10,025 research outputs found
Velocity correlations in dense granular flows
Velocity fluctuations of grains flowing down a rough inclined plane are
experimentally studied. The grains at the free surface exhibit fluctuating
motions, which are correlated over few grains diameters. The characteristic
correlation length is shown to depend on the inclination of the plane and not
on the thickness of the flowing layer. This result strongly supports the idea
that dense granular flows are controlled by a characteristic length larger than
the particle diameter
Enskog Theory for Polydisperse Granular Mixtures II. Sonine Polynomial Approximation
The linear integral equations defining the Navier-Stokes (NS) transport
coefficients for polydisperse granular mixtures of smooth inelastic hard disks
or spheres are solved by using the leading terms in a Sonine polynomial
expansion. Explicit expressions for all the NS transport coefficients are given
in terms of the sizes, masses, compositions, density and restitution
coefficients. In addition, the cooling rate is also evaluated to first order in
the gradients. The results hold for arbitrary degree of inelasticity and are
not limited to specific values of the parameters of the mixture. Finally, a
detailed comparison between the derivation of the current theory and previous
theories for mixtures is made, with attention paid to the implication of the
various treatments employed to date.Comment: 26 pages, to be published in Phys. Rev.
A scalable architecture for quantum computation with molecular nanomagnets
A proposal for a magnetic quantum processor that consists of individual
molecular spins coupled to superconducting coplanar resonators and transmission
lines is carefully examined. We derive a simple magnetic quantum
electrodynamics Hamiltonian to describe the underlying physics. It is shown
that these hybrid devices can perform arbitrary operations on each spin qubit
and induce tunable interactions between any pair of them. The combination of
these two operations ensures that the processor can perform universal quantum
computations. The feasibility of this proposal is critically discussed using
the results of realistic calculations, based on parameters of existing devices
and molecular qubits. These results show that the proposal is feasible,
provided that molecules with sufficiently long coherence times can be developed
and accurately integrated into specific areas of the device. This architecture
has an enormous potential for scaling up quantum computation thanks to the
microscopic nature of the individual constituents, the molecules, and the
possibility of using their internal spin degrees of freedom.Comment: 27 pages, 6 figure
Shocks in supersonic sand
We measure time-averaged velocity, density, and temperature fields for steady
granular flow past a wedge and calculate a speed of granular pressure
disturbances (sound speed) equal to 10% of the flow speed. The flow is
supersonic, forming shocks nearly identical to those in a supersonic gas.
Molecular dynamics simulations of Newton's laws and Monte Carlo simulations of
the Boltzmann equation yield fields in quantitative agreement with experiment.
A numerical solution of Navier-Stokes-like equations agrees with a molecular
dynamics simulation for experimental conditions excluding wall friction.Comment: 4 pages, 5 figure
Granular Rheology in Zero Gravity
We present an experimental investigation on the rheological behavior of model
granular media made of nearly elastic spherical particles. The experiments are
performed in a cylindrical Couette geometry and the experimental device is
placed inside an airplane undergoing parabolic flights to cancel the effect of
gravity. The corresponding curves, shear stress versus shear rate, are
presented and a comparison with existing theories is proposed. The quadratic
dependence on the shear rate is clearly shown and the behavior as a function of
the solid volume fraction of particles exhibits a power law function. It is
shown that theoretical predictions overestimate the experiments. We observe, at
intermediate volume fractions, the formation of rings of particles regularly
spaced along the height of the cell. The differences observed between
experimental results and theoretical predictions are discussed and related to
the structures formed in the granular medium submitted to the external shear.Comment: 10 pages, 6 figures to be published in Journal of Physics : Condensed
Matte
Avian malaria and bird humoral immune response.
Plasmodium parasites are known to impose fitness costs on their vertebrate hosts. Some of these costs are due to the activation of the immune response, which may divert resources away from self-maintenance. Plasmodium parasites may also immuno-deplete their hosts. Thus, infected individuals may be less able to mount an immune response to a new pathogen than uninfected ones. However, this has been poorly investigated.
The effect of Plasmodium infection on bird humoral immune response when encountering a novel antigen was tested. A laboratory experiment was conducted on canaries (Serinus canaria) experimentally infected with Plasmodium relictum (lineage SGS1) under controlled conditions. Birds were immune challenged with an intra-pectoral injection of a novel non-pathogenic antigen (keyhole limpet haemocyanin, KLH). One week later they were challenged again. The immune responses to the primary and to the secondary contacts were quantified as anti-KLH antibody production via enzyme-linked immunosorbent assay (ELISA).
There was no significant difference in antibody production between uninfected and Plasmodium infected birds at both primary and secondary contact. However, Plasmodium parasite intensity in the blood increased after the primary contact with the antigen.
There was no effect of Plasmodium infection on the magnitude of the humoral immune response. However, there was a cost of mounting an immune response in infected individuals as parasitaemia increased after the immune challenge, suggesting a trade-off between current control of chronic Plasmodium infection and investment against a new immune challenge
Masses, Oxygen and Carbon abundances in CHEPS dwarf stars
Reproduced with permission from Astronomy & Astrophysics. © 2019 ESOContext. We report the results from the determination of stellar masses, carbon, and oxygen abundances in the atmospheres of 107 stars from the Calan-Hertfordshire Extrasolar Planet Search (CHEPS) programme. Our stars are drawn from a population with a significantly super-solar metallicity. At least 10 of these stars are known to host orbiting planets. Aims. In this work, we set out to understand the behaviour of carbon and oxygen abundance in stars with different spectral classes, metallicities, and V sin i within the metal-rich stellar population. Methods. Masses of these stars were determined using data from Gaia DR2. Oxygen and carbon abundances were determined by fitting the absorption lines. We determined oxygen abundances with fits to the 6300.304 Å O I line, and we used 3 lines of the C I atom and 12 lines of the C 2 molecule for the determination of carbon abundances. Results. We determine masses and abundances of 107 CHEPS stars. There is no evidence that the [C/O] ratio depends on V sin i or the mass of the star within our constrained range of masses, i.e. 0.82 5 km s -1) are massive stars.Peer reviewedFinal Published versio
Hyperon Nonleptonic Decays in Chiral Perturbation Theory Reexamined
We recalculate the leading nonanalytic contributions to the amplitudes for
hyperon nonleptonic decays in chiral perturbation theory. Our results partially
disagree with those calculated before, and include new terms previously omitted
in the P-wave amplitudes. Although these modifications are numerically
significant, they do not change the well-known fact that good agreement with
experiment cannot be simultaneously achieved using one-loop S- and P-wave
amplitudes.Comment: 14 pages, latex, 3 figures, uses axodraw.sty, minor additions, to
appear in Phys. Rev.
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