17,331 research outputs found
Mass transfer, fluid flow and membrane properties in flat and corrugated plate hyperfiltration modules
Concentration polarisation, decreasing the efficiency in membrane separation processes, can be reduced by increasing mass transfer between membrane surface and bulk of the feed stream. Analogous to techniques used in plate heat exchangers efforts have been made to enhance mass transfer in a plate hyperfiltration module by using a corrugated membrane in stead of a flat one. The corrugations are pressed into an originally flat membrane. These corrugations do not only have an influence on the mass transfer, but also on such membrane properties as salt and water permeability. Corrugations enhance mass transfer in a more effective way than increase of flow rate does.\ud
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The effect of the corrugations on membrane properties shows a large spread. For corrugated membranes prepared by our group, flux increases of 100% at almost the same or even slightly higher retentions have been obtained
Impact and extinction signatures in complete Cretaceous-Tertiary (K-T) boundary sections
The Zumaya, Caravaca and Agost sections in Spain, the El Kef section in Tunisia and the Negev (Nahal Avdat) sections in Israel are among the most continuous, expanded and complete K-T boundary sections. The distribution patterns of the planktic faunas were quantitatively analyzed in closely spaced samples across the K-T boundary in these sections, in conjuction with the geochemistry, stable isotopes, mineralogy and magnetostratigraphy. Three hundred foraminiferal specimens were randomly selected and determined. Reliable estimates for the foraminiferal productivity changes across the K-T boundary and for the 1 to 2 Ma interval preceding the K-T boundary were made from the numbers of individuals/gram of sediment corrected for the sedimentation rates (calculated from magnetic reversals and lithology). No gradual or stepwise extinction is seen below the K-T boundary nor any productivity decrease. Stable isotope analyses show a warming just after deposition of the ejecta layer, not cooling as predicted by nuclear winter scenarios, although the duration of such cooling may be too short to be observed even in these complete sections. Low REE values and cpx spherules with quench textures idential to quench-textures in diagenetically altered spherules, strongly indicate an oceanic site of (one of) the impactor(s)
Diffusion in a crowded environment
We analyze a pair of diffusion equations which are derived in the infinite
system--size limit from a microscopic, individual--based, stochastic model.
Deviations from the conventional Fickian picture are found which ultimately
relate to the depletion of resources on which the particles rely. The
macroscopic equations are studied both analytically and numerically, and are
shown to yield anomalous diffusion which does not follow a power law with time,
as is frequently assumed when fitting data for such phenomena. These anomalies
are here understood within a consistent dynamical picture which applies to a
wide range of physical and biological systems, underlining the need for clearly
defined mechanisms which are systematically analyzed to give definite
predictions.Comment: 4 pages, 3 figures, minor change
Shear flow, viscous heating, and entropy balance from dynamical systems
A consistent description of a shear flow, the accompanied viscous heating,
and the associated entropy balance is given in the framework of a deterministic
dynamical system, where a multibaker dynamics drives two fields: the velocity
and the temperature distributions. In an appropriate macroscopic limit their
transport equations go over into the Navier-Stokes and the heat conduction
equation of viscous flows. The inclusion of an artificial heat sink can
stabilize steady states with constant temperatures. It mimics a thermostating
algorithm used in non-equilibrium molecular-dynamics simulations.Comment: LaTeX 2e (epl.cls + sty-files for Europhys Lett included); 7 pages +
1 eps-figur
Relativistic Nucleus-Nucleus Collisions: Zone of Reactions and Space-Time Structure of a Fireball
A zone of reactions is determined and then exploited as a tool in studying
the space-time structure of an interacting system formed in a collision of
relativistic nuclei. The time dependence of the reaction rates integrated over
spatial coordinates is also considered. Evaluations are made with the help of
the microscopic transport model UrQMD. The relation of the boundaries of
different zones of reactions and the hypersurfaces of sharp chemical and
kinetic freeze-outs is discussed.Comment: 6 pages, 5 figure
Tiling strategies for optical follow-up of gravitational wave triggers by wide field of view telescopes
Binary neutron stars are among the most promising candidates for joint
gravitational-wave and electromagnetic astronomy. The goal of this work is to
investigate the strategy of using gravitational wave sky-localizations for
binary neutron star systems, to search for electromagnetic counterparts using
wide field of view optical telescopes. We examine various strategies of
scanning the gravitational wave sky-localizations on the mock 2015-16
gravitational-wave events. We propose an optimal tiling-strategy that would
ensure the most economical coverage of the gravitational wave sky-localization,
while keeping in mind the realistic constrains of transient optical astronomy.
Our analysis reveals that the proposed tiling strategy improves the
sky-localization coverage over naive contour-covering method. The improvement
is more significant for observations conducted using larger field of view
telescopes, or for observations conducted over smaller confidence interval of
gravitational wave sky-localization probability distribution. Next, we
investigate the performance of the tiling strategy for telescope arrays and
compare their performance against monolithic giant field of view telescopes. We
observed that distributing the field of view of the telescopes into arrays of
multiple telescopes significantly improves the coverage efficiency by as much
as 50% over a single large FOV telescope in 2016 localizations while scanning
around 100 sq. degrees. Finally, we studied the ability of optical counterpart
detection by various types of telescopes. In Our analysis for a range of wide
field-of-view telescopes we found improvement in detection upon sacrificing
coverage of localization in order to achieve greater observation depth for very
large field-of-view - small aperture telescopes, especially if the intrinsic
brightness of the optical counterparts are weak.Comment: Accepted for publication in A&A. 10 pages, 10 figure
Kinematics of the ultracompact helium accretor AM canum venaticorum
We report on the results from a five-night campaign of high-speed spectroscopy of the 17-min binary AM Canum Venaticorum (AM CVn), obtained with the 4.2-m William Herschel Telescope on La Palma.
We detect a kinematic feature that appears to be entirely analogous to the 'central spike' known from the long-period, emission-line AM CVn stars GP Com, V396 Hya and SDSS J124058.03-015919.2, which has been attributed to the accreting white dwarf. Assuming that the feature indeed represents the projected velocity amplitude and phase of the accreting white dwarf, we derive a mass ratio q = 0.18 +/- 0.01 for AM CVn. This is significantly higher than the value found in previous, less direct measurements. We discuss the implications for AM CVn's evolutionary history and show that a helium star progenitor scenario is strongly favoured. We further discuss the implications for the interpretation of AM CVn's superhump behaviour, and for the detectability of its gravitational-wave signal with the Laser Interferometer Space Antenna (LISA).
In addition, we demonstrate a method for measuring the circularity or eccentricity of AM CVn's accretion disc, using stroboscopic Doppler tomography. We test the predictions of an eccentric, precessing disc that are based on AM CVn's observed superhump behaviour. We limit the effective eccentricity in the outermost part of the disc, where the resonances that drive the eccentricity are thought to occur, to e = 0.04 +/- 0.01, which is smaller than previous models indicated
Density dynamics from current auto-correlations at finite time- and length-scales
We consider the increase of the spatial variance of some inhomogeneous,
non-equilibrium density (particles, energy, etc.) in a periodic quantum system
of condensed matter-type. This is done for a certain class of initial quantum
states which is supported by static linear response and typicality arguments.
We directly relate the broadening to some current auto-correlation function at
finite times. Our result is not limited to diffusive behavior, however, in that
case it yields a generalized Einstein relation. These findings facilitate the
approximation of diffusion constants/conductivities on the basis of current
auto-correlation functions at finite times for finite systems. Pursuing this,
we quantitatively confirm the magnetization diffusion constant in a spin chain
which was recently found from non-equilibrium bath scenarios.Comment: 4 pages, 1 figure, accepted for publication in Europhys. Let
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