5 to 7 percent decrease in drag force was
detected, explained by the rupture of force chains near the intruder due to its rotation.
