Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D30: Nanofluids: Computations I |
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Chair: Nikolai Priezjev, Michigan State University Room: 33A |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D30.00001: Controlling Surface Roughness to Enhance Mass Flow Rates in Nanochannels Malgorzata Zimon, David Emerson, Jason Reese A very active field of research in fluid mechanics and material science is predicting the behavior of Newtonian fluids flowing over porous media with different wettabilities. Opposite effects have been observed: some state that wall roughness always suppresses fluid-slip, whereas others show that for some cases roughness may reduce the surface friction. In this work, MD simulations were carried out to further investigate physical mechanisms for liquid slip, and factors affecting it. A rough wall was formed by either periodically spaced rectangular protrusions or was represented by a cosine wave. The MD simulations were conducted to study Poiseuille and Couette flow of liquid argon in a nanochannel with hydrophilic kryptonian walls. The effect of wall roughness and interface wettability on the streaming velocity, and the slip-length at the walls, is observed to be significant. Our results show a dependency of mass flow rate on the type of flow and topography of the channel walls. For a fixed magnitude of the driving force, an increase in the mass flow rate, compared to the smooth surface, was observed for the wavy roughness, whereas the opposite effect was observed for Couette flow where a higher slip was obtained for rectangular gaps. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D30.00002: Properties of Water/Gold Nanofluids Gianluca Puliti, Samuel Paolucci, Mihir Sen Nanofluids are believed to have enhanced thermophysical properties and heat transfer performance. The novelty of this work is in a comprehensive approach to understanding nanofluids through the use of molecular dynamics simulations with accurate potentials to model realistic materials. Specifically, this study treats the case of water confined between gold nanolayers to examine interfacial interactions and a water-based nanofluid with spherical gold nanoparticles. Properties are discussed for both systems. While the thermodynamic properties of the mixture are typically predicted using ideal mixture theory, such predictions are found to be generally poor for nanofluids. The anisotropy induced by the gold-water interface, and its effects, appear to be responsible for the disagreement. Transport properties, show a much more peculiar trend: the presence of nanoparticles has no effect on self-diffusion while viscosity increases drastically with respect to the prediction of classical theories. It is interesting to note that the thermal conductivity of nanofluids is enhanced for low particle concentrations, but it is below the prediction of classical theories for higher volume fractions. Interfacial effects appear, once again, to be responsible for such trend in transport properties. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D30.00003: Nitrogen Flow in a Nanonozzle with Heat Addition Sergey Averkin, Zetian Zhang, Nikolaos Gatsonis The nitrogen flow in conical nanonozzles at atmospheric pressures are investigated using a three-dimensional unstructured direct simulation Monte Carlo (U3DSMC) method. The DSMC simulations are performed in computational domains that feature the plenum, the nanonozzle region and the external plume expansion region. The inlet and outlet boundaries are modeled by the Kinetic-Moment (KM) boundary conditions method. This methodology is based on the local one dimensional inviscid (LODI) formulation used in compressible (continuous) flow computations. The cross section for elastic collisions is based on the variable hard sphere (VHS) model. The Larsen--Borgnakke (L-B) model is used to simulate the exchange of the internal energy in the collision pair. Solid surfaces are modeled as being either diffuse or specularly reflecting. The effects of Knudsen number, aspect ratio, and nanonozzle scale on the heat transfer are investigating by ranging the throat diameters from 100-500 nm, exit diameter from 100-1000 nm, stagnation pressure from 1-10atm, and wall temperature from 300K-500K. Finite backpressure and vacuum conditions are considered. Macroscopic flow variables are obtained and compared with continuum predictions in order to elucidate the impacts of nanoscale. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D30.00004: Molecular Dynamics Simulation of Gas Separation using Nanoporous Graphene Harold Au, Michael Boutilier, Pietro Poesio, Nicolas Hadjiconstantinou, Rohit Karnik We present molecular dynamics simulations of gas transport through nanoporous graphene to evaluate the latter's potential as a gas-separation membrane. Due to their very small thickness, such membranes are expected to exhibit high permeance. Provided precise tuning of the pore sizes is possible, graphene membranes have the potential to combine high permeance with high selectivity through molecular size exclusion. In the present study, we focus on separation of methane from hydrogen. Our results show that graphene with pores that are large compared to the kinetic diameters of both species are permeable to both gases. As pore size is reduced, we observe a greater decrease in the permeance of methane that results in a size exclusion effect for a range of pore sizes that are still permeable to hydrogen. Our results indicate that hydrogen permeance over this range of pores sizes is sufficiently large to offer considerable improvement compared to state of the art polymeric membranes. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D30.00005: Polarization as a field variable from molecular dynamics simulations Kranthi K. Mandadapu, Jeremy Templeton, Jonathan Lee In this talk, we show that polarization density, an important quantity in electromagnetism, can be obtained from molecular dynamics simulations. We show that the Irving and Kirkwood procedure used for obtaining stresses and heat fluxes in terms of the microscopic quantities can be extended to the case of electrostatics where the macroscopic electrostatic equation can be derived starting with the microscopic electrostatic equation, microscopic density of charges and using a phase-space distribution function and a suitable localization function. As a result, we obtain an expression for polarization density as a field variable in terms of the microscopic dipole moments and quadrupole moments and higher order terms depending upon the degree of the polynomial used for the localization function. Finally, we apply this method to obtain the dielectric constant of bulk water and to study the polarization effects in electric double layer calculations. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D30.00006: Thermal Resistance and Temperature Jumps at Liquid/Solid Interfaces: Insights from Molecular Dynamics Simulations Sandra Troian At macroscale dimensions, it is normally assumed that two distinct materials maintain equal temperature across the surface of contact. Even in the presence of a thermal flux across the interface, the contacting boundary is assumed to maintain thermal equilibrium so long as the interfacial resistance is negligible in comparison to that of the bulk. This has long been assumed an excellent approximation for liquid/solid interfaces, since liquids will conform in shape even to roughened surfaces. Recent molecular dynamics simulations of nanoscale films, however, have revealed the existence of intrinsic temperature jumps at liquid/solid interfaces. While previous studies have shown how stronger interaction energy between the liquid and solid will diminish temperature jumps, they cannot be altogether eliminated due to commensurability mismatch. Here we show a non-local effect in which the magnitude of the thermal jump is controlled by the thermal flux in the bulk. This finding suggests that temperature jumps across a liquid/solid interface are not simply a local effect due to the density mismatch across the interface. These jumps are also controlled by the rate of heat transfer, underscoring the importance of thermal resistance effects in nanoscale hydrodynamic systems. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D30.00007: A multiscale method for modeling high-aspect-ratio micro/nano flows Duncan Lockerby, Matthew Borg, Jason Reese In this paper we present a new multiscale scheme for simulating micro/nano flows of high aspect ratio in the flow direction, e.g. within long ducts, tubes, or channels, of varying section. The scheme consists of applying a simple hydrodynamic description over the entire domain, and allocating micro sub-domains in very small ``slices'' of the channel. Every micro element is a molecular dynamics simulation (or other appropriate model, e.g., a direct simulation Monte Carlo method for micro-channel gas flows) over the local height of the channel/tube. The number of micro elements as well as their streamwise position is chosen to resolve the geometrical features of the macro channel. While there is no direct communication between individual micro elements, coupling occurs via an iterative imposition of mass and momentum-flux conservation on the macro scale. The greater the streamwise scale of the geometry, the more significant is the computational speed-up when compared to a full MD simulation. We test our new multiscale method on the case of a converging/diverging nanochannel conveying a simple Lennard-Jones liquid. We validate the results from our simulations by comparing them to a full MD simulation of the same test case. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D30.00008: Influence of Slip Boundary Conditions and Confinement on Molecular Diffusion in Nanochannels: A Molecular Dynamics Simulation Study Ali Kharazmi, Nikolai Priezjev We investigate the effects of confinement and slip boundary conditions on diffusion of solvent molecules in a nanochannel using the LAMMPS molecular dynamics program. In our simulations, the Lennard-Jones fluid is confined by crystalline substrates which allow fine adjustment of the slip length without changing the interfacial fluid structure. In the absence of flow, the molecular trajectories are used to compute the probability density of molecular displacements. The rate matrix that describes the time evolution of the probability density is then estimated by maximizing the likelihood function. Finally, the position-dependent diffusion coefficient is computed numerically from the Smoluchowski equation. We found that the local diffusion coefficient in the directions parallel and perpendicular to confining walls is a function of the distance from the confining walls and the degree of slip. These results are discussed in the context of nano-PIV (Particle Image Velocimetry) measurements of slip flows in nanochannels. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D30.00009: Molecular dynamics simulations of oscillatory Couette flows with slip boundary conditions Nikolai Priezjev The effect of interfacial slip on steady-state and time-periodic flows of monatomic liquids is investigated using non-equilibrium molecular dynamics simulations. The simulations were performed in a wide range of oscillation frequencies; namely, when the Stokes boundary layer thickness is smaller than the channel width at the highest frequency, and, on the other hand, at lower frequencies that correspond to quasi-steady flows. It was found that the velocity profiles computed in MD simulations are well described by the continuum solution with the slip length as a fitting parameter that depends on the local shear rate. Interestingly, the shear rate dependence of the slip length obtained in steady-state shear flows is reproduced in oscillatory flows when the slip length is measured as a function of the absolute value of the local shear rate. Finally, for both types of flows, the friction coefficient at the liquid-solid interface correlates well with the structure factor and the contact density of the first fluid layer. Financial support from the National Science Foundation (CBET-1033662) is gratefully acknowledged. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D30.00010: Effective Translational Diffusion of Nanorotors as Rotary Powered Random Walkers Amir Nourhani, Paul Lammert, Ali Borhan, Vincent Crespi The coupling of the orientational stochastic dynamics and rotary powered dynamics at different dimensions leads to an effective translational diffusion of a rotary powered random walker. In a conventional nanorotor system, moving in two-dimension close to a substrate, the one-dimensional orientational stochastic dynamics couples to the rotary deterministic motion and leads to an effective two-dimensional translational diffusion, which is chiral in short to medium time scales. If a nanorotor can have three-dimensional dynamics, an emergent three-dimensional effective diffusion would be the outcome of the coupling between three one-dimensional orientational stochastic processes and a two-dimensional deterministic rotation in the plane of motion. Such effective diffusion processes are a property of nanoscale where the deterministic and stochastic dynamics are both significant. [Preview Abstract] |
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