14,498 research outputs found
Dynamic simulations of water at constant chemical potential
The grand molecular dynamics (GMD) method has been extended and applied to examine the density dependence of the chemical potential of a three-site water model. The method couples a classical system to a chemical potential reservoir of particles via an ansatz Lagrangian. Equilibrium properties such as structure and thermodynamics, as well as dynamic properties such as time correlations and diffusion constants, in open systems at a constant chemical potential, are preserved with this method. The average number of molecules converges in a reasonable amount of computational effort and provides a way to estimate the chemical potential of a given model force field
Vlasov versus N-body: the H\'enon sphere
We perform a detailed comparison of the phase-space density traced by the
particle distribution in Gadget simulations to the result obtained with a
spherical Vlasov solver using the splitting algorithm. The systems considered
are apodized H\'enon spheres with two values of the virial ratio, R ~ 0.1 and
0.5. After checking that spherical symmetry is well preserved by the N-body
simulations, visual and quantitative comparisons are performed. In particular
we introduce new statistics, correlators and entropic estimators, based on the
likelihood of whether N-body simulations actually trace randomly the Vlasov
phase-space density. When taking into account the limits of both the N-body and
the Vlasov codes, namely collective effects due to the particle shot noise in
the first case and diffusion and possible nonlinear instabilities due to finite
resolution of the phase-space grid in the second case, we find a spectacular
agreement between both methods, even in regions of phase-space where nontrivial
physical instabilities develop. However, in the colder case, R=0.1, it was not
possible to prove actual numerical convergence of the N-body results after a
number of dynamical times, even with N=10 particles.Comment: 19 pages, 11 figures, MNRAS, in pres
Non-diffusive transport in plasma turbulence: a fractional diffusion approach
Numerical evidence of non-diffusive transport in three-dimensional, resistive
pressure-gradient-driven plasma turbulence is presented. It is shown that the
probability density function (pdf) of test particles' radial displacements is
strongly non-Gaussian and exhibits algebraic decaying tails. To model these
results we propose a macroscopic transport model for the pdf based on the use
of fractional derivatives in space and time, that incorporate in a unified way
space-time non-locality (non-Fickian transport), non-Gaussianity, and
non-diffusive scaling. The fractional diffusion model reproduces the shape, and
space-time scaling of the non-Gaussian pdf of turbulent transport calculations.
The model also reproduces the observed super-diffusive scaling
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