Impact and Shocks in Granular Material
Understanding the force exerted on a projectile upon penetration into a granular material is both key to a wide range of applications, and also a topic of scientific controversy: current attempts at formulating "unified granular drag laws" vary widely in form and parameterization, and are frequently only applicable in a specific experimental context. In our group we are seeking to understand the underlying physical processes involved in impact for a wide range of speeds by combining 3D molecular dynamics simulations with constant-speed penetration experiments.
These simulations allow us to probe the internal, grain-scale dynamics of the impact process that are extremely challenging to probe experimentally, and also reveal new features that occur at previously inaccessible timescales. Our results indicate that competing time- and depth-dependent effects combine to produce the wide range of observed behavior across velocity regimes, and indicate the need for a new approach to thinking about granular impact. The results of this research could provoke a fundamental shift in how projectile impact in granular material is modeled and provide predictions for the effect of projectile shape.
Though a pervasive and practical issue, research into the nature of shock waves in granular material is still poorly understood. Among the most critical questions are: What sets the speed of the shock front? What is the mechanism for the shock front propagation? and, What effect does the presence of a shock have on the force felt by an intruding object? Though an analytical solution exists in 1D, our preliminary analysis has shown that the situation in higher dimensions is much different and does not conform to the expected patterns.
Minimally jammed systems provide the best basis in which to develop a fundamental understanding of shocks by minimizing the effect of a preexisting stress network. Experimental systems – due to friction between grains and boundaries, complex grain-to- grain contact forces, and the presence of gravity – are currently too noisy and inhomogeneous to be capable of yielding sufficiently sensitive measurements of shocks. We have developed simulations in both two and three dimensions that approach the minimal jamming limit and have produced surprising and useful results on shock propagation that already provide insight into previously opaque experimental results.