Granular dynamics in auger sampling

TL;DR

This study investigates the granular flow dynamics in auger drilling, combining experiments, simulations, and a continuum model to predict sampling efficiency and its dependence on operational parameters, with implications for space exploration.

Contribution

It introduces a continuum model for granular flow in auger drilling and reveals the linear decrease of sampling efficiency with rotation to penetration speed ratio, independent of gravity.

Findings

Sampling efficiency decreases linearly with rotation to penetration speed ratio.

The steady state of granular flow is characterized by constant flow speeds in both channels.

Sampling efficiency is independent of gravity in the steady state.

Abstract

From geotechnical applications to space exploration, auger drilling is often used as a standard tool for soil sample collection, instrument installation, and others. Focusing on granular flow associated with the rotary drilling process, we investigate the performance of auger drilling in terms of sampling efficiency, defined as the mass ratio of the soil sample collected in the coring tube to its total volume at a given penetration depth, by means of experiments, numerical simulations, as well as theoretical analysis. The ratio of rotation to penetration speed is found to play a crucial role in the sampling process. A continuum model for the coupled granular flow in both coring and discharging channels is proposed to elucidate the physical mechanism behind the sampling process. Supported by a comparison to experimental results, the continuum model provides a practical way to predict the performance of auger drilling. Further analysis reveals that the drilling process approaches a steady state with constant granular flow speeds in both channels. In the steady state, sampling efficiency decreases linearly with the growth of the rotation to penetration speed ratio, which can be well captured by the analytical solution of the model. The analytical solution also suggests that the sampling efficiency is independent of gravity in the steady state, which has profound implications for extraterrestrial sample collection in future space missions.