Current research interests
I am currenty part of the US Department of Energy's Hydrogen Storage Engineering Center of Excellence (HSECoE), which focuses on designing systems to store hydrogen for light-duty vehicles that can satisfy DOE's targets (introduction, explanation of the targets).
The HSECoE's charter is to focus on materials-based storage, i.e., the hydrogen will be stored inside a material, absorbed either chemically or physically. The variety of material types makes the system designs very different. Some operate at cryogenic temperatures, others at room temperature, and yet others at high temperature. Some release heat when releasing hydrogen while others absorb it. Some can be refueled with hydrogen gas at the station while others need to be sent to a central plant to chemically recharge the material with hydrogen.
In order to decide which systems are worth building we use models, with the system embedded in a model of a fuel cell vehicle. But comparing models of systems that are so different can be difficult: how do we ensure that this is an apples-to-apples comparison? At the HSECoE we developed a simulation framework (paper) that allows us to compare models of these systems while explicitly keeping other assumptions for the fuel cell vehicle and driving conditions constant. The development of this framework earned us the 2012 DOE Hydrogen and Fuel Cells Program R&D Award.
Fuel cell modeling
At UTRC I've worked on modeling fuel cell performance, water management, as well as chemical and mechanical degradation.
Uncertainty quantification (UQ)
UQ for hybrid systems
In collaboration with Tuhin Sahai.
UQ of systems with a large number of interacting agents
This work was part of DARPA's Robust Uncertainty Management program.
Past research interests
Aeolian sediment transport and dune evolution
Modeling of steady aeolian transport
Study of granular boundary conditions
In collaboration with the Groupe Matière Condensée et Matériaux at the University of Rennes 1 in France.
Fluid dynamics, sediment transport, and scour
In collaboration with Philip Liu.
Kinetic theory for granular media
In collaboration with the Molecular Dynamics & Kinetic Theory Group at the University of Chile.
Detail of the velocity distribution function for a 1–d granular gas between two walls at different temperatures. This is a numerical solution of the Boltzmann equation in the limit of infinite number of particles N, but making the system increasingly more elastic, such that the product of N and the inelasticity q is kept constant. This is conceptually similar to the Boltzmann-Grad limit, if we replace the mean free path (which cannot be tuned while keeping the density constant, since the concept of cross-section breaks down in 1–d) with the persistence length for the velocity of a test particle.
Note that the distribution is discontinuous, and the jump is larger at the colder wall (x = 0). The onset of clustering is given by the moment when the slope df/dx at (x,c) = (0,0) becomes infinite.
The distribution f(x,c) is the solution to the following nonlinear integrodifferential problem:
In the case shown, T1 = 0.5, T2 = 2, and qN = 0.35.
Force networks and wave propagation in granular packings
Equilibrium force network for a hexagonal packing of frictionless polydisperse spheres under anisotropic compression. The spheres interact through Hertzian contacts, and the thickness of each line is proportional to the contact force.
Elasticity of thin sheets and draping
In collaboration with Enrique Cerda and L. Mahadevan.
A circular inextensible elastic sheet hung from its center may take the shape of a cylindrical surface or a developable cone (d-cone) with one or more folds, depending on its size. The d-cone shown here corresponds to the solution for a small tablecloth. This solution will compete with the cylindrical one, and for it to be realized it must have lower energy.