Postdoctoral Researcher, UC Santa Cruz
Visiting Researcher, Naval Postgraduate School
Motivation
Granular materials are central to a wide range of earth science processes, and natural shear flows are composed of particles with a variety of irregular shapes, sizes, and material properties. For hazards involving granular media, like debris flows and earthquakes, it is important to understand how the complexity of natural grains affects the transition between nearly static slow moving shear flow regimes and dilatational fast moving shear flow regimes. Such an understanding will result in effective predictions of rheological behavior based on certain material characteristics and boundary conditions and will answer an enduring and fundamental physics question of how energy entering into a granular system is dissipated.
Laboratory Experiments
Experimental measurements are required to resolve how energy is transferred between work applied to a granular system, fluctuating kinetic energy and mean flow characteristics and rheology. I use acoustic energy to measure fluctuations in kinetic energy in sheared granular samples, and particle image velocimetry to observe velocity profiles at depth in the shear flow.
As granular material is sheared, grains bounce into and roll past one another, producing noise from collisions or frictional slip. Granular materials are sensitive to vibrations, and can rearrange, compact or dilate depending on vibrational amplitude and frequency. I seek to understand how noise and fluctuating kinetic energy in granular shear flows feeds back into the system to change its rheology.
Recent relevant publications:
Energy partitioning in granular flow depends on mineralogy via nano-scale plastic work, Journal of Geophysical Research: Solid Earth, 2019.
Granular temperature measured experimentally in a shear flow by acoustic energy, Physical Review E, 2017.
Discrete Element Method Models
Discrete element method models follow individual grains as they interact during shear. Simulations built with grains of different shape and surface friction properties can capture how vibrations and perturbations affect grain networks, and how these complex interactions change the bulk rheology of the flow.
TA Instruments AR2000ex torsional rheometer customized to measure vibration, stress, and thickness of granular sample during shear flow. Glass sample cell allows monitoring of velocity profiles and grain motion.
Results from 2D DEM simulations. Change in frictional resistance to flow at low velocity for disks and trimers in the presence of vibrations that produce a given acceleration. Varying the surface friction and shape of the particles in the shear flow changes how the flow responds to vibration.