The influence of gravity on granular impacts

Abstract

Context. Impacts on small-body surfaces can occur naturally during cratering events or even strategically during carefully planned impact experiments, sampling maneuvers, and landing attempts. A proper interpretation of impact dynamics allows for a better understanding of the physical properties and the dynamical process of their regolith-covered surfaces and their general evolution. Aims. This work aims to first validate low-velocity, low-gravity impact simulations against experimental results, and then to discuss the observed collision behaviors in terms of a popular phenomenological collision model and a commonly referenced scaling relationship. Methods. We performed simulations using the soft-sphere discrete element method and two different codes, Chrono and pkdgrav. The simulations consist of a 10-cm-diameter spherical projectile impacting a bed of approximately 1-cm-diameter glass beads at collision velocities up to 1 m s−1. The impact simulations and experiments were conducted under terrestrial and low-gravity conditions, and the experimental results were used to calibrate the simulation parameters. Results. Both Chrono and pkdgrav succeed in replicating the terrestrial gravity impact experiments with high and comparable computational performance, allowing us to simulate impacts in other gravity conditions with confidence. Low-gravity impact simulations with Chrono show that the penetration depth and collision duration both increase when the gravity level decreases. However, the presented collision model and scaling relationship fail to describe the projectile’s behavior over the full range of impact cases. Conclusions. The impact simulations reveal that the penetration depth is a more reliable metric than the peak acceleration for assessing collision behavior in a coarse-grained material. This observation is important to consider when analyzing lander-regolith interactions using the accelerometer data from small-body missions. The objective of future work will be to determine the correct form and applicability of the cited collision models for different impact velocity and gravity regimes.

The influence of gravity on granular impacts

“…Here, we investigate the general behavior of a spherical projectile impacting a bed of spherical grains at low speeds. In accordance with our previous experiments, we assume that the projectile comes to a rest without rebounding (Murdoch et al 2017(Murdoch et al , 2021Sunday et al 2021).…”

“…A detailed description of the code, the simulation setup, the boundary conditions, and the simulation parameters can be found in Sect. 2.2 of Sunday et al (2021).…”

“…The exact method and criteria that was used to identify a peak , z stop , and t stop can be found in Sect. 2.3 of Sunday et al (2021). Additional information regarding the filtering and treatment of the acceleration data for this particular study can be found in Appendix B.…”

“…2.2 and 2.3, we introduce a revised collision model, and we reformulate the model in terms of the dimensionless Froude number, which takes into account both impact velocity and gravity. Then, we extend the work of Sunday et al (2021) by performing impact simulations using the soft-sphere discrete element method (SSDEM) and the open-source code Chrono (Tasora et al 2016;Sunday et al 2020). In Sect.…”

Chrono::GPU: An Open-Source Simulation Package for Granular Dynamics Using the Discrete Element Method

“…To further validate the collision behavior of impact tests, quantities including the peak acceleration of the projectile, penetration depth, and collision time were evaluated and compared against experimental and numerical results [71,72]. Note that the simulation herein used monodisperse particles, whereas polydisperse ones were employed in [72]. A projectile of mass 1 kg was dropped into a cylinder of settled granular material.…”