774 research outputs found
Discontinuities without discontinuity: The Weakly-enforced Slip Method
Tectonic faults are commonly modelled as Volterra or Somigliana dislocations
in an elastic medium. Various solution methods exist for this problem. However,
the methods used in practice are often limiting, motivated by reasons of
computational efficiency rather than geophysical accuracy. A typical
geophysical application involves inverse problems for which many different
fault configurations need to be examined, each adding to the computational
load. In practice, this precludes conventional finite-element methods, which
suffer a large computational overhead on account of geometric changes. This
paper presents a new non-conforming finite-element method based on weak
imposition of the displacement discontinuity. The weak imposition of the
discontinuity enables the application of approximation spaces that are
independent of the dislocation geometry, thus enabling optimal reuse of
computational components. Such reuse of computational components renders
finite-element modeling a viable option for inverse problems in geophysical
applications. A detailed analysis of the approximation properties of the new
formulation is provided. The analysis is supported by numerical experiments in
2D and 3D.Comment: Submitted for publication in CMAM
On the singular nature of the elastocapillary ridge
The functionality of soft interfaces is crucial to many applications in
biology and surface science. Recent studies have used liquid drops to probe the
surface mechanics of elastomeric networks. Experiments suggest an intricate
surface elasticity, also known as the Shuttleworth effect, where surface
tension is not constant but depends on substrate deformation. However,
interpretations have remained controversial due to singular elastic
deformations, induced exactly at the point where the droplet pulls the network.
Here we reveal the nature of the elastocapillary singularity on a hyperelastic
substrate with various constitutive relations for the interfacial energy.
First, we finely resolve the vicinity of the singularity using goal-adaptive
finite element simulations. This confirms the universal validity, also at large
elastic deformations, of the previously disputed Neumann's law for the contact
angles. Subsequently, we derive exact solutions of nonlinear elasticity that
describe the singularity analytically. These solutions are in perfect agreement
with numerics, and show that the stretch at the contact line, as previously
measured experimentally, consistently points to a strong Shuttleworth effect.
Finally, using Noether's theorem we provide a quantitative link between wetting
hysteresis and Eshelby-like forces, and thereby offer a complete framework for
soft wetting in the presence of the Shuttleworth effect.Comment: 17 Pages, 7 figure
Electrovibrostimulation during the training of sportsmen, an experimental set-up
In this study the effects of electrovibrostimulation training are compared with classical powertraining, in a group of 18-24 years old healthy sportsmen, during the spring season of 2007. Former studies showed the effectiveness of electrovibrostimulation on the functional condition of muscles: increase of the strength in the active insufficiency zone and increase of the elasticity in the passive insufficiency zone, consequently improving joint strength and mobility. After four weeks of electrovibrostimulation, applied twice a week, significant increases of isometric strength and explosive strength were observed, whereas the flexibility remained the same. Some hypotheses are discussed concerning the possible working mechanisms of electrovibrostimulatio
Morphology and function of dog arterial grafts preserved in UW-solution
Objectives:To assess the function of arterial grafts after prolonged preservation in the University of Wisconsin solution (UW), in vitro and in vivo.Methods:Carotid arteries were harvested from dogs and stored for 1–21 days at 4°C in UW (n = 10) or in PBS (0.9% NaCl, pH 7.4), (PBS) (n = 10). Slices were examined by lightmicroscopy (LM) and scanning electron microscopy (SEM). For viability testing, specimens were connected to an isometric force transducer (2 × n = 9). Contractile and relaxation responses were examined by adding phenylephrine (200μM) and metacholine (200μM), respectively. For in vivo studies (n = 41), 2.5cm carotid artery segments were implanted orthotopically, as autografts and allografts, after 14 days of storage in UW or in PBS. Autologous veins were used as controls. After 28 days or 56 days, arteriography was performed and the grafts were excised for LM and SEM.Results:The arterial endothelial layer remained intact after up to 14 days of storage in UW. In PBS, the endothelium was lost after 3 days. The functional response after 14 days storage in UW was approximately 50% vs. 0% after 14 days in PBS. In the autografts, total patencies (28 days + 56 days) were 100% (8/8) and 63% (5/8) for UW and PBS stored grafts, respectively. In the allografts, the UW and PBS preserved grafts showed total patencies of 86% (12/14) and 83% (5/6), respectively. Microscopically, the allografts showed fibrotic degeneration.Conclusions:Arteries are well preserved in UW up to 14 days of storage. Arterial autografts preserved in UW showed good patency and better integrity of the vessel wall after implantation, than grafts stored in PBS or allografts (without immunosuppressive therapy)
First LOFAR results on galaxy clusters
Deep radio observations of galaxy clusters have revealed the existence of
diffuse radio sources related to the presence of relativistic electrons and
weak magnetic fields in the intracluster volume. The role played by this
non-thermal intracluster component on the thermodynamical evolution of galaxy
clusters is debated, with important implications for cosmological and
astrophysical studies of the largest gravitationally bound structures of the
Universe. The low surface brightness and steep spectra of diffuse cluster radio
sources make them more easily detectable at low-frequencies. LOFAR is the first
instrument able to detect diffuse radio emission in hundreds of massive galaxy
clusters up to their formation epoch. We present the first observations of
clusters imaged by LOFAR and the huge perspectives opened by this instrument
for non-thermal cluster studies.Comment: Proceedings of the 2012 week of the French Society of Astronomy and
Astrophysics (SF2A) held in Nice, June 5th-8t
Challenges of a transition to a sustainably managed shrimp culture agro-ecosystem in the Mahakam delta, East Kalimantan, Indonesia
Around 1990, when in other countries mangrove protection took off, massive conversion of mangrove forest into shrimp ponds started in the Mahakam delta. To identify constraints to and options for sustainable management we analysed institutions and constraints with stakeholders. In 3 sites we used participatory tools and a complementary survey to assess the livelihood framework. Since 1970, ponds for shrimp farming gradually replaced 75% of mangrove forested area. After 2004, recovery of mangrove took off, as, mainly due to low shrimp yields, ponds were abandoned. In 2008, 54% of the delta was dedicated to ponds for shrimp production. Around 80% of livelihood activities of pond-farmers, pond caretakers, and fishermen was related to mangroves. The involvement of men and women in these activities varied between sites and types. Poor households depended more on mangroves. Most activities resulted in seasonal income peaks; only a few activities resulted in a full daily livelihood. Ponds, on the other hand, provide 50% of households’ livelihood, but this remains vulnerable in the context of the risky shrimp production. Skewed land holding, unequal sharing of benefits, competing claims and vested interests of stakeholders pose a great challenge to a transition to a more sustainable use of the mangrove area. In particular, ponds located on peat soils are non-sustainable and would require full restoration into mangrove; ponds on other soils could best be transformed into a mixed mangrove-pond system using a ‘green-water’ technology
Nile perch distribution in south-east Lake Victoria is more strongly driven by abiotic factors, than by prey densities
Abstract We studied the effects of environmental driving factors (maximum depth, visibility, oxygen, temperature, and prey densities) on the distribution and diet composition of Nile perch (Lates niloticus) in south-east Lake Victoria from 2009 to 2011. We tested the hypotheses that (i) Nile perch distribution is regulated by the same environmental factors on a local scale (Mwanza Gulf) and on a regional scale (Mwanza Gulf, Speke Gulf and the open lake in Sengerema district), and (ii) driving factors act differently on different Nile perch size classes. Fish were sampled with gillnets. Nile perch densities were highest in the shallow part of the Mwanza Gulf and during the wet seasons, mainly caused by high densities of juveniles. The environmental driving factors explained Nile perch distributions on both regional and local scales in a similar way, often showing non-linear relationships. Maximum depth and temperature were the best predictors of Nile perch densities. Prey densities of shrimp and haplochromines did not strongly affect Nile perch distributions, but did explain Nile perch diet on a local and regional scale. We conclude that abiotic variables drive Nile perch distributions more strongly than prey densities and that feeding takes place opportunistically
Chronic ethanol attenuates centrally-mediated hypotension elicited via alpha-2-adrenergic, but not I1-imidazoline, receptor activation in female rats
Aims—This study dealt with the effect of chronic ethanol administration on hemodynamic responses
elicited by α2-adrenergic (α-methyldopa) or I1-imidazoline (rilmenidine) receptor activation in
telemetered female rats.
Main methods—The effects of α-methyldopa or rilmenidine on blood pressure (BP), heart rate
(HR) and their variability were investigated in rats that received liquid diet without or with ethanol
(5% w/v) for 12 weeks. To evaluate the effect of each drug on cardiovascular autonomic control (BP
and HR variability) in the absence or presence of ethanol, three time-domain indices of hemodynamic
variability were measured: (i) standard deviation of mean arterial pressure (SDMAP), (ii) standard
deviation of beat-to-beat intervals, and (iii) root mean square of successive differences in R-R
intervals.
Key findings—In liquid diet-fed control rats, i.p. rilmenidine (600 μg/kg) or α-methyldopa (100
mg/kg) reduced BP along with decreases and increases, respectively, in HR. Both drugs had no effect
on HR variability but reduced BP variability (SDMAP), suggesting a reduced vasomotor sympathetic
tone. Ethanol feeding attenuated reductions in BP and SDMAP evoked by α-methyldopa but not by
rilmenidine.
Significance—We conclude that chronic ethanol preferentially compromises α2- but not I1-
receptor-mediated hypotension in female rats probably via modulation of vasomotor sympathetic
activity. These findings highlight the adequacy of rilmenidine use to lower BP in hypertensive
alcoholic females
Optimized Trigger for Ultra-High-Energy Cosmic-Ray and Neutrino Observations with the Low Frequency Radio Array
When an ultra-high energy neutrino or cosmic ray strikes the Lunar surface a
radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to
detect these pulses. In this work we propose an efficient trigger
implementation for LOFAR optimized for the observation of short radio pulses.Comment: Submitted to Nuclear Instruments and Methods in Physics Research
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