11,215 research outputs found
Generation of large-scale winds in horizontally anisotropic convection
We simulate three-dimensional, horizontally periodic Rayleigh-B\'enard
convection between free-slip horizontal plates, rotating about a distant
horizontal axis. When both the temperature difference between the plates and
the rotation rate are sufficiently large, a strong horizontal wind is generated
that is perpendicular to both the rotation vector and the gravity vector. The
wind is turbulent, large-scale, and vertically sheared. Horizontal anisotropy,
engendered here by rotation, appears necessary for such wind generation. Most
of the kinetic energy of the flow resides in the wind, and the vertical
turbulent heat flux is much lower on average than when there is no wind
Habitable Climates: The Influence of Eccentricity
In the outer regions of the habitable zone, the risk of transitioning into a
globally frozen "snowball" state poses a threat to the habitability of planets
with the capacity to host water-based life. We use a one-dimensional energy
balance climate model (EBM) to examine how obliquity, spin rate, orbital
eccentricity, and ocean coverage might influence the onset of such a snowball
state. For an exoplanet, these parameters may be strikingly different from the
values observed for Earth. Since, for constant semimajor axis, the annual mean
stellar irradiation scales with (1-e^2)^(-1/2), one might expect the greatest
habitable semimajor axis (for fixed atmospheric composition) to scale as
(1-e^2)^(-1/4). We find that this standard ansatz provides a reasonable lower
bound on the outer boundary of the habitable zone, but the influence of
obliquity and ocean fraction can be profound in the context of planets on
eccentric orbits. For planets with eccentricity 0.5, our EBM suggests that the
greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending
on obliquity, with higher obliquity worlds generally more stable against
snowball transitions. One might also expect that the long winter at an
eccentric planet's apoastron would render it more susceptible to global
freezing. Our models suggest that this is not a significant risk for Earth-like
planets around Sun-like stars since such planets are buffered by the thermal
inertia provided by oceans covering at least 10% of their surface. Since
planets on eccentric orbits spend much of their year particularly far from the
star, such worlds might turn out to be especially good targets for direct
observations with missions such as TPF-Darwin. Nevertheless, the extreme
temperature variations achieved on highly eccentric exo-Earths raise questions
about the adaptability of life to marginally or transiently habitable
conditions.Comment: References added, text and figures updated, accepted by Ap
Higher-order Continuum Approximation for Rarefied Gases
The Hilbert-Chapman-Enskog expansion of the kinetic equations in mean flight
times is believed to be asymptotic rather than convergent. It is therefore
inadvisable to use lower order results to simplify the current approximation as
is done in the traditional Chapman-Enskog procedure, since that is an iterative
method. By avoiding such recycling of lower order results, one obtains
macroscopic equations that are asymptotically equivalent to the ones found in
the Chapman-Enskog approach. The new equations contain higher order terms that
are discarded in the Chapman-Enskog method. These make a significant impact on
the results for such problems as ultrasound propagation. In this paper, it is
shown that these results turn out well with relatively little complication when
the expansions are carried to second order in the mean free time, for the
example of the relaxation or BGK model of kinetic theory.Comment: 20 pages, 2 figures, RevTeX 4 macro
Constraints on Supersymmetric Dark Matter for Heavy Scalar Superpartners
We study the constraints on neutralino dark matter in minimal low energy
supersymmetry models and the case of heavy lepton and quark scalar
superpartners. For values of the Higgsino and gaugino mass parameters of the
order of the weak scale, direct detection experiments are already putting
strong bounds on models in which the dominant interactions between the dark
matter candidates and nuclei are governed by Higgs boson exchange processes,
particularly for positive values of the Higgsino mass parameter mu. For
negative values of mu, there can be destructive interference between the
amplitudes associated with the exchange of the standard CP-even Higgs boson and
the exchange of the non-standard one. This leads to specific regions of
parameter space which are consistent with the current experimental constraints
and a thermal origin of the observed relic density. In this article we study
the current experimental constraints on these scenarios, as well as the future
experimental probes, using a combination of direct and indirect dark matter
detection and heavy Higgs and electroweakino searches at hadron colliders.Comment: 32 pages, 13 figure
Dynamics of a liquid dielectric attracted by a cylindrical capacitor
The dynamics of a liquid dielectric attracted by a vertical cylindrical
capacitor is studied. Contrary to what might be expected from the standard
calculation of the force exerted by the capacitor, the motion of the dielectric
is different depending on whether the charge or the voltage of the capacitor is
held constant. The problem turns out to be an unconventional example of
dynamics of a system with variable mass, whose velocity can, in certain
circumstances, suffer abrupt changes. Under the hypothesis that the voltage
remains constant the motion is described in qualitative and quantitative
details, and a very brief qualitative discussion is made of the constant charge
case.Comment: To appear in European Journal of Physic
The tachocline revisited
The solar tachocline is a shear layer located at the base of the solar
convection zone. The horizontal shear in the tachocline is likely turbulent,
and it is often assumed that this turbulence would be strongly anisotropic as a
result of the local stratification. What role this turbulence plays in the
tachocline dynamics, however, remains to be determined. In particular, it is
not clear whether it would result in a turbulent eddy diffusivity, or
anti-diffusivity, or something else entirely. In this paper, we present the
first direct numerical simulations of turbulence in horizontal shear flows at
low Prandtl number, in an idealized model that ignores rotation and magnetic
fields. We find that several regimes exist, depending on the relative
importance of the stratification, viscosity and thermal diffusivity. Our
results suggest that the tachocline is in the stratified turbulence regime,
which has very specific properties controlled by a balance between buoyancy,
inertia, and thermal diffusion.Comment: Invited review for the meeting Dynamics of the Sun and Stars:
Honoring the Life and Work of Michael J. Thompson (Boulder, Colorado, 24-26
September 2019
Gypsum-DL: an open-source program for preparing small-molecule libraries for structure-based virtual screening
Computational techniques such as structure-based virtual screening require carefully prepared 3D models of potential small-molecule ligands. Though powerful, existing commercial programs for virtual-library preparation have restrictive and/or expensive licenses. Freely available alternatives, though often effective, do not fully account for all possible ionization, tautomeric, and ring-conformational variants. We here present Gypsum-DL, a free, robust open-source program that addresses these challenges. As input, Gypsum-DL accepts virtual compound libraries in SMILES or flat SDF formats. For each molecule in the virtual library, it enumerates appropriate ionization, tautomeric, chiral, cis/trans isomeric, and ring-conformational forms. As output, Gypsum-DL produces an SDF file containing each molecular form, with 3D coordinates assigned. To demonstrate its utility, we processed 1558 molecules taken from the NCI Diversity Set VI and 56,608 molecules taken from a Distributed Drug Discovery (D3) combinatorial virtual library. We also used 4463 high-quality protein-ligand complexes from the PDBBind database to show that Gypsum-DL processing can improve virtual-screening pose prediction. Gypsum-DL is available free of charge under the terms of the Apache License, Version 2.0
Continuous unidirectional fiber reinforced composites: Fabrication and testing
The study of the anisotropic mechanical properties of an inexpensively fabricated composite with continuous unidirectional fibers and a clear matrix was investigated. A method has been developed to fabricate these composites with aluminum fibers and a polymer matrix. These composites clearly demonstrate the properties of unidirectional composites and cost less than five dollars each to fabricate
Diffusive transport in networks built of containers and tubes
We developed analytical and numerical methods to study a transport of
non-interacting particles in large networks consisting of M d-dimensional
containers C_1,...,C_M with radii R_i linked together by tubes of length l_{ij}
and radii a_{ij} where i,j=1,2,...,M. Tubes may join directly with each other
forming junctions. It is possible that some links are absent. Instead of
solving the diffusion equation for the full problem we formulated an approach
that is computationally more efficient. We derived a set of rate equations that
govern the time dependence of the number of particles in each container
N_1(t),N_2(t),...,N_M(t). In such a way the complicated transport problem is
reduced to a set of M first order integro-differential equations in time, which
can be solved efficiently by the algorithm presented here. The workings of the
method have been demonstrated on a couple of examples: networks involving
three, four and seven containers, and one network with a three-point junction.
Already simple networks with relatively few containers exhibit interesting
transport behavior. For example, we showed that it is possible to adjust the
geometry of the networks so that the particle concentration varies in time in a
wave-like manner. Such behavior deviates from simple exponential growth and
decay occurring in the two container system.Comment: 21 pages, 18 figures, REVTEX4; new figure added, reduced emphasis on
graph theory, additional discussion added (computational cost, one
dimensional tubes
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