1,501 research outputs found
Non-equilibrium concentration fluctuations in binary liquids with realistic boundary conditions
Because of the spatially long-ranged nature of spontaneous fluctuations in
thermal non-equilibrium systems, they are affected by boundary conditions for
the fluctuating hydrodynamic variables. In this paper we consider a liquid
mixture between two rigid and impervious plates with a stationary concentration
gradient resulting from a temperature gradient through the Soret effect. For
liquid mixtures with a large Lewis number, we are able to obtain explicit
analytical expressions for the intensity of the non-equilibrium concentration
fluctuations as a function of the frequency and the wave number of
the fluctuations. In addition we elucidate the spatial dependence of the
intensity of the non-equilibrium fluctuations responsible for a non-equilibrium
Casimir effect.Comment: 9 pages, 2 figure
Inelastic lifetimes of hot electrons in real metals
We report a first-principles description of inelastic lifetimes of excited
electrons in real Cu and Al, which we compute, within the GW approximation of
many-body theory, from the knowledge of the self-energy of the excited
quasiparticle. Our full band-structure calculations indicate that actual
lifetimes are the result of a delicate balance between localization, density of
states, screening, and Fermi-surface topology. A major contribution from
-electrons participating in the screening of electron-electron interactions
yields lifetimes of excited electrons in copper that are larger than those of
electrons in a free-electron gas with the electron density equal to that of
valence () electrons. In aluminum, a simple metal with no -bands,
splitting of the band structure over the Fermi level results in electron
lifetimes that are smaller than those of electrons in a free-electron gas.Comment: 4 papes, 2 figures, to appear in Phys. Rev. Let
The Stokes-Einstein Relation at Moderate Schmidt Number
The Stokes-Einstein relation for the self-diffusion coefficient of a
spherical particle suspended in an incompressible fluid is an asymptotic result
in the limit of large Schmidt number, that is, when momentum diffuses much
faster than the particle. When the Schmidt number is moderate, which happens in
most particle methods for hydrodynamics, deviations from the Stokes-Einstein
prediction are expected. We study these corrections computationally using a
recently-developed minimally-resolved method for coupling particles to an
incompressible fluctuating fluid in both two and three dimensions. We find that
for moderate Schmidt numbers the diffusion coefficient is reduced relative to
the Stokes-Einstein prediction by an amount inversely proportional to the
Schmidt number in both two and three dimensions. We find, however, that the
Einstein formula is obeyed at all Schmidt numbers, consistent with linear
response theory. The numerical data is in good agreement with an approximate
self-consistent theory, which can be used to estimate finite-Schmidt number
corrections in a variety of methods. Our results indicate that the corrections
to the Stokes-Einstein formula come primarily from the fact that the particle
itself diffuses together with the momentum. Our study separates effects coming
from corrections to no-slip hydrodynamics from those of finite separation of
time scales, allowing for a better understanding of widely observed deviations
from the Stokes-Einstein prediction in particle methods such as molecular
dynamics.Comment: Submitte
Violation of action--reaction and self-forces induced by nonequilibrium fluctuations
We show that the extension of Casimir-like forces to fluctuating fluids
driven out of equilibrium can exhibit two interrelated phenomena forbidden at
equilibrium: self-forces can be induced on single asymmetric objects and the
action--reaction principle between two objects can be violated. These effects
originate in asymmetric restrictions imposed by the objects' boundaries on the
fluid's fluctuations. They are not ruled out by the second law of
thermodynamics since the fluid is in a nonequilibrium state. Considering a
simple reaction--diffusion model for the fluid, we explicitly calculate the
self-force induced on a deformed circle. We also show that the action--reaction
principle does not apply for the internal Casimir forces exerting between a
circle and a plate. Their sum, instead of vanishing, provides the self-force on
the circle-plate assembly.Comment: 4 pages, 1 figure. V2: New title; Abstract partially rewritten;
Largely enhanced introductory and concluding remarks (incl. new Refs.
Lattice Boltzmann - Langevin simulations of binary mixtures
We report a hybrid numerical method for the solution of the model H
fluctuating hydrodynamic equations for binary mixtures. The momentum
conservation equations with Landau-Lifshitz stresses are solved using the
fluctuating lattice Boltzmann equation while the order parameter conservation
equation with Langevin fluxes are solved using the stochastic method of lines.
Two methods, based on finite difference and finite volume, are proposed for
spatial discretisation of the order parameter equation. Special care is taken
to ensure that the fluctuation-dissipation theorem is maintained at the lattice
level in both cases. The methods are benchmarked by comparing static and
dynamic correlations and excellent agreement is found between analytical and
numerical results. The Galilean invariance of the model is tested and found to
be satisfactory. Thermally induced capillary fluctuations of the interface are
captured accurately, indicating that the model can be used to study nonlinear
fluctuations
Spatial correlations of hydrodynamic fluctuations in simple fluids under shear flow: a mesoscale simulation study
Hydrodynamic fluctuations in simple fluids under shear flow are demonstrated to be spatially correlated, in contrast to the fluctuations at equilibrium, using mesoscopic hydrodynamic simulations. The simulation results for the equal-time hydrodynamic correlations in a multiparticle collision dynamics (MPC) fluid in shear flow are compared with the explicit expressions obtained from fluctuating hydrodynamics calculations. For large wave vectors
k
, the nonequilibrium contributions to transverse and longitudinal velocity correlations decay as
k
−
4
for wave vectors along the flow direction and as
k
−
2
for the off-flow directions. For small wave vectors, a crossover to a slower decay occurs, indicating long-range correlations in real space. The coupling between the transverse velocity components, which vanishes at equilibrium, also exhibits a
k
−
2
dependence on the wave vector. In addition, we observe a quadratic dependency on the shear rate of the nonequilibrium contribution to pressure
Boltzmann equation and hydrodynamic fluctuations
We apply the method of invariant manifolds to derive equations of generalized
hydrodynamics from the linearized Boltzmann equation and determine exact
transport coefficients, obeying Green-Kubo formulas. Numerical calculations are
performed in the special case of Maxwell molecules. We investigate, through the
comparison with experimental data and former approaches, the spectrum of
density fluctuations and address the regime of finite Knudsen numbers and
finite frequencies hydrodynamics.Comment: This is a more detailed version of a related paper: I.V. Karlin, M.
Colangeli, M. Kroger, PRL 100 (2008) 214503, arXiv:0801.2932. It contains
comparison between predictions and experiment, in particular. 11 pages, 6
figures, 2 table
An evaluation of management strategies for Atlantic tuna stocks
International agreements for the International Commission for the Conservation of Atlantic Tunas (ICCAT) convention area imply that Atlantic tuna stocks should be managed by strategies based on maximum sustainable yield (MSY); however, there is concern whether this will actually ensure sustainability with sufficiently high probability consistent with the principals of the precautionary approach. Therefore, the performance of MSY management strategies based on current assessment procedures was evaluated using a computer simulation framework. The framework includes the data collection, assessment, prediction, and management processes, as well as the implementation of management regulations. It therefore provides an integrated way to evaluate the relative importance of and the interactions between each component of the system with regard to the overall success of the management strategy. The study elucidates guidelines about assessment and management that are general enough to be applied to all tunas in the Atlantic Ocean. It does so by comparing different hypotheses about management and assessment for three stocks (North Atlantic albacore, Atlantic bigeye and East Atlantic skipjack), which are representative of the variety encountered (i.e. from data rich to poor and tropical to temperate waters) in ICCAT stocks. Management performance was especially sensitive to the carrying capacity of the stock. The type of proxy used for MSY was more important to the success of the procedure than the frequency of assessment or the number of indices used in the assessment. Whilst the procedure was successful at achieving the management objectives for albacore, it was only partially successful for bigeye and was too conservative for skipjack.No disponibl
An assessment of pediatric residency applicant perceptions of Fit during the virtual interview era
PURPOSE: Residency recruitment events and interviews are widely considered an integral component of the residency match experience. Due to the COVID-19 pandemic, residency recruitment and interviewing throughout the 2020-2021 academic year were performed virtually, which created challenges for applicants\u27 ability to discern fit to a program. Given this change, it is reasonable to suspect that applicants would be less able to discern program fit. Therefore, this study evaluated how virtual interviews impacted pediatric residency applicants\u27 ability to assess factors contributing to fit and subsequently how applicants assessed their self-perceived fit to their top-ranked programs.
METHODS: An online, anonymous survey was distributed to all residency applicants who applied to any specialty at our large academic institution. The survey utilized a 5-point Likert-type scale to evaluate qualities of fit as well as the applicants\u27 self-perceived ability to assess these qualities through a virtual platform.
RESULTS: 1,840 surveys were distributed, of which 473 residency applicants responded (25.7% response rate). Among these responses, 81 were pediatric applicants (27.6%). Factors deemed most important in determining fit included how well the residents get along with one another (98.8%), how much the program appeared to care about its trainees (97.5%), and how satisfied residents were with their program (97.5%). Qualities deemed most difficult for applicants to discern included the quality of facilities (18.6%), patient diversity (29.4%), and how well the residents got along with one another (30.2%). When compared to all other residency applicants, pediatric applicants placed more value on whether a program was family-friendly (p = 0.015), the quality of the facilities (p = 0.009), and the on-call system (p = 0.038).
CONCLUSION: This study highlights factors that influence pediatric applicants\u27 perception of fit into a program. Unfortunately, many factors deemed most important for pediatric applicants were also among the most difficult to assess virtually. These include resident camaraderie, whether a program cares about its residents, and overall resident satisfaction. Taken together, these findings and the recommendations presented should be considered by all residency program leaders to ensure the successful recruitment of a pediatric residency class
Preserved Flora and Organics in Impact Melt Breccias
At least seven glass-bearing strata of varying ages occur at different horizons in the Pampean sediments of Argentina dating back to the Miocene. In a strict sense, these impact glasses are melt-matrix breccias composed of partially digested minerals clasts and basement fragments indicative of crater excavation. Ar-40/Ar-39 dating yield ages (+/- 2 sigma) of 6 +/- 2 Ka, 114 +/- 26 Ka, 230 +/- 30 Ka, 445 +/- 21 Ka, 3.27 +/- 0.08 Ma (near Mar del Plata = MdP), 5.28 +/- 0.04 Ma, and 9.21 +/- 0.08 Ma (near Chasico = CH) Where found in place (not reworked), these ages are consistent with the local stratigraphy and faunal assemblages. A striking property of some of these impact glasses is the encapsulation of preserved fragments of floral (and even soft-tissue faunal remains). Here we identify retained organics and describe a likely process of encapsulation and preservation
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