4,808 research outputs found
2D and 3D Dense-Fluid Shear Flows via Nonequilibrium Molecular Dynamics. Comparison of Time-and-Space-Averaged Tensor Temperature and Normal Stresses from Doll's, Sllod, and Boundary-Driven Shear Algorithms
Homogeneous shear flows (with constant strainrate du/dy) are generated with
the Doll's and Sllod algorithms and compared to corresponding inhomogeneous
boundary-driven flows. We use one-, two-, and three-dimensional smooth-particle
weight functions for computing instantaneous spatial averages. The nonlinear
stress differences are small, but significant, in both two and three space
dimensions. In homogeneous systems the sign and magnitude of the shearplane
stress difference, P(xx) - P(yy), depend on both the thermostat type and the
chosen shearflow algorithm. The Doll's and Sllod algorithms predict opposite
signs for this stress difference, with the Sllod approach definitely wrong, but
somewhat closer to the (boundary-driven) truth. Neither of the homogeneous
shear algorithms predicts the correct ordering of the kinetic temperatures,
T(xx) > T(zz) > T(yy).Comment: 34 pages with 12 figures, under consideration by Physical Review
Nonequilibrium Temperature and Thermometry in Heat-Conducting Phi-4 Models
We analyze temperature and thermometry for simple nonequilibrium
heat-conducting models. We show in detail, for both two- and three-dimensional
systems, that the ideal gas thermometer corresponds to the concept of a local
instantaneous mechanical kinetic temperature. For the Phi-4 models investigated
here the mechanical temperature closely approximates the local thermodynamic
equilibrium temperature. There is a significant difference between kinetic
temperature and the nonlocal configurational temperature. Neither obeys the
predictions of extended irreversible thermodynamics. Overall, we find that
kinetic temperature, as modeled and imposed by the Nos\'e-Hoover thermostats
developed in 1984, provides the simplest means for simulating, analyzing, and
understanding nonequilibrium heat flows.Comment: 20 pages with six figures, revised following review at Physical
Review
Microscopic and Macroscopic Stress with Gravitational and Rotational Forces
Many recent papers have questioned Irving and Kirkwood's atomistic expression
for stress. In Irving and Kirkwood's approach both interatomic forces and
atomic velocities contribute to stress. It is the velocity-dependent part that
has been disputed. To help clarify this situation we investigate [1] a fluid in
a gravitational field and [2] a steadily rotating solid. For both problems we
choose conditions where the two stress contributions, potential and kinetic,
are significant. The analytic force-balance solutions of both these problems
agree very well with a smooth-particle interpretation of the atomistic
Irving-Kirkwood stress tensor.Comment: Fifteen pages with seven figures, revised according to referees'
suggestions at Physical Review E. See also Liu and Qiu's arXiv contribution
0810.080
Well-Posed Two-Temperature Constitutive Equations for Stable Dense Fluid Shockwaves using Molecular Dynamics and Generalizations of Navier-Stokes-Fourier Continuum Mechanics
Guided by molecular dynamics simulations, we generalize the
Navier-Stokes-Fourier constitutive equations and the continuum motion equations
to include both transverse and longitudinal temperatures. To do so we partition
the contributions of the heat transfer, the work done, and the heat flux vector
between the longitudinal and transverse temperatures. With shockwave boundary
conditions time-dependent solutions of these equations converge to give
stationary shockwave profiles. The profiles include anisotropic temperature and
can be fitted to molecular dynamics results, demonstrating the utility and
simplicity of a two-temperature description of far-from-equilibrium states.Comment: 19 pages with 10 figures, revised following review at Physical Review
E and with additional figure/discussion, for presentation at the
International Summer School and Conference "Advanced Problems in Mechanics"
[Saint Petersburg, Russia] 1-5 July 2010
Nonlinear Stresses and Temperatures in Transient Adiabatic and Shear Flows via Nonequilibrium Molecular Dynamics -- Three Definitions of Temperature
We compare nonlinear stresses and temperatures for adiabatic shear flows,
using up to 262,144 particles, with those from corresponding homogeneous and
inhomogeneous flows. Two varieties of kinetic temperature tensors are compared
to the configurational temperatures. This comparison leads to an improved form
for the local and instantaneous smooth-particle averaged stream velocity and to
a recognition of rotational contributions to the configurational temperature.Comment: 16 pages, 8 figures, stimulated by Denis Evans' comments on Hoover et
alii, Physical Review E 78, 046701 (2008). Augmented 30 January 2009 in
response to referees' comments at Physical Review
Remarks on NonHamiltonian Statistical Mechanics: Lyapunov Exponents and Phase-Space Dimensionality Loss
The dissipation associated with nonequilibrium flow processes is reflected by
the formation of strange attractor distributions in phase space. The
information dimension of these attractors is less than that of the equilibrium
phase space, corresponding to the extreme rarity of nonequilibrium states. Here
we take advantage of a simple model for heat conduction to demonstrate that the
nonequilibrium dimensionality loss can definitely exceed the number of
phase-space dimensions required to thermostat an otherwise Hamiltonian system.Comment: 5 pages, 2 figures, minor typos correcte
Time-reversal focusing of an expanding soliton gas in disordered replicas
We investigate the properties of time reversibility of a soliton gas,
originating from a dispersive regularization of a shock wave, as it propagates
in a strongly disordered environment. An original approach combining
information measures and spin glass theory shows that time reversal focusing
occurs for different replicas of the disorder in forward and backward
propagation, provided the disorder varies on a length scale much shorter than
the width of the soliton constituents. The analysis is performed by starting
from a new class of reflectionless potentials, which describe the most general
form of an expanding soliton gas of the defocusing nonlinear Schroedinger
equation.Comment: 7 Pages, 6 Figure
Lyapunov instability for a periodic Lorentz gas thermostated by deterministic scattering
In recent work a deterministic and time-reversible boundary thermostat called
thermostating by deterministic scattering has been introduced for the periodic
Lorentz gas [Phys. Rev. Lett. {\bf 84}, 4268 (2000)]. Here we assess the
nonlinear properties of this new dynamical system by numerically calculating
its Lyapunov exponents. Based on a revised method for computing Lyapunov
exponents, which employs periodic orthonormalization with a constraint, we
present results for the Lyapunov exponents and related quantities in
equilibrium and nonequilibrium. Finally, we check whether we obtain the same
relations between quantities characterizing the microscopic chaotic dynamics
and quantities characterizing macroscopic transport as obtained for
conventional deterministic and time-reversible bulk thermostats.Comment: 18 pages (revtex), 7 figures (postscript
A Study of Activated Processes in Soft Sphere Glass
On the basis of long simulations of a binary mixture of soft spheres just
below the glass transition, we make an exploratory study of the activated
processes that contribute to the dynamics. We concentrate on statistical
measures of the size of the activated processes.Comment: 17 pages, 9 postscript figures with epsf, uses harvmac.te
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