261 research outputs found
Path Integral Monte Carlo Simulation of the Low-Density Hydrogen Plasma
Restricted path integral Monte Carlo simulations are used to calculate the
equilibrium properties of hydrogen in the density and temperature range of
and . We test the accuracy of the pair density matrix and
analyze the dependence on the system size, on the time step of the path
integral and on the type of nodal surface. We calculate the equation of state
and compare with other models for hydrogen valid in this regime. Further, we
characterize the state of hydrogen and describe the changes from a plasma to an
atomic and molecular liquid by analyzing the pair correlation functions and
estimating the number of atoms and molecules present.Comment: 12 pages, 21 figures, submitted for Phys. Rev.
All-Electron Path Integral Monte Carlo Simulations of Warm Dense Matter: Application to Water and Carbon Plasmas
We develop an all-electron path integral Monte Carlo (PIMC) method with
free-particle nodes for warm dense matter and apply it to water and carbon
plasmas. We thereby extend PIMC studies beyond hydrogen and helium to elements
with core electrons. PIMC pressures, internal energies, and pair-correlation
functions compare well with density functional theory molecular dynamics
(DFT-MD) at temperatures of (2.5-7.5) K and both methods together
form a coherent equation of state (EOS) over a density-temperature range of
3--12 g/cm and 10--10 K
Numerical Investigation of the Influence of Span-wise Force Variation in Circular Cylinders Undergoing Vortex Induced Vibrations at High Reynolds Number
The focus of this research is on the development
of a new approach for simulating vortex induced vibrations on
marine risers at high Reynolds numbers. This method considers
the span-wise variation of the lift and drag forces, and determines
the moment acting on the cylinder. The predicted motion then
consists of a rotational component to accompany the traditional
cross-stream and stream-wise translations normally associated
with vortex induced vibrations. This was accomplished by describing
the motion of the cylinder using a set of springs and
dampers. A moment acting on the cylinder causes the springs on
one end to compress, and stretch on the other, thus rotating the
cylinder.
A Large Eddy Simulation (LES) computational fluid dynamics
code running on 16 3Ghz processors was used to calculate the
unsteady flow and at each time step the hydrodynamic forces
acting on the cylinder were calculated in a separate routine based
on the pressure distribution around the cylinder. This information
was then used to solve two second-order ordinary differential
equations, which gave the velocity and displacement of the
cylinder in cross-flow and rotational planes. This information was
transferred back to the code where the cylinder was displaced
and another cycle of calculations was started.
The simulated results showed that the correlation length was
higher for a cylinder subject to pure translation compared to a
cylinder free to translate and rotate in the cross-stream direction.
This has implications for current numerical and experimental
techniques since it has been traditionally assumed that the
flow around a circular cylinder becomes two-dimensional during
vortex induced vibrations. Consequently, empirical,numerical
and experimental models have generally only considered cross
stream and/or stream-wise translation. The extent to which the
experimental apparatus or harmonic model may have influenced
the behavior of the riser by eliminating span-wise amplitude
variation is important information that should be considered for
future riser designs
Equations of state and stability of MgSiO perovskite and post-perovskite phases from quantum Monte Carlo simulations
We have performed quantum Monte Carlo (QMC) simulations and density
functional theory (DFT) calculations to study the equations of state of
MgSiO perovskite (Pv) and post-perovskite (PPv), up to the pressure and
temperature conditions of the base of Earth's lower mantle. The ground state
energies were derived using QMC and the temperature dependent Helmholtz free
energies were calculated within the quasi-harmonic approximation and density
functional perturbation theory. The equations of state for both phases of
MgSiO agree well with experiments, and better than those from generalized
gradient approximation (GGA) calculations. The Pv-PPv phase boundary calculated
from our QMC equations of states is also consistent with experiments, and
better than previous LDA calculations. We discuss the implications for double
crossing of the Pv-PPv boundary in the Earth
Variational Density Matrix Method for Warm Condensed Matter and Application to Dense Hydrogen
A new variational principle for optimizing thermal density matrices is
introduced. As a first application, the variational many body density matrix is
written as a determinant of one body density matrices, which are approximated
by Gaussians with the mean, width and amplitude as variational parameters. The
method is illustrated for the particle in an external field problem, the
hydrogen molecule and dense hydrogen where the molecular, the dissociated and
the plasma regime are described. Structural and thermodynamic properties
(energy, equation of state and shock Hugoniot) are presented.Comment: 26 pages, 13 figures. submitted to Phys. Rev. E, October 199
Rocky core solubility in Jupiter and giant exoplanets
Gas giants are believed to form by the accretion of hydrogen-helium gas
around an initial protocore of rock and ice. The question of whether the rocky
parts of the core dissolve into the fluid H-He layers following formation has
significant implications for planetary structure and evolution. Here we use ab
initio calculations to study rock solubility in fluid hydrogen, choosing MgO as
a representative example of planetary rocky materials, and find MgO to be
highly soluble in H for temperatures in excess of approximately 10000 K,
implying significant redistribution of rocky core material in Jupiter and
larger exoplanets
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Austenite to ferrite transformation kinetics during continuous cooling
The austenite decomposition has been investigated in a hypo-eutectoid plain carbon steel under continuous cooling conditions using a dilatometer and a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary systems Fe-C-Mn. The austenite to ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates. The formation of ferrite nuclei takes place with equilibrium composition on austenite grain boundaries. The nuclei are assumed to have a pill box shape in accordance with minimal interfacial energy. For higher cooling rates, early growth has to be taken into account to describe the transformation start. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium; i.e. only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute drag effect on the boundary movement. Thus, growth kinetics is controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements
The effect of differential rotation on Jupiter's low-degree even gravity moments
The close-by orbits of the ongoing Juno mission allow measuring with unprecedented accuracy Jupiter's low-degree even gravity moments J(2), J(4), J(6), and J(8). These can be used to better determine Jupiter's internal density profile and constrain its core mass. Yet the largest unknown on these gravity moments comes from the effect of differential rotation, which gives a degree of freedom unaccounted for by internal structure models. Here considering a wide range of possible internal flow structures and dynamical considerations, we provide upper bounds to the effect of dynamics (differential rotation) on the low-degree gravity moments. In light of the recent Juno gravity measurements and their small uncertainties, this allows differentiating between the various models suggested for Jupiter's internal structure.Israeli Ministry of Science; Minerva foundation; Federal German Ministry of Education and Research; Helen Kimmel Center for Planetary Science at the Weizmann Institute of Science; CNES; BSF; NSF; Juno project6 month embargo; Published Online: 19 June 2017This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
Path integral Monte Carlo simulation of charged particles in traps
This chapter is devoted to the computation of equilibrium (thermodynamic)
properties of quantum systems. In particular, we will be interested in the
situation where the interaction between particles is so strong that it cannot
be treated as a small perturbation. For weakly coupled systems many efficient
theoretical and computational techniques do exist. However, for strongly
interacting systems such as nonideal gases or plasmas, strongly correlated
electrons and so on, perturbation methods fail and alternative approaches are
needed. Among them, an extremely successful one is the Monte Carlo (MC) method
which we are going to consider in this chapter.Comment: 18 pages, based on talks on Hareaus school on computational methods,
Greifswald, September 200
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