Background
Common to all solid planetary bodies is surface regolith, unconsolidated material typically comprised of dust and broken rock fragments. On Earth, the regolith contains high amounts of organic material and biological weathering, and is subjected to effective aeolian and fluvial processes that serve to round individual grains and sort sediments by size. Less well understood is the behavior of regolith materials on airless bodies, where minimal erosional processes keep individual grains at a high degree of non-sphericity. Gully formations and fluvial-like features on the small asteroids Vesta and Helene have been attributed to several hypotheses, but none have explored the role of grain size and shape distribution on the dynamic and static behavior of the regolith on airless planetary bodies.
Understanding the effects of particle shape and size has significance beyond geology, including targeted drug delivery. The role of particle shape is typically ignored in applied problems due to computational limits and fundamental lack of knowledge on particle behavior. This project sought to use a novel new approach – a fluid dynamics method – to solve a geomechanical problem. Recent Institute advances in multi-phase lattice-Boltzmann numerical modeling, including simulating non-Newtonian fluids, suggests that 2D simulations of multiple densely-packed angular-shaped particles is not only feasible, but may have wide ranging modeling applications in both geological and biomedical fields.
Approach
The objectives of this project were twofold: (1) to understand how grain size and shape affect material properties such as cohesion, friction angle, and porosity, as well as dynamic behavior such as angle of repose and (2) improve existing Institute modeling capabilities of complex geologic and biologic problems by including arbitrary shape particles and Bingham fluid flow behavior. Physical model experiments established real-world geomechanical behavior to compare to numerical simulations.
Existing SwRI numerical models were translated to allow for computational optimization of the code, including increasing the number of modeled particles, size, shape, and fluid type. Upgrades to the lattice-Boltzmann modeling approach, including non-Newtonian viscosities such as dilatant, pseudoplastic, and viscoeleastic fluid flows, have been augmented with Bingham fluid capabilities, allowing for modeling of a wide range of conditions, from creeping (low Reynolds numbers) to moderately turbulent flow regimes.
Accomplishments
Experimental results of spherical and irregular shaped dust-to-sand sized particles demonstrated that particle shape has a significant influence on several geomechanical properties such as porosity and angle of repose. This project also successfully replicated non-spherical particle behavior in microfluidic environments for potential drug delivery simulations. We developed different particle shapes, and coded for ease of incorporating additional subroutines for more complicated particle geometries. We validated our new lattice-Boltzmann model against previously published simulation results with a circular and an ellipsoid particle in a Newtonian fluid, and then modeled the settling trajectories and velocities of particles of different shapes. This new modeling approach is currently resolving outstanding questions regarding the fundamental behavior of particle shapes in a wide variety of applications, including the space science and biomedical fields.