406 research outputs found
Minimal Model for Sand Dunes
We propose a minimal model for aeolian sand dunes. It combines an analytical
description of the turbulent wind velocity field above the dune with a
continuum saltation model that allows for saturation transients in the sand
flux. The model provides a qualitative understanding of important features of
real dunes, such as their longitudinal shape and aspect ratio, the formation of
a slip face, the breaking of scale invariance, and the existence of a minimum
dune size.Comment: 4 pages, 4 figures, replaced with publishd versio
Linear stability analysis of transverse dunes
Sand-moving winds blowing from a constant direction in an area of high sand
availability form transverse dunes, which have a fixed profile in the direction
orthogonal to the wind. Here we show, by means of a linear stability analysis,
that transverse dunes are intrinsically unstable. Any along-axis perturbation
on a transverse dune amplify in the course of dune migration due to the
combined effect of two main factors, namely: the lateral transport through
avalanches along the dune's slip-face, and the scaling of dune migration
velocity with the inverse of the dune height. Our calculations provide a
quantitative explanation for recent observations from experiments and numerical
simulations, which showed that transverse dunes moving on the bedrock cannot
exist in a stable form and decay into a chain of crescent-shaped barchans.Comment: 8 pages, 4 figure
Topically applied vitamin C increases the density of dermal papillae in aged human skin
BACKGROUND: The influence of ageing on the density of the functional entities of the papillae containing nutritive capillaries, here in terms as the papillary index, and the effect of topically applied vitamin C were investigated by confocal laser scanning microscopy (CLSM) in vivo. METHODS: The age dependency of the papillary index was determined by CLSM on 3 different age groups. Additionally, we determined the effect of a topical cream containing 3% vitamin C against the vehicle alone using daily applications for four months on the volar forearm of 33 women. RESULTS: There were significant decreases in the papillary index showing a clear dependency on age. Topical vitamin C resulted in a significant increase of the density of dermal papillae from 4 weeks onward compared to its vehicle. Reproducibility was determined in repeated studies. CONCLUSIONS: Vitamin C has the potential to enhance the density of dermal papillae, perhaps through the mechanism of angiogenesis. Topical vitamin C may have therapeutical effects for partial corrections of the regressive structural changes associated with the aging process
On the shape of barchan dunes
Barchans are crescent-shaped sand dunes forming in aride regions with
unidirectional wind and limited sand supply. We report analytical and numerical
results for dune shapes under different environmental conditions as obtained
from the so-called `minimal model' of aeolian sand dunes. The profiles of
longitudinal vertical slices (i.e. along the wind direction) are analyzed as a
function of wind speed and sand supply. Shape transitions can be induced by
changes of mass, wind speed and sand supply. Within a minimal extension of the
model to the transverse direction the scale-invariant profile of transverse
vertical cuts can be derived analytically.Comment: to appear in J. Phys.: Condens. Matter 17 (2005
Corridors of barchan dunes: stability and size selection
Barchans are crescentic dunes propagating on a solid ground. They form dune
fields in the shape of elongated corridors in which the size and spacing
between dunes are rather well selected. We show that even very realistic models
for solitary dunes do not reproduce these corridors. Instead, two instabilities
take place. First, barchans receive a sand flux at their back proportional to
their width while the sand escapes only from their horns. Large dunes
proportionally capture more than they loose sand, while the situation is
reversed for small ones: therefore, solitary dunes cannot remain in a steady
state. Second, the propagation speed of dunes decreases with the size of the
dune: this leads -- through the collision process -- to a coarsening of barchan
fields. We show that these phenomena are not specific to the model, but result
from general and robust mechanisms. The length scales needed for these
instabilities to develop are derived and discussed. They turn out to be much
smaller than the dune field length. As a conclusion, there should exist further
- yet unknown - mechanisms regulating and selecting the size of dunes.Comment: 13 pages, 13 figures. New version resubmitted to Phys. Rev. E.
Pictures of better quality available on reques
Wer bremst verliert - Antimon in Straßenrandböden
Seit dem weitreichenden Verbot von Asbest in den 1980er Jahren wurde verstärkt das Halbmetall Antimon (Sb) in Bremsbelägen von PKW und LKW verwendet. Im Zuge des Bremsvorganges wird dieses in Form von Feinstaub freigesetzt und straßennah in die Böden eingetragen. Grundsätzlich sind Antimoneinträge in Böden negativ zu bewerten, da das Halbmetall toxische Wirkungen hervorrufen kann. Um die räumliche Verteilung von Antimon und seinen Bindungsformen in Straßenrandböden zu untersuchen, wurden 5 parallele Transekte mit jeweils 7 Entfernungen und 4 Probentiefen entlang einer Kölner Kreisstraße (Verkehrsaufkommen ca. 9.600 Fahrzeuge pro Tag) beprobt. Neben einer allgemeinen physikochemischen Charakterisierung der Bodenproben wurden ihre königswasser-löslichen Antimongehalte bestimmt. Zusätzlich wurde an einem Transekt eine sequentielle Extraktion nach Wenzel et al. 2001 durchgeführt, um die Bindungsformen von Antimon zu ermitteln. Die Antimongehalte nahmen von bis zu 6,18 g kg–1 unmittelbar am Straßenrand bis auf 0,513 g kg–1 in 10 m Entfernung ab. Ebenfalls konnte eine deutliche Abnahme mit der Bodentiefe (0 bis 30 cm) festgestellt werden. Dieses Verteilungsmuster deutet auf den Verkehr als Antimoneintragsquelle hin. Als Hinweis auf einen durch den Bremsabrieb metallener Bremsscheiben dominierten Eintrag wurde die magnetische Suszeptibilität gemessen. Allerdings lieferte der Korrelationskoeffizient (Spearman) von Antimon gegenüber der magnetischen Suszeptibilität 0,423 (p = 0,023) nur einen eher schwachen Beleg. Die ursprünglich für Arsen entwickelte sequentielle Extraktion ließ sich reproduzierbar ebenfalls für Antimon anwenden. Anhand dieser konnten im Wesentlichen drei Trends der Bindungsformen erfasst werden: i) spezifisch gebundenes Antimon (Fraktion 2: 0,05 mol l–1 (NH4)H2P2O4) konnte lediglich innerhalb 1 m Entfernung zur Straße und mit der Bodentiefen abnehmenden Gehalten nachgewiesen werden, ii) der Anteil an schwachkristallinen Eisenoxiden gebundenem Antimon (Fraktion 3: 0,2 mol l–1 (NH4)2-Oxalatpuffer; pH 3,25) nahm signifikant mit steigender Entfernung zur Straße ab, während iii) die prozentualen Gehalte in den starkkristallinen Eisenoxiden (Fraktion 4: 0,2 mol l–1 (NH4)2-Oxalatpuffer + 0,1 mol l–1 Ascorbinsäure; pH 3,25) und in der Residualphase (Fraktion 5: Königswasser) zunahmen
A Continuum Saltation Model for Sand Dunes
We derive a phenomenological continuum saltation model for aeolian sand
transport that can serve as an efficient tool for geomorphological
applications. The coupled differential equations for the average density and
velocity of sand in the saltation layer reproduce both known equilibrium
relations for the sand flux and the time evolution of the sand flux as
predicted by microscopic saltation models. The three phenomenological
parameters of the model are a reference height for the grain-air interaction,
an effective restitution coefficient for the grain-bed interaction, and a
multiplication factor characterizing the chain reaction caused by the impacts
leading to a typical time or length scale of the saturation transients. We
determine the values of these parameters by comparing our model with wind
tunnel measurements. Our main interest are out of equilibrium situations where
saturation transients are important, for instance at phase boundaries
(ground/sand) or under unsteady wind conditions. We point out that saturation
transients are indispensable for a proper description of sand flux over
structured terrain, by applying the model to the windward side of an isolated
dune, thereby resolving recently reported discrepancies between field
measurements and theoretical predictions.Comment: 11 pages, 7 figure
Chiral SU(3) Dynamics with Coupled Channels: Inclusion of P-Wave Multipoles
We extend our recent non-perturbative chiral SU(3) coupled channel approach
to pion- and photon-induced - and -meson production off protons by
including all strong and electromagnetic p-wave multipoles. We identify the
p-wave amplitudes of the next-to-leading order SU(3) chiral meson-baryon
Lagrangian with a coupled channel potential which is iterated to infinite
orders in a separable Lippmann-Schwinger equation. Our approach to - and
-photoproduction introduces no additional free parameters. By adjusting a
few finite range parameters and the unknown parameters in the Lagrangian, we
are able to simultaneously describe a very large amount of low-energy data.
These include the total and differential cross sections of the -induced
reactions and
as well as those of photoproduction . The polarization observables
measured in - and -photoproduction are particularly sensitive to
interference terms between the s- and p-wave multipoles. The total cross
section data are remarkably well reproduced in all channels. There remain,
however, some open questions concerning details of angular distributions and
polarization observables.Comment: 20 pages, 5 figures, accepted for publication in Nucl. Phys.
Mucosal prior to systemic application of recombinant adenovirus boosting is more immunogenic than systemic application twice but confers similar protection against SIV-challenge in DNA vaccine-primed macaques
AbstractWe investigated the immunogenicity and efficacy of a bimodal prime/boost vaccine regimen given by various routes in the Simian immunodeficiency virus (SIV) rhesus monkey model for AIDS. Twelve animals were immunized with SIV DNA-vectors followed by the application of a recombinant adenovirus (rAd5) expressing the same genes either intramuscularly (i.m.) or by oropharyngeal spray. The second rAd5-application was given i.m. All vaccinees plus six controls were challenged orally with SIVmac239 12 weeks post-final immunization.Both immunization strategies induced strong SIV Gag-specific IFN-γ and T-cell proliferation responses and mediated a conservation of CD4+ memory T-cells and a reduction of viral load during peak viremia following infection. Interestingly, the mucosal group was superior to the systemic group regarding breadth and strength of SIV-specific T-cell responses and exhibited lower vector specific immune responses. Therefore, our data warrant the inclusion of mucosal vector application in a vaccination regimen which makes it less invasive and easier to apply
10 simple rules to create a serious game, illustrated with examples from structural biology
Serious scientific games are games whose purpose is not only fun. In the
field of science, the serious goals include crucial activities for scientists:
outreach, teaching and research. The number of serious games is increasing
rapidly, in particular citizen science games, games that allow people to
produce and/or analyze scientific data. Interestingly, it is possible to build
a set of rules providing a guideline to create or improve serious games. We
present arguments gathered from our own experience ( Phylo , DocMolecules ,
HiRE-RNA contest and Pangu) as well as examples from the growing literature on
scientific serious games
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