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Soutenance de Charles-Édouard Lecomte

Instabilities in a granular material: washboard road and segregation into rubble-pile asteroids
When Jul 13, 2018
from 02:00 to 04:00
Where Amphi A
Attendees Charles-Édouard Lecomte
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This thesis, consisting of two parts, is focusing on instabilities into a granular assembly.

The first part deals with washboard road instability, which is the growth of a ripple pattern on a track subjected to repeated passages of vehicles. Our experimental work as well as soft spheres numerical simulations provide us a better understanding of this phenomenon. In the case of a rigid wheel dragged at constant velocity, we quantitatively studied the impact of the cohesion into the granular media on the main features of the instability: critical velocity, wavelength and growth rates. Finally, we have begun preliminary studies on a yield stress fluid, namely a carbopol microgel.

The second part is devoted to the granular segregation in asteroids. Several keys show that a significant part of them are rubble-piles, without internal cohesion and held together by their weak self-gravity. The distribution of grains is heterogeneous: while some regions consist in fine sand or powder, large boulders seem to accumulate in other parts. We numerically simulated a granular pile and subjected it to repeated quakes: we managed to reproduce segregation. We studied the influence of physical and numerical parameters on the segregation level and the dynamics of the phenomenon. We also investigate various causes of the segregation: granular convection, kinetic sieving, depletion pressure, etc. Finally, we highlighted an azimuthal segregation which can explain observations during spatial missions.