Penetration of Rigid Rods, Flexible Rods, and Granular Jets into Low-Density Granular Media

TL;DR

This study investigates how rigid rods, flexible rods, and granular jets penetrate low-density granular media, revealing differences in deviation, rotation, buckling, and penetration depth through experiments and simulations.

Contribution

It provides new insights into the dynamics of rod penetration and the effects of flexibility and length, contrasting with spherical projectile behavior.

Findings

Rigid rods penetrate deeper before deviation.

Flexible rods experience buckling and lose vertical alignment.

Granular arrays stop vertically and transfer momentum during collisions.

Abstract

The penetration of projectiles into granular materials has been mainly studied using spherical intruders. Here we explore the dynamics of rods penetrating vertically in a two-dimensional granular bed composed of expanded polystyrene spheres. The experiments were performed using rigid rods, flexible rods and vertical arrays of non-cohesive particles, and the dynamics for the three cases was compared. In contrast to the vertical penetration observed for a single spherical projectile, high speed videos reveal that a rod rapidly deviates from its initial vertical direction due to inhomogeneities of the bed packing fraction. Then, the rod rotates due to the torque induced by the resistance force and follows a curved trajectory until be aligned horizontally at a final depth. A short rod tends to deviate faster than a longer rod due to the smaller moment of inertia. Moreover, long flexible rods always lose their vertical alignment and experience buckling, whereas rigid rods of the same size penetrate deeper before being deviated. On the other hand, experiments and molecular dynamics simulations show that a initially vertical array of grains also loses its verticality and stops adopting a final horizontal configuration. The granular array penetrates considerably less than the rods of equivalent mass, and the stopping mechanism is based on vertical-to-horizontal momentum transfer during a collisional process of the constituting particles.