2,255 research outputs found
Stratified horizontal flow in vertically vibrated granular layers
A layer of granular material on a vertically vibrating sawtooth-shaped base
exhibits horizontal flow whose speed and direction depend on the parameters
specifying the system in a complex manner. Discrete-particle simulations reveal
that the induced flow rate varies with height within the granular layer and
oppositely directed flows can occur at different levels. The behavior of the
overall flow is readily understood once this novel feature is taken into
account.Comment: 4 pages, 6 figures, submitte
Using Available Volume to Predict Fluid Diffusivity in Random Media
We propose a simple equation for predicting self-diffusivity of fluids
embedded in random matrices of identical, but dynamically frozen, particles
(i.e., quenched-annealed systems). The only nontrivial input is the volume
available to mobile particles, which also can be predicted for two common
matrix types that reflect equilibrium and non-equilibrium fluid structures. The
proposed equation can account for the large differences in mobility exhibited
by quenched-annealed systems with indistinguishable static pair correlations,
illustrating the key role that available volume plays in transport.Comment: to appear in Physical Review E (12 pages, 4 figures
Electrostatic traps for dipolar excitons
We consider the design of two-dimensional electrostatic traps for dipolar
indirect excitons. We show that the excitons dipole-dipole interaction,
combined with the in-plane electric fields that arise due to the trap geometry,
constrain the maximal density and lifetime of trapped excitons. We derive an
analytic estimate of these values and determine their dependence on the trap
geometry, thus suggesting the optimal design for high density trapping as a
route for observing excitonic Bose-Einstein condensation.Comment: 5 pages, 3 figures. This 2nd version contains a revised Fig.3 + minor
revisions to the discussion and abstrac
Free Thermal Convection Driven by Nonlocal Effects
We report and explain a convective phenomenon observed in molecular dynamics
simulations that cannot be classified either as a hydrodynamics instability nor
as a macroscopically forced convection. Two complementary arguments show that
the velocity field by a thermalizing wall is proportional to the ratio between
the heat flux and the pressure. This prediction is quantitatively corroborated
by our simulations.Comment: RevTex, figures is eps, submited for publicatio
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 , where 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.Comment: revised version, 15 pages, 15 figures, to appear in J. Chem. Phy
Using the fractional interaction law to model the impact dynamics in arbitrary form of multiparticle collisions
Using the molecular dynamics method, we examine a discrete deterministic
model for the motion of spherical particles in three-dimensional space. The
model takes into account multiparticle collisions in arbitrary forms. Using
fractional calculus we proposed an expression for the repulsive force, which is
the so called fractional interaction law. We then illustrate and discuss how to
control (correlate) the energy dissipation and the collisional time for an
individual article within multiparticle collisions. In the multiparticle
collisions we included the friction mechanism needed for the transition from
coupled torsion-sliding friction through rolling friction to static friction.
Analysing simple simulations we found that in the strong repulsive state binary
collisions dominate. However, within multiparticle collisions weak repulsion is
observed to be much stronger. The presented numerical results can be used to
realistically model the impact dynamics of an individual particle in a group of
colliding particles.Comment: 17 pages, 8 figures, 1 table; In review process of Physical Review
Generalized Rosenfeld scalings for tracer diffusivities in not-so-simple fluids: Mixtures and soft particles
Rosenfeld [Phys. Rev. A 15, 2545 (1977)] noticed that casting transport
coefficients of simple monatomic, equilibrium fluids in specific dimensionless
forms makes them approximately single-valued functions of excess entropy. This
has predictive value because, while the transport coefficients of dense fluids
are difficult to estimate from first principles, excess entropy can often be
accurately predicted from liquid-state theory. Here, we use molecular
simulations to investigate whether Rosenfeld's observation is a special case of
a more general scaling law relating mobility of particles in mixtures to excess
entropy. Specifically, we study tracer diffusivities, static structure, and
thermodynamic properties of a variety of one- and two-component model fluid
systems with either additive or non-additive interactions of the hard-sphere or
Gaussian-core form. The results of the simulations demonstrate that the effects
of mixture concentration and composition, particle-size asymmetry and
additivity, and strength of the interparticle interactions in these fluids are
consistent with an empirical scaling law relating the excess entropy to a new
dimensionless (generalized Rosenfeld) form of tracer diffusivity, which we
introduce here. The dimensionless form of the tracer diffusivity follows from
knowledge of the intermolecular potential and the transport / thermodynamic
behavior of fluids in the dilute limit. The generalized Rosenfeld scaling
requires less information, and provides more accurate predictions, than either
Enskog theory or scalings based on the pair-correlation contribution to the
excess entropy. As we show, however, it also suffers from some limitations,
especially for systems that exhibit significant decoupling of individual
component tracer diffusivities.Comment: 15 pages, 10 figure
Relationship between speaking English as a second language and agitation in people with dementia living in care homes: Results from the MARQUE (Managing Agitation and Raising Quality of life) English national care home survey
OBJECTIVE:
As not speaking English as a first language may lead to increased difficulties in communication with staff and other residents, we (1) tested our primary hypotheses that care home residents with dementia speaking English as a second language experience more agitation and overall neuropsychiatric symptoms, and (2) explored qualitatively how staff consider that residents' language, ethnicity, and culture might impact on how they manage agitation.
METHODS:
We interviewed staff, residents with dementia, and their family carers from 86 care homes (2014–2015) about resident's neuropsychiatric symptoms, agitation, life quality, and dementia severity. We qualitatively interviewed 25 staff.
RESULTS:
Seventy-one out of 1420 (5%) of care home residents with dementia interviewed spoke English as a second language. After controlling for dementia severity, age, and sex, and accounting for care home and staff proxy clustering, speaking English as a second language compared with as a first language was associated with significantly higher Cohen-Mansfield Agitation Inventory (adjusted difference in means 8.3, 95% confidence interval 4.1 to 12.5) and Neuropsychiatric inventory scores (4.1, 0.65 to 7.5). Staff narratives described how linguistic and culturally isolating being in a care home where no residents or staff share your culture or language could be for people with dementia, and how this sometimes caused or worsened agitation.
CONCLUSIONS:
Considering a person with dementia's need to be understood when selecting a care home and developing technology resources to enable dementia-friendly translation services could be important strategies for reducing distress of people with dementia from minority ethnic groups who live in care homes
Airports at Risk: The Impact of Information Sources on Security Decisions
Security decisions in high risk organizations such as airports involve obtaining ongoing and frequent information about potential threats. Utilizing questionnaire survey data from a sample of airport
employees in European Airports across the continent, we analyzed
how both formal and informal sources of security information affect employee's decisions to comply with the security rules and
directives. This led us to trace information network flows to assess its impact on the degree employees making security decisions comply or deviate with the prescribed security rules. The results of the multivariate analysis showed that security information obtained through formal and informal networks differentially determine if employee will comply or not with the rules. Information sources emanating from the informal network tends to encourage employees to be more flexible in their security decisions
while formal sources lead to be more rigid with complying with rules and protocols. These results suggest that alongside the formal administrative structure of airports, there exists a diverse and pervasiveness set of informal communications networks that are a potent factor in determining airport security levels
A relative entropy rate method for path space sensitivity analysis of stationary complex stochastic dynamics
We propose a new sensitivity analysis methodology for complex stochastic
dynamics based on the Relative Entropy Rate. The method becomes computationally
feasible at the stationary regime of the process and involves the calculation
of suitable observables in path space for the Relative Entropy Rate and the
corresponding Fisher Information Matrix. The stationary regime is crucial for
stochastic dynamics and here allows us to address the sensitivity analysis of
complex systems, including examples of processes with complex landscapes that
exhibit metastability, non-reversible systems from a statistical mechanics
perspective, and high-dimensional, spatially distributed models. All these
systems exhibit, typically non-gaussian stationary probability distributions,
while in the case of high-dimensionality, histograms are impossible to
construct directly. Our proposed methods bypass these challenges relying on the
direct Monte Carlo simulation of rigorously derived observables for the
Relative Entropy Rate and Fisher Information in path space rather than on the
stationary probability distribution itself. We demonstrate the capabilities of
the proposed methodology by focusing here on two classes of problems: (a)
Langevin particle systems with either reversible (gradient) or non-reversible
(non-gradient) forcing, highlighting the ability of the method to carry out
sensitivity analysis in non-equilibrium systems; and, (b) spatially extended
Kinetic Monte Carlo models, showing that the method can handle high-dimensional
problems
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