171 research outputs found
Reducing multiphoton ionization in a linearly polarized microwave field by local control
We present a control procedure to reduce the stochastic ionization of
hydrogen atom in a strong microwave field by adding to the original Hamiltonian
a comparatively small control term which might consist of an additional set of
microwave fields. This modification restores select invariant tori in the
dynamics and prevents ionization. We demonstrate the procedure on the
one-dimensional model of microwave ionization.Comment: 8 page
Far-from-equilibrium Ostwald ripening in electrostatically driven granular powders
We report the first experimental study of cluster size distributions in
electrostatically driven granular submonolayers. The cluster size distribution
in this far-from-equilibrium process exhibits dynamic scaling behavior
characteristic of the (nearly equilibrium) Ostwald ripening, controlled by the
attachment and detachment of the "gas" particles. The scaled size distribution,
however, is different from the classical Wagner distribution obtained in the
limit of a vanishingly small area fraction of the clusters. A much better
agreement is found with the theory of Conti et al. [Phys. Rev. E 65, 046117
(2002)] which accounts for the cluster merger.Comment: 5 pages, to appear in PR
Fluctuations of Current in Non-Stationary Diffusive Lattice Gases
We employ the macroscopic fluctuation theory to study fluctuations of
integrated current in one-dimensional lattice gases with a step-like initial
density profile. We analytically determine the variance of the current
fluctuations for a class of diffusive processes with a density-independent
diffusion coefficient, but otherwise arbitrary. Our calculations rely on a
perturbation theory around the noiseless hydrodynamic solution. We consider
both quenched and annealed types of averaging (the initial condition is allowed
to fluctuate in the latter situation). The general results for the variance are
specialized to a few interesting models including the symmetric exclusion
process and the Kipnis-Marchioro-Presutti model. We also probe large deviations
of the current for the symmetric exclusion process. This is done by numerically
solving the governing equations of the macroscopic fluctuation theory using an
efficient iteration algorithm.Comment: Slightly extended version. 12 pages, 6 figure
On population extinction risk in the aftermath of a catastrophic event
We investigate how a catastrophic event (modeled as a temporary fall of the
reproduction rate) increases the extinction probability of an isolated
self-regulated stochastic population. Using a variant of the Verhulst logistic
model as an example, we combine the probability generating function technique
with an eikonal approximation to evaluate the exponentially large increase in
the extinction probability caused by the catastrophe. This quantity is given by
the eikonal action computed over "the optimal path" (instanton) of an effective
classical Hamiltonian system with a time-dependent Hamiltonian. For a general
catastrophe the eikonal equations can be solved numerically. For simple models
of catastrophic events analytic solutions can be obtained. One such solution
becomes quite simple close to the bifurcation point of the Verhulst model. The
eikonal results for the increase in the extinction probability caused by a
catastrophe agree well with numerical solutions of the master equation.Comment: 11 pages, 11 figure
Thermal Instability-Induced Interstellar Turbulence
We study the dynamics of phase transitions in the interstellar medium by
means of three-dimensional hydrodynamic numerical simulations. We use a
realistic cooling function and generic nonequilibrium initial conditions to
follow the formation history of a multiphase medium in detail in the absence of
gravity. We outline a number of qualitatively distinct stages of this process,
including a linear isobaric evolution, transition to an isochoric regime,
formation of filaments and voids (also known as "thermal" pancakes), the
development and decay of supersonic turbulence, an approach to pressure
equilibrium, and final relaxation of the multiphase medium. We find that 1%-2%
of the initial thermal energy is converted into gas motions in one cooling
time. The velocity field then randomizes into turbulence that decays on a
dynamical timescale E_k ~ t^-n, 1 < n < 2. While not all initial conditions
yield a stable two-phase medium, we examine such a case in detail. We find that
the two phases are well mixed with the cold clouds possessing a fine-grained
structure near our numerical resolution limit. The amount of gas in the
intermediate unstable phase roughly tracks the rms turbulent Mach number,
peaking at 25% when M_rms ~ 8, decreasing to 11% when M_rms ~ 0.4.Comment: To appear in the ApJ Letters, April 2002; 5 pages, 3 color figures,
mpeg animations available at http://akpc.ucsd.edu/ThermalLetter/thermal.htm
Psychological Safety and Communication Difficulties of Teachers and Students During Long-term Online Training
Due to the growing concerns related to the psychological well-being of students and teachers during a long and intensive online training, it becomes necessary for teachers, psychologists, practitioners to take measures to prevent threats to online communication and identify personal resources of psychological security in the online environment.The purpose of the study was to identify the communicative difficulties of long-term online learning during the COVID-19 pandemic, and the personal resources of students and teachers that contribute to ensuring their psychological safety.The study was conducted in February-March 2022. The study sample included 132 students and 40 teachers of the Faculty of Psychology of the Russian State Social University (Moscow). The following techniques were used: “The test of hardiness” (S. Muddy, in the Russian-language adaptation of E.N. Osin, E.I. Rasskazova), “The scale of subjective well-being” (A. Perrudet-Badoux, G.A. Mendelssohn, J. Chiche, in the Russian-language adaptation of M.V. Sokolova), “Methodology for assessing the level of sociability” (V.F. Ryakhovsky), questionnaires “Difficulties of online communication” for students and teachers. The empirical data obtained were interpreted and processed using qualitative and quantitative methods of analysis, including: descriptive statistics, frequency analysis, Spearman correlation analysis. The study showed that during the long-term distance learning, students and teachers experienced significant difficulties in online educational communication, had low levels of subjective well-being, resilience and sociability. These personal qualities are systemic in nature, interrelated and can act as resources to ensure the psychological safety of subjects of education, prevention or coping with difficulties of online communication and hybrid forms of learning.The data obtained make it necessary for teachers to create psychodidactic conditions for a safe online educational environment in which students will be involved as subjects of education, will be able to freely share their opinions and not be afraid to make a mistake, will feel belonging to a group and protected from verbal aggression
The Knudsen temperature jump and the Navier-Stokes hydrodynamics of granular gases driven by thermal walls
Thermal wall is a convenient idealization of a rapidly vibrating plate used
for vibrofluidization of granular materials. The objective of this work is to
incorporate the Knudsen temperature jump at thermal wall in the Navier-Stokes
hydrodynamic modeling of dilute granular gases of monodisperse particles that
collide nearly elastically. The Knudsen temperature jump manifests itself as an
additional term, proportional to the temperature gradient, in the boundary
condition for the temperature. Up to a numerical pre-factor of order unity,
this term is known from kinetic theory of elastic gases. We determine the
previously unknown numerical pre-factor by measuring, in a series of molecular
dynamics (MD) simulations, steady-state temperature profiles of a gas of
elastically colliding hard disks, confined between two thermal walls kept at
different temperatures, and comparing the results with the predictions of a
hydrodynamic calculation employing the modified boundary condition. The
modified boundary condition is then applied, without any adjustable parameters,
to a hydrodynamic calculation of the temperature profile of a gas of inelastic
hard disks driven by a thermal wall. We find the hydrodynamic prediction to be
in very good agreement with MD simulations of the same system. The results of
this work pave the way to a more accurate hydrodynamic modeling of driven
granular gases.Comment: 7 pages, 3 figure
A nonlinear theory of non-stationary low Mach number channel flows of freely cooling nearly elastic granular gases
We use hydrodynamics to investigate non-stationary channel flows of freely
cooling dilute granular gases. We focus on the regime where the sound travel
time through the channel is much shorter than the characteristic cooling time
of the gas. As a result, the gas pressure rapidly becomes almost homogeneous,
while the typical Mach number of the flow drops well below unity. Eliminating
the acoustic modes, we reduce the hydrodynamic equations to a single nonlinear
and nonlocal equation of a reaction-diffusion type in Lagrangian coordinates.
This equation describes a broad class of channel flows and, in particular, can
follow the development of the clustering instability from a weakly perturbed
homogeneous cooling state to strongly nonlinear states. If the heat diffusion
is neglected, the reduced equation is exactly soluble, and the solution
develops a finite-time density blowup. The heat diffusion, however, becomes
important near the attempted singularity. It arrests the density blowup and
brings about novel inhomogeneous cooling states (ICSs) of the gas, where the
pressure continues to decay with time, while the density profile becomes
time-independent. Both the density profile of an ICS, and the characteristic
relaxation time towards it are determined by a single dimensionless parameter
that describes the relative role of the inelastic energy loss and heat
diffusion. At large values of this parameter, the intermediate cooling dynamics
proceeds as a competition between low-density regions of the gas. This
competition resembles Ostwald ripening: only one hole survives at the end.Comment: 20 pages, 15 figures, final versio
Weak selection and stability of localized distributions in Ostwald ripening
We support and generalize a weak selection rule predicted recently for the
self-similar asymptotics of the distribution function (DF) in the
zero-volume-fraction limit of Ostwald ripening (OR). An asymptotic perturbation
theory is developed that, when combined with an exact invariance property of
the system, yields the selection rule, predicts a power-law convergence towards
the selected self-similar DF and agrees well with our numerical simulations for
the interface- and diffusion-controlled OR.Comment: 4 pages, 2 figures, submitted to PR
Normal scaling in globally conserved interface-controlled coarsening of fractal clusters
Globally conserved interface-controlled coarsening of fractal clusters
exhibits dynamic scale invariance and normal scaling. This is demonstrated by a
numerical solution of the Ginzburg-Landau equation with a global conservation
law. The sharp-interface limit of this equation is volume preserving motion by
mean curvature. The scaled form of the correlation function has a power-law
tail accommodating the fractal initial condition. The coarsening length
exhibits normal scaling with time. Finally, shrinking of the fractal clusters
with time is observed. The difference between global and local conservation is
discussed.Comment: 4 pages, 3 eps figure
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