3,945 research outputs found
Observation of mesospheric air inside the arctic stratospheric polar vortex in early 2003
During several balloon flights inside the Arctic polar vortex in early 2003, unusual trace gas distributions were observed, which indicate a strong influence of mesospheric air in the stratosphere. The tuneable diode laser (TDL) instrument SPIRALE (Spectroscopie InFrarouge par Absorption de Lasers Embarqués) measured unusually high CO values (up to 600 ppb) on 27 January at about 30 km altitude. The cryosampler BONBON sampled air masses with very high molecular Hydrogen, extremely low SF6 and enhanced CO values on 6 March at about 25 km altitude. Finally, the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) Fourier Transform Infra-Red (FTIR) spectrometer showed NOy values which are significantly higher than NOy* (the NOy derived from a correlation between N2O and NOy under undisturbed conditions), on 21 and 22 March in a layer centred at 22 km altitude. Thus, the mesospheric air seems to have been present in a layer descending from about 30 km in late January to 25 km altitude in early March and about 22 km altitude on 20 March. We present corroborating evidence from a model study using the KASIMA (KArlsruhe Simulation model of the Middle Atmosphere) model that also shows a layer of mesospheric air, which descended into the stratosphere in November and early December 2002, before the minor warming which occurred in late December 2002 lead to a descent of upper stratospheric air, cutting of a layer in which mesospheric air is present. This layer then descended inside the vortex over the course of the winter. The same feature is found in trajectory calculations, based on a large number of trajectories started in the vicinity of the observations on 6 March. Based on the difference between the mean age derived from SF6 (which has an irreversible mesospheric loss) and from CO2 (whose mesospheric loss is much smaller and reversible) we estimate that the fraction of mesospheric air in the layer observed on 6 March, must have been somewhere between 35% and 100%
Wave-piercing catamaran transom stern ventilation process
The new class of highly fuel-efficient medium-speed catamarans operate at speeds where the transom is partially or fully ventilated, hence it is important to understand the characteristics of the wake for resistance prediction. Unsteady Reynolds-Averaged Navier Stokes simulations were used to simulate the flow around a 98 m catamaran, at both model and full scale, and compared to model test results for a 1:22 scale model. A non-shedding squashed horseshoe vortex was found to build up in the stagnant zone past the vessel, with the transom running dry at transom draft Froude numbers of 2.5 in model test experiments and at transom draft Froude numbers of 2.4 in numerical simulations. For full-scale Reynolds numbers, ventilation occurred at transom draft Froude numbers of 2.2. Finally, unsteady Reynolds-Averaged Navier Stokes simulations are capable of accurately predicting the recirculating flow in the wake of the vessel and the state of transom ventilation
Rapid dynamics of cell-shape recovery in response to local deformations
It is vital that cells respond rapidly to mechanical cues within their microenvironment through changes
in cell shape and volume, which rely upon the mechanical properties of cells’ highly interconnected
cytoskeletal networks and intracellular fluid redistributions. While previous research has largely
investigated deformation mechanics, we now focus on the immediate cell-shape recovery response
following mechanical perturbation by inducing large, local, and reproducible cellular deformations using
AFM. By continuous imaging within the plane of deformation, we characterize the membrane and
cortical response of HeLa cells to unloading, and model the recovery via overdamped viscoelastic
dynamics. Importantly, the majority (90%) of HeLa cells recover their cell shape in o1 s. Despite actin
remodelling on this time scale, we show that cell-shape recovery time is not affected by load duration,
nor magnitude for untreated cells. To further explore this rapid recovery response, we expose cells to
cytoskeletal destabilizers and osmotic shock conditions, which uncovers the interplay between actin and
osmotic pressure. We show that the rapid dynamics of recovery depend crucially on intracellular
pressure, and provide strong evidence that cortical actin is the key regulator in the cell-shape recovery
processes, in both cancerous and non-cancerous epithelial cell
Phase Separation and the Phase Diagram in Cuprates Superconductors
We show that the main features of the cuprates superconductors phase diagram
can be derived considering the disorder as a key property of these materials.
Our basic point is that the high pseudogap line is an onset of phase separation
which generates compounds made up of regions with distinct doping levels. We
calculate how this continuous temperature dependent phase separation process
occurs in high critical temperature superconductors (HTSC) using the
Cahn-Hilliard approach, originally applied to study alloys. Since the level of
phase separation varies for different cuprates, it is possible that different
systems with average doping level pm exhibit different degrees of charge and
spin segregation. Calculations on inhomogeneous charge distributions in form of
stripes in finite clusters performed by the Bogoliubov-deGennes superconducting
approach yield good agreement to the pseudogap temperature T*(pm), the onset of
local pairing amplitudes with phase locked and concomitantly, how they develop
at low temperatures into the superconducting phase at Tc(pm) by percolation.Comment: 9 pages, 9 figures. Submitted to Phys. Rev.
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