Showing papers by "Paul Sánchez published in 2017"

Rolling resistance of a spherical pod on a granular bed

Abstract: The motion of a spherical ball rolling without slipping or plowing on a granular bed is studied. We propose a definition of the rolling resistance force and torque, and carry out experiments with a basketball and medicine ball rolling on a bed of gravel to measure the corresponding rolling resistance coefficients. These experiments reveal, in good agreement with literature, coefficients that are velocity-independent, and show that little to no plowing of the spheres into the granular substrate occurs. This indicates a regime of motion distinctly different from those treated in previous works. A simplified model correctly predicts the velocity independence and suggests an inverse dependence of the rolling resistance coefficient on the reduced inertia of the ball. These predictions match the experimentally observed behavior. Numerical simulations based on soft-sphere DEM shed more light on the mechanics of energy dissipation that occur in this no-plowing regime, and reveal a mass dependency that is not captured by the model. Our results provide insight into an unstudied regime of motion, and are of interest to the mission design of spacecraft to explore the surfaces of asteroids and comets.

Looking into the evolution of granular asteroids in the Solar System

Abstract: By now it has been accepted that most of the small asteroids in the Solar System are granular aggregates kept together by gravitational and possibly, cohesive forces. These aggregates can form, deform and disrupt over millennia subjected to different internal and external factors that would ultimately determine how they evolve over time. Parameters such as porosity, cohesive and tensile strength, angles of friction, particle size distributions, stress states, heterogeneity and yield criteria among others, determine how these granular systems will react when subjected to different, changing, external factors. These external factors include solar photon momentum, gravitational tides, micro- and macro-impacts and are believed to have produced and shaped the current asteroid population. In our research we use a combination of Soil Mechanics theory, Soft-Sphere Discrete Element Method (SSDEM) Simulations and Orbital Mechanics in order to understand how simulated, homogeneous and heterogeneous, ellipsoidal and spherical gravitational aggregates, a crude but useful representation of an asteroid, evolve when rotated to the point of disruption. Then, we compare our results to the shapes of observed asteroids as well as to the disruption patterns of a few active asteroids. Our results lead us to believe that the different shapes of observed asteroids as well as their unique disruption patterns could give us clues about their internal structure, strength and geophysical properties in general.

Small solar system bodies as granular systems

Abstract: Asteroids and other Small Solar System Bodies (SSSBs) are currently of great scientific and even industrial interest. Asteroids exist as the permanent record of the formation of the Solar System and therefore hold many clues to its understanding as a whole, as well as insights into the formation of planetary bodies. Additionally, SSSBs are being investigated in the context of impact risks for the Earth, space situational awareness and their possible industrial exploitation (asteroid mining). In all these aspects, the knowledge of the geophysical characteristics of SSSB surface and internal structure are of great importance. Given their size, constitution, and the evidence that many SSSBs are not simple monoliths, these bodies should be studied and modelled as self-gravitating granular systems in general, or as granular systems in micro-gravity environments in particular contexts. As such, the study of the geophysical characteristics of SSSBs is a multi-disciplinary effort that lies at the crossroads between Granular Mechanics, Celestial Mechanics, Soil Mechanics, Aerospace Engineering and Computer Sciences.