215,951 research outputs found
Interplay of air and sand: Faraday heaping unravelled
We report on numerical simulations of a vibrated granular bed including the effect of the ambient air, generating the famous Faraday heaps known from experiment. A detailed analysis of the forces shows that the heaps are formed and stabilized by the airflow through the bed while the gap between bed and vibrating bottom is growing, confirming the pressure gradient mechanism found experimentally by Thomas and Squires [Phys. Rev. Lett. 81, 574 (1998)], with the addition that the airflow is partly generated by isobars running parallel to the surface of the granular bed. Importantly, the simulations also explain the heaping instability of the initially flat surface and the experimentally observed coarsening of a number of small heaps into a larger one
RTNN: The new parallel machine in Zaragoza
I report on the development of RTNN, a parallel computer designed as a 4^4
hypercube of 256 T9000 transputer nodes, each with 8 MB memory. The peak
performance of the machine is expected to be 2.5 Gflops.Comment: 10 pages PostScript, including 5 figures. Write-up (June 1995) of
talk at the International Workshop ``QCD on Massively Parallel Computers'',
Yamagata, Japan, 16-18 March 1995. To appear in the Proceedings, Suppl.
Progr. Theor. Phys. (Kyoto
Biofuel scenarios in a water perspective: the global blue and green water footprint of road transport in 2030
The trend towards substitution of conventional transport fuels by biofuels requires additional water. The EU aims In the last two centuries, fossil fuels have been our major source of energy. However, issues concerning energy security and the quality of the environment have given an impulse to the development of alternative, renewable fuels. Particularly the transport sector is expected to steadily switch from fossil fuels to a larger fraction of biofuels - liquid transport fuels derived from biomass. Many governments believe that biofuels can replace substantial volumes of crude oil and that they will play a key role in diversifying the sources of energy supply in the coming decades. The growth of biomass requires water, a scarce resource. The link between water resources and (future) biofuel consumption, however, has not been analyzed in great detail yet. Existing scenarios on the use of water resources usually only consider the changes in food and livestock production, industry and domestic activity. The aim of this research is to assess the change in water use related to the expected increase in the use of biofuels for road transport in 2030, and subsequently evaluate the contribution to potential water scarcity. The study builds on earlier research on the relation between energy and water and uses the water footprint (WF) methodology to investigate the change in water demand related to a transition to biofuels in road transport. Information about this transition in each country is based on a compilation of different energy scenarios. The study distinguishes between two different bio-energy carriers, bio-ethanol and biodiesel, and assesses the ratio of fuel produced from selected first-generation energy crops per country. For ethanol these crops are sugar cane, sugar beet, sweet sorghum, wheat and maize. For biodiesel they are soybean, rapeseed, jatropha, and oil palm
Some views on monopoles and confinement
Aspects of the monopole condensation picture of confinement are discussed.
First, the nature of the monopole singularities in the abelian projection
approach is analysed. Their apparent gauge dependence is shown to have a
natural interpretation in terms of 't~Hooft-Polyakov-like monopoles in
euclidean SU(2) gauge theory. Next, the results and predictions of a
realization of confinement through condensation of such monopoles are
summarized and compared with numerical data.Comment: Talk at the International RCNP Workshop on COLOR CONFINEMENT AND
HADRONS --- CONFINEMENT 95 (March 22--24, 1995, RCNP Osaka, Japan), to appear
in the proceedings. 9 pages latex, 1 PostScript figure in uufiles format,
uses epsf.te
Electromagnons and instabilities in magnetoelectric materials with non-collinear spin orders
We show that strong electromagnon peaks can be found in absorption spectra of
non-collinear magnets exhibiting a linear magnetoelectric effect. The
frequencies of these peaks coincide with the frequencies of antiferromagnetic
resonances and the ratio of the spectral weights of the electromagnon and
antiferromagnetic resonance is related to the ratio of the static
magnetoelectric constant and magnetic susceptibility. Using a Kagome lattice
antiferromagnet as an example, we show that frustration of spin ordering gives
rise to magnetoelastic instabilities at strong spin-lattice coupling, which
transform a non-collinear magnetoelectric spin state into a collinear
multiferroic state with a spontaneous electric polarization and magnetization.
The Kagome lattice antiferromagnet also shows a ferroelectric
incommensurate-spiral phase, where polarization is induced by the exchange
striction mechanism.Comment: 9 pages, 4 figure
Residual entropy in a model for the unfolding of single polymer chains
We study the unfolding of a single polymer chain due to an external force. We
use a simplified model which allows to perform all calculations in closed form
without assuming a Boltzmann-Gibbs form for the equilibrium distribution.
Temperature is then defined by calculating the Legendre transform of the
entropy under certain constraints. The application of the model is limited to
flexible polymers. It exhibits a gradual transition from compact globule to
rod. The boundary line between these two phases shows reentrant behavior. This
behavior is explained by the presence of residual entropy.Comment: 5 pages, 4 figures, extended version of arXiv:cond-mat/061225
A macroscopic model for sessile droplet evaporation on a flat surface
The evaporation of sessile droplets on a flat surface involves a complex
interplay between phase change, diffusion, advection and surface forces. In an
attempt to significantly reduce the complexity of the problem and to make it
manageable, we propose a simple model hinged on a surface free energy-based
relaxation dynamics of the droplet shape, a diffusive evaporation model and a
contact line pinning mechanism governed by a yield stress. Our model reproduces
the known dynamics of droplet shape relaxation and of droplet evaporation, both
in the absence and in the presence of contact line pinning. We show that shape
relaxation during evaporation significantly affects the lifetime of a drop. We
find that the dependence of the evaporation time on the initial contact angle
is a function of the competition between the shape relaxation and evaporation,
and is strongly affected by any contact line pinning.Comment: 13 pages, 8 figure
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