8,835 research outputs found
Influence of pore-scale disorder on viscous fingering during drainage
We study viscous fingering during drainage experiments in linear Hele-Shaw
cells filled with a random porous medium. The central zone of the cell is found
to be statistically more occupied than the average, and to have a lateral width
of 40% of the system width, irrespectively of the capillary number . A
crossover length separates lower scales where the
invader's fractal dimension is identical to capillary fingering,
and larger scales where the dimension is found to be . The lateral
width and the large scale dimension are lower than the results for Diffusion
Limited Aggregation, but can be explained in terms of Dielectric Breakdown
Model. Indeed, we show that when averaging over the quenched disorder in
capillary thresholds, an effective law relates the
average interface growth rate and the local pressure gradient.Comment: 4 pages, 4 figures, submitted to Phys Rev Letter
Charge-coupled devices with fast timing for astrophysics and space physics research
A charge coupled device is under development with fast timing capability (15 millisecond full frame readout, 30 microsecond resolution for measuring the time of individual pixel hits). The fast timing CCD will be used in conjunction with a CsI microfiber array or segmented scintillator matrix detector to detect x rays and gamma rays with submillimeter position resolution. The initial application will be in conjunction with a coded aperture hard x ray/gamma ray astronomy instrument. We describe the concept and the readout architecture of the device
The Highly Eccentric Pre-Main Sequence Spectroscopic Binary RX J0529.3+1210
The young system RX J0529.3+1210 was initially identified as a single-lined
spectroscopic binary. Using high-resolution infrared spectra, acquired with
NIRSPEC on Keck II, we measured radial velocities for the secondary. The method
of using the infrared regime to convert single-lined spectra into double-lined
spectra, and derive the mass ratio for the binary system, has been successfully
used for a number of young, low-mass binaries. For RX J0529.3+1210, a long-
period(462 days) and highly eccentric(0.88) binary system, we determine the
mass ratio to be 0.78+/-0.05 using the infrared double-lined velocity data
alone, and 0.73+/-0.23 combining visible light and infrared data in a full
orbital solution. The large uncertainty in the latter is the result of the
sparse sampling in the infrared and the high eccentricity: the stars do not
have a large velocity separation during most of their ~1.3 year orbit. A mass
ratio close to unity, consistent with the high end of the one sigma uncertainty
for this mass ratio value, is inconsistent with the lack of a visible light
detection of the secondary component. We outline several scenarios for a color
difference in the two stars, such as one heavily spotted component, higher
order multiplicity, or a unique evolutionary stage, favoring detection of only
the primary star in visible light, even in a mass ratio ~1 system. However, the
evidence points to a lower ratio. Although RX J0529.3+1210 exhibits no excess
at near-infrared wavelengths, a small 24 micron excess is detected, consistent
with circumbinary dust. The properties of this binary and its membership in
Lambda Ori versus a new nearby stellar moving group at ~90 pc are discussed. We
speculate on the origin of this unusual system and on the impact of such high
eccentricity on the potential for planet formation.Comment: 4 Figure
Magnetic Order and Dynamics in an Orbitally Degenerate Ferromagnetic Insulator
Neutron scattering was used to determine the spin structure and the magnon
spectrum of the Mott--Hubbard insulator YTiO. The magnetic structure is
complex, comprising substantial G-type and A-type antiferromagnetic components
in addition to the predominant ferromagnetic component. The magnon spectrum, on
the other hand, is gapless and nearly isotropic. We show that these findings
are inconsistent with the orbitally ordered states thus far proposed for
YTiO and discuss general implications for a theoretical description of
exchange interactions in orbitally degenerate systems.Comment: to appear in Phys. Rev. Let
PROBAVNI SUSTAV KOĆ TUNJAÄA PERSORBIRA MIKROÄESTICE
The ability of the teleost gut to absorb microparticulate material was examined following rectal intubation (3.5 g kg -1 ) of commercial grade cornstarch (â21 mm diameter), or potato starch (â43 mm diameter). Tissue samples were taken from the mid - and hind-gut of control and treated fish 18 h postintubation. Collected samples were processed using standard plastic and staining protocols and resultant photomicrographs examined by computer-assisted image analysis. Cornstarch particles (8-14 mm), were observed to pass from gut lumen to the lamina propria via a paracellular or persorptive route only. No evidence for the like passage of potato starch was found.MoguÄnost probavnog sustava koĆĄtunjaviÄastih vrsta riba da apsorbiraju sitne Äestice pojedinih tvari bila je istraĆŸivana s pomoÄu rektalne intubacije (3, 5 g kg -1 ) komercijalnoga ĆĄkroba kukuruza (promjera â21 mm) i ĆĄkroba krumpira (promjera â 43 mm). Uzorci tkiva uzeti su iz srednjeg i straĆŸnjeg dijela crijeva kontrolne i tretirane skupine riba, i to 18 sati nakon intubacije. Sakupljeni su uzorci spremljeni u standardne plastike i obojeni prema protokolu, te fotomikrografski snimljeni na raÄunalu. Äestice ĆĄkroba kukuruza (8-14 mm) zapaĆŸene su da iz lumena crijeva prolaze u laminu propriju samo paracelularnim ili persorptivnim putem. No, nije zapaĆŸen prolazak Äestica ĆĄkroba krumpira
Scaling Relations of Viscous Fingers in Anisotropic Hele-Shaw Cells
Viscous fingers in a channel with surface tension anisotropy are numerically
studied. Scaling relations between the tip velocity v, the tip radius and the
pressure gradient are investigated for two kinds of boundary conditions of
pressure, when v is sufficiently large. The power-law relations for the
anisotropic viscous fingers are compared with two-dimensional dendritic growth.
The exponents of the power-law relations are theoretically evaluated.Comment: 5 pages, 4 figure
The K\"ahler-Ricci flow on surfaces of positive Kodaira dimension
The existence of K\"ahler-Einstein metrics on a compact K\"ahler manifold has
been the subject of intensive study over the last few decades, following Yau's
solution to Calabi's conjecture. The Ricci flow, introduced by Richard Hamilton
has become one of the most powerful tools in geometric analysis.
We study the K\"ahler-Ricci flow on minimal surfaces of Kodaira dimension one
and show that the flow collapses and converges to a unique canonical metric on
its canonical model. Such a canonical is a generalized K\"ahler-Einstein
metric. Combining the results of Cao, Tsuji, Tian and Zhang, we give a metric
classification for K\"aher surfaces with a numerical effective canonical line
bundle by the K\"ahler-Ricci flow. In general, we propose a program of finding
canonical metrics on canonical models of projective varieties of positive
Kodaira dimension
Clarifying the distal to proximal tephrochronology of the Millennium (BâTm) eruption, Changbaishan Volcano, northeast China
Tephra dispersed during the Millennium eruption (ME), Changbaishan Volcano, NE China provides one of the key stratigraphic links between Asia and Greenland for the synchronization of palaeoenvironmental records. However, controversy surrounds proximal-distal tephra correlations because (a) the proposed proximal correlatives of the distal ME tephra (i.e. B-Tm) lack an unequivocal chronostratigraphic context, and (b) the ME tephra deposits have not been chemically characterized for a full spectrum of element using grain-specific techniques. Here we present grain-specific glass chemistry, including for the first time, single grain trace element data, for a composite proximal sequence and a distal tephra from Lake Kushu, northern Japan (ca. 1100 km away from Changbaishan). We demonstrate a robust proximal-distal correlation and that the Kushu tephra is chemically associated with the ME/B-Tm. We propose that three of the proximal pyroclastic fall units were erupted as part of the ME. The radiocarbon chronology of the Kushu sedimentary record has been utilised to generate a Bayesian age-depth model, providing an age for the Kushu tephra which is consistent with high resolution ages determined for the eruption and therefore supports our geochemical correlation. Two further Bayesian age-depth models were independently constructed each incorporating one of two ice-core derived ages for the B-Tm tephra, providing Bayesian modelled ages of 933-949 and 944-947 cal AD (95.4%) for the Kushu tephra. The high resolution ice-core tephra ages imported into the deposition models help test and ultimately constrain the radiocarbon chronology in this interval of the Lake Kushu sedimentary record. The observed geochemical diversity between proximal and distal ME tephra deposits clearly evidences the interaction of two compositionally distinct magma batches during this caldera forming eruption
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