Jeremy B. Lechman

Evaporation of Lennard-Jones fluids

Evaporation and condensation at a liquid/vapor interface are ubiquitous interphase mass and energy transfer phenomena that are still not well understood. We have carried out large scale molecular dynamics simulations of Lennard-Jones (LJ) fluids composed of monomers, dimers, or trimers to investigate these processes with molecular detail. For LJ monomers in contact with a vacuum, the evaporation rate is found to be very high with significant evaporative cooling and an accompanying density gradient in the liquid domain near the liquid/vapor interface. Increasing the chain length to just dimers significantly reduces the evaporation rate. We confirm that mechanical equilibrium plays a key role in determining the evaporation rate and the density and temperature profiles across the liquid/vapor interface. The velocity distributions of evaporated molecules and the evaporation and condensation coefficients are measured and compared to the predictions of an existing model based on kinetic theory of gases. Our results indicate that for both monatomic and polyatomic molecules, the evaporation and condensation coefficients are equal when systems are not far from equilibrium and smaller than one, and decrease with increasing temperature. For the same reduced temperature T/T(c), where T(c) is the critical temperature, these two coefficients are higher for LJ dimers and trimers than for monomers, in contrast to the traditional viewpoint that they are close to unity for monatomic molecules and decrease for polyatomic molecules. Furthermore, data for the two coefficients collapse onto a master curve when plotted against a translational length ratio between the liquid and vapor phase.

Three-Dimensional Shear in Granular Flow

The evolution of granular shear flow is investigated as a function of height in a split-bottom Couette cell. Using particle tracking, magnetic-resonance imaging, and large-scale simulations, we find a transition in the nature of the shear as a characteristic height H* is exceeded. Below H* there is a central stationary core; above H* we observe the onset of additional axial shear associated with torsional failure. Radial and axial shear profiles are qualitatively different: the radial extent is wide and increases with height, while the axial width remains narrow and fixed.

Velocity correlations in dense gravity-driven granular chute flow

We report numerical results for velocity correlations in dense, gravity-driven granular flow down an inclined plane. For the grains on the surface layer, our results are consistent with experimental measurements reported by Pouliquen. We show that the correlation structure within planes parallel to the surface persists in the bulk. The two-point velocity correlation function exhibits exponential decay for small to intermediate values of the separation between spheres. The correlation lengths identified by exponential fits to the data show nontrivial dependence on the averaging time Deltat used to determine grain velocities. We discuss the correlation length dependence on averaging time, incline angle, pile height, depth of the layer, system size, and grain stiffness and relate the results to other length scales associated with the rheology of the system. We find that correlation lengths are typically quite small, of the order of a particle diameter, and increase approximately logarithmically with a minimum pile height for which flow is possible, hstop, contrary to the theoretical expectation of a proportional relationship between the two length scales.

Mesoscale hydrodynamics via stochastic rotation dynamics: Comparison with Lennard-Jones fluid

Stochastic rotation dynamics (SRD) is a relatively recent technique, closely related to lattice Boltzmann, for capturing hydrodynamic fluid flow at the mesoscale. The SRD method is based on simple constituent fluid particle interactions and dynamics. Here we parametrize the SRD fluid to provide a one to one match in the shear viscosity of a Lennard-Jones fluid and present viscosity measurements for a range of such parameters. We demonstrate how to apply the Müller-Plathe reverse perturbation method for determining the shear viscosity of the SRD fluid and discuss how finite system size and momentum exchange rates effect the measured viscosity. The implementation and performance of SRD in a parallel molecular dynamics code is also described.

Stresses in smooth flows of dense granular media

The form of the stress tensor is investigated in smooth, dense granular flows which are generated in split-bottom shear geometries. We find that, within a fluctuation fluidized spatial region, the form of the stress tensor is directly dictated by the flow field: The stress and strain-rate tensors are co-linear. The effective friction, defined as the ratio between shear and normal stresses acting on a shearing plane, is found not to be constant but to vary throughout the flowing zone. This variation cannot be explained by inertial effects, but appears to be set by the local geometry of the flow field. This is in agreement with a recent prediction, but in contrast with most models for slow grain flows, and points to there being a subtle mechanism that selects the flow profiles.

Liquid-vapor coexistence for nanoparticles of various size.

We present molecular dynamics simulations of the liquid-vapor phase coexistence of pure nanoparticle systems with three different model nanoparticle interactions. Our simulations show that the form of the interaction potential between nanoparticles strongly influences their coexistence behavior. For nanoparticles interacting with an integrated Lennard-Jones potential, the critical temperature and critical density increase with increasing particle size. In contrast, nanoparticles interacting via a Lennard-Jones potential shifted to the surface of the nanoparticle do not exhibit the expected size dependence of the phase diagram. For this model, the critical temperature decreases with increasing nanoparticle size. Similar results were observed for composite nanoparticles, with the interactions truncated at a finite distance.

Performance of mesoscale modeling methods for predicting rheological properties of charged polystyrene/water suspensions

We examine the accuracy and performance of several leading discrete-element-modeling approaches to predicting equilibrium and dynamic rheological properties of an aqueous, polystyrene suspension. What distinguishes each approach presented is the methodology of handling the solvent hydrodynamics. Specifically, we compare stochastic rotation dynamics (SRD), fast lubrication dynamics (FLD), and dissipative particle dynamics (DPD) methods of including solvent hydrodynamics against each other and against experimental data. Quantities examined are equilibrium structure properties (e.g., pair-distribution function), equilibrium dynamic properties (e.g., long-time diffusivities), and dynamic response (e.g., steady shear). In all approaches, we deploy the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential for colloid–colloid interactions. Comparisons are made over a range of volume fractions and salt concentrations. Long-time diffusivities are especially difficult to compute and exhibit clear discrepancies across m...

Multiscale models of nuclear waste reprocessing : from the mesoscale to the plant-scale.

Nuclear waste reprocessing and nonproliferation models are needed to support the renaissance in nuclear energy. This report summarizes an LDRD project to develop predictive capabilities to aid the next-generation nuclear fuel reprocessing, in SIERRA Mechanics, Sandia’s high performance computing multiphysics code suite and Cantera, an open source software product for thermodynamics and kinetic modeling. Much of the focus of the project has been to develop a moving conformal decomposition finite element method (CDFEM) method applicable to mass transport at the water/oil droplet interface that occurs in the turbulent emulsion of droplets within the contactor. Contactor-scale models were developed using SIERRA Mechanics turbulence modeling capability. Unit operations occur at the column-scale where many contactors are connected in series. Population balance models

Thermal conduction in particle packs via finite elements

Conductive transport in heterogeneous materials composed of discrete particles is a fundamental problem for a number of applications. While analytical results and rigorous bounds on effective conductivity in mono-sized particle dispersions are well established in the literature, the methods used to arrive at these results often fail when the average size of particle clusters becomes large (i.e., near the percolation transition where particle contact networks dominate the bulk conductivity). Our aim is to develop general, efficient numerical methods that would allow us to explore this behavior and compare to a recent microstructural description of conduction in this regime. To this end, we present a finite element analysis approach to modeling heat transfer in granular media with the goal of predicting effective bulk thermal conductivities of particle-based heterogeneous composites. Our approach is verified against theoretical predictions for random isotropic dispersions of mono-disperse particles at vario...

Theory and modeling of active brazing.

Active brazes have been used for many years to produce bonds between metal and ceramic objects. By including a relatively small of a reactive additive to the braze one seeks to improve the wetting and spreading behavior of the braze. The additive modifies the substrate, either by a chemical surface reaction or possibly by alloying. By its nature, the joining process with active brazes is a complex nonequilibrium non-steady state process that couples chemical reaction, reactant and product diffusion to the rheology and wetting behavior of the braze. Most of the these subprocesses are taking place in the interfacial region, most are difficult to access by experiment. To improve the control over the brazing process, one requires a better understanding of the melting of the active braze, rate of the chemical reaction, reactant and product diffusion rates, nonequilibrium composition-dependent surface tension as well as the viscosity. This report identifies ways in which modeling and theory can assist in improving our understanding.