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Impact cratering with yield-stress fluids


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Authors

Ioannou, Georgia 

Abstract

Impact cratering is the process where a moving object hits a deformable target, causing material to be ejected away from the impact point, at least most of the times, and opening a crater on the target surface. This process has been studied extensively to understand the dynamics of planetary impact cratering and other similar natural or industrial processes. Most of the relevant experimental works involve water and granular media, and very few yield-stress fluids like soft materials. Except for the common experiment of a water drop impacting a water pool, in most works the impactor is a solid, non-deformable sphere. However, in the relevant geological process, and in most other applications, both target and impactor are deformable. Also, the material used in lab experiments to mimic the relevant geological process must have rheological properties that allow for it to hold a shape at the end of the process so that the resulted crater does not vanish. Ideal experimental materials are the yield-stress fluids, which behave as solids when low stresses are applied, but deform as fluids when the applied stresses exceed a threshold value.

In this work, we conduct impact cratering experiments with a yield-stress fluid as both target and impactor. We explore many aspects of the time-dependent features of this highly transient process by recording the dynamics with high-speed cameras. The transient features we study are the transient cavity (air-gel interface) dimensions and shape, the spreading of the drop material upon impact, and the duration of the cavity growth. The dynamics of this transient process are considered using an energy balance. We find that only a small percentage of the impactor kinetic energy is converted into potential energy of the cavity, unlike Newtonian fluids. Here, most of the impactor kinetic energy is converted into elastic energy stored in the material.

A particle tracking method is employed to visualise the response of the target material upon impact. Interestingly, the cavity does not grow radially as a hemi-sphere, like in Newtonian fluids, but growth is faster in horizontal than in vertical direction. Additionally, growth in vertical direction ceases before that in the horizontal direction. After the crater is formed, the target material undergoes a damped oscillation for a time period 50 times greater than the duration of cavity growth. We explore the dependence of the period of oscillation on material properties and examine whether the material oscillates in phase everywhere in the target.

Our study of the transient features expands to the ejecta sheet that emerges from the target, which is primarily material expelled from the point of impact. We perform a qualitative study of sheet shapes, categorising the ejecta into regimes according to the instabilities that arise at the edge of the sheet. These regimes are determined by a single dimensionless number that compares the inertial stresses to the dissipative stresses of the flow. Additionally, we study the dimensions and shape of the ejecta sheet and how these quantities evolve with time and compare our findings with the ejecta emerging from water and granular impact cratering.

When the transient part of the process finishes, a final crater that has a static shape in time is formed on the surface of the target. Using laser profilometry, we acquire the three-dimensional shape of the crater formed from which we categorise the different morphological regimes and examine how the final dimensions of the crater are related to its transient conformation. Moreover, we compare the size and shape of our craters with those reported in the literature when the target is a granular bed or a planetary body.

We augment our experimental study of impact cratering with simulations that imitate the laboratory experiments. For the simulations we use OpenFOAM, an open-source software package, and investigate various constitutive models for non-Newtonian fluids. Only the cavity growth stage is studied, when the flow is presumed to be stable and axisymmetric. The size and shape of the transient cavity for the different models are compared with each other and with the experimental results.

We conclude with a summary of our findings and a discussion of future directions of research.

Description

Date

2023-09-30

Advisors

Dalziel, Stuart
Neufeld, Jerome

Keywords

drop, drop impact, drop impact dynamics, impact cratering, non-Newtonian fluids

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge