814 research outputs found
Development of a Computationally Efficient Fabric Model for Optimization of Gripper Trajectories in Automated Composite Draping
An automated prepreg fabric draping system is being developed which consists
of an array of actuated grippers. It has the ability to pick up a fabric ply
and place it onto a double-curved mold surface. A previous research effort
based on a nonlinear Finite Element model showed that the movements of the
grippers should be chosen carefully to avoid misplacement and induce of
wrinkles in the draped configuration. Thus, the present study seeks to develop
a computationally efficient model of the mechanical behavior of a fabric based
on 2D catenaries which can be used for optimization of the gripper
trajectories. The model includes bending stiffness, large deflections, large
ply shear and a simple contact formulation. The model is found to be quick to
evaluate and gives very reasonable predictions of the displacement field
Survival of the Mýrdalsjökull ice cap through the Holocene thermal maximum: evidence from sulphur contents in Katla tephra layers (Iceland) from the last ∼8400 years
International audienceThe climate in Iceland was drier and warmer during the Holocene thermal maximum than it is today and it has been suggested that ice caps disappeared entirely. Katla, a volcano covered by the Mýrdalsjökull ice cap in southern Iceland, has erupted rather steadily throughout the Holocene. Preand post-eruption sulphur concentrations in its products have been determined in previous studies, through melt inclusions trapped in phenocrysts (pre-eruption mean values of 2155 ± 165 ppm) and fully degassed magmatic tephra (post-eruption mean values of 445 ± 130 ppm). The phreatomagmatic tephra has much more variable S contents (550-1775 ppm) and spans the compositional gap between magmatic tephra and melt inclusions. These variable sulphur values are attributed to arresting of degassing as the magma is quenched upon contact with external water in the shallow levels of the volcano conduit. Sulphur in Katla tephra can thus be used to evaluate whether Mýrdalsjökull survived the warm spells of the Holocene. In this study, sulphur concentrations in tephra layers representing the last ∼8400 years of the volcano's eruption history were measured, revealing concentrations in the phreatomagmatic range (600-1600 ppm). Hence, we conclude that over the last ∼8400 years, explosive activity at Katla has been dominated by phreatomagmatic eruptions, implying that the Mýrdalsjökull ice cap has been present throughout the Holocene
Afterglow Light Curves and Broken Power Laws: A Statistical Study
In gamma-ray burst research it is quite common to fit the afterglow light
curves with a broken power law to interpret the data. We apply this method to a
computer simulated population of afterglows and find systematic differences
between the known model parameters of the population and the ones derived from
the power law fits. In general, the slope of the electron energy distribution
is overestimated from the pre-break light curve slope while being
underestimated from the post-break slope. We also find that the jet opening
angle derived from the fits is overestimated in narrow jets and underestimated
in wider ones. Results from fitting afterglow light curves with broken power
laws must therefore be interpreted with caution since the uncertainties in the
derived parameters might be larger than estimated from the fit. This may have
implications for Hubble diagrams constructed using gamma-ray burst data.Comment: 4 pages, 5 figures, accepted for publication in ApJ Letter
Energy Injection Episodes in Gamma Ray Bursts: The Light Curves and Polarization Properties of GRB 021004
Several GRB afterglow light curves deviate strongly from the power law decay
observed in most bursts. We show that these variations can be accounted for by
including refreshed shocks in the standard fireball model previously used to
interpret the overall afterglow behavior. As an example we consider GRB 021004
that exhibited strong light curve variations and has a reasonably well
time-resolved polarimetry. We show that the light curves in the R-band, X-rays
and in the radio can be accounted for by four energy injection episodes in
addition to the initial event. The polarization variations are shown to be a
consequence of the injections.Comment: 4 pages, 2 figures. To appear in ApJ
Fiabilidad diagnóstica comparativa de la mielografía y de la tomografía axial computarizada en la hernia discal lumbar
Un grupo de 137 pacientes con hernia discal lumbar fue estudiado retrospectivamente
, contrastando los hallazgos quirúrgicos con los proporcionado s por la
mielografía y la tomografía axial computarizada (TAC), con objeto de comparar la fiabilidad
de ambas técnicas de diagnóstico. La hernia discal correspondió al nivel L5-S1
en 76 pacientes y al L4-L5 en 61. Preoperatoriamente se había efectuado una TAC lumbar
en 99 paciente s y una mielografía en 77. En 39 casos se efectuaron ambas exploraciones.
La fiabilidad diagnóstica cuantificada por el porcentaje de resultados "verdaderos
positivos" fue similar en ambas exploraciones, elevándose al 89% y 90% para la TAC
y la mielografía respectivamente . De los 39 paciente s a quiene s se le s practicó ambas
exploraciones, en 12 (39%) de ellos no existía concordancia en el diagnóstico. No se registraron
"falsos negativos" con la TAC. Como conclusión, ambas técnicas diagnósticas
tienen una alta y similar fiabilidad para detectar hernias discale s a nivel lumbar. No
obstante, las ventajas que aporta la TAC respecto a la mielografía en cuanto a confort
para el pacient e y ausencia de reaccione s adversas hac e que esta técnica sea considerada
como el método diagnóstico de elección. La mielografía queda relegada a casos
con imágene s dudosas con la TAC.In orde r to compar e the diagnosti c accurac y of CT sca n and myelo -
graphy, a series of 137 patients with herniated lumbar disc was retrospectively analyzed
matching the surgical findings with the images obtained by both diagnostic techniques.
Ther e wer e 7 6 herniate d disc s a t th e L5-S1 leve l an d 6 1 a t th e L4-L5.
Preoperatively, 99 patients wer e assessed by CT scan and 77 by mielography. In 39 cases,
both technique s wer e performed. The diagnosti c accuracy, quantified by the percentage
of "true positive" results wa s similar in both techniques: 89% in CT scan and
90% in mielography . In 12 of the 39 patients explore d by bot h CT scan and myelo -
graphy, the results wer e discordant. "False negative" case s wer e not found using CT
scan. In conclusion, both CT scan and myelography provide equal high diagnosti c accuracy
for lumbar herniated disc. However, CT scan has become the primary diagnostic
tool because of several advantages, such as the confort for the patients and the lack
of adverse reactions
Surface elevation change and mass balance of Icelandic ice caps derived from swath mode CryoSat-2 altimetry
We apply swath processing to CryoSat-2 interferometric mode data acquired over the Icelandic ice caps to generate maps of rates of surface elevation change at 0.5 km postings. This high-resolution mapping reveals complex surface elevation changes in the region, related to climate, ice dynamics, and subglacial geothermal and magmatic processes. We estimate rates of volume and mass change independently for the six major Icelandic ice caps, 90% of Iceland's permanent ice cover, for five glaciological years between October 2010 and September 2015. Annual mass balance is highly variable; during the 2014/2015 glaciological year, the Vatnajökull ice cap (~70% of the glaciated area) experienced positive mass balance for the first time since 1992/1993. Our results indicate that between glaciological years 2010/2011and 2014/2015 Icelandic ice caps have lost 5.8 ± 0.7 Gt a−1 on average, ~40% less than the preceding 15 years, contributing 0.016 ± 0.002 mm a−1 to sea level rise
Performance of a sisal fibre fixed-bed anaerobic digester for biogas production from sisal pulp waste
A single stage anaerobic digester employing a sisal fibre waste fixed bed was studied for biogas production from sisal pulp waste. The fibre was colonized by microorganisms involved in biogas production. The sisal pulp waste to be digested was fed from the top and was sprinkled intermittently with recirculating leachate from the material. Organic loading rates of 0.1-10 kg volatile solids (VS) m-3d-1 could be applied and methane yields in the range of 0.13-0.48 m3 CH4 kg-1 VS added were obtained. The average methane content in the biogas produced from sisal pulp waste was 55%, and the biogas production rate was 0.15-0.54 m3m-3d-1. The methane yield obtained and the highest organic loading rate that could be sustained by this simple, fixed-bed digester are indications of an attractive system in terms of performance and reliability. It is concluded that the sisal fibre waste fixed bed is a promising carrier for microbes and can be employed for long-term operation without changing the bed. Tanzania Journal of Science Vol. 31 (2) 2005: pp. 41-5
Using stable isotopes and continuous meltwater river monitoring to investigate the hydrology of a rapidly retreating Icelandic outlet glacier
Virkisjökull is a rapidly retreating outlet glacier draining the western flanks of Öræfajökull in SE Iceland. Since 2011 there have been continuous measurements of flow in the proglacial meltwater channel and regular campaigns to sample stable isotopes δ2H and δ18O from the river, ice, moraine springs and groundwater. The stable isotopes provide reliable end members for glacial ice and shallow groundwater. Analysis of data from 2011 to 2014 indicates that although ice and snowmelt dominate summer riverflow (mean 5.3–7.9 m3 s−1), significant flow is also observed in winter (mean 1.6–2.4 m3 s−1) due primarily to ongoing glacier icemelt. The stable isotope data demonstrate that the influence of groundwater discharge from moraines and the sandur aquifer increases during winter and forms a small (15–20%) consistent source of baseflow to the river. The similarity of hydrological response across seasons reflects a highly efficient glacial drainage system, which makes use of a series of permanent englacial channels within active and buried ice throughout the year. The study has shown that the development of an efficient year round drainage network within the lower part of the glacier has been coincident with the stagnation and subsequent rapid retreat of the glacier
Upper limit on spontaneous supercurrents in SrRuO
It is widely believed that the perovskite SrRuO is an unconventional
superconductor with broken time reversal symmetry. It has been predicted that
superconductors with broken time reversal symmetry should have spontaneously
generated supercurrents at edges and domain walls. We have done careful imaging
of the magnetic fields above SrRuO single crystals using scanning Hall
bar and SQUID microscopies, and see no evidence for such spontaneously
generated supercurrents. We use the results from our magnetic imaging to place
upper limits on the spontaneously generated supercurrents at edges and domain
walls as a function of domain size. For a single domain, this upper limit is
below the predicted signal by two orders of magnitude. We speculate on the
causes and implications of the lack of large spontaneous supercurrents in this
very interesting superconducting system.Comment: 9 page
Associating Liver Partition and Portal Vein Ligation for Staged Hepatectomy (ALPPS) procedure for colorectal liver metastasis
Since first described, Associating Liver Partition and Portal Vein Ligation for Staged Hepatectomy (ALPPS) has garnered boisterous praise and fervent criticism. Its rapid adoption and employment for a variety of indications resulted in high perioperative morbidity and mortality. However recent risk stratification, refinement of technique to reduce the impact of stage I and progression along the learning curve have resulted in improved outcomes. The first randomized trial comparing ALPPS to two stage hepatectomy (TSH) for colorectal liver metastases (CRLM) was recently published demonstrating comparable perioperative morbidity and mortality with improved resectability and survival following ALPPS. In this review, as ALPPS enters the thirteenth year since conception, the current status of this contentious two stage technique is presented and best practices for deployment in the treatment of CRLM is codified
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