1,571 research outputs found
Stochastic Perturbations in Vortex Tube Dynamics
A dual lattice vortex formulation of homogeneous turbulence is developed,
within the Martin-Siggia-Rose field theoretical approach. It consists of a
generalization of the usual dipole version of the Navier-Stokes equations,
known to hold in the limit of vanishing external forcing. We investigate, as a
straightforward application of our formalism, the dynamics of closed vortex
tubes, randomly stirred at large length scales by gaussian stochastic forces.
We find that besides the usual self-induced propagation, the vortex tube
evolution may be effectively modeled through the introduction of an additional
white-noise correlated velocity field background. The resulting
phenomenological picture is closely related to observations previously reported
from a wavelet decomposition analysis of turbulent flow configurations.Comment: 16 pages + 2 eps figures, REVTeX
Stratified gravity currents in porous media
We consider theoretically and experimentally the propagation in porous media of variable-density gravity currents containing a stably stratified density field, with most previous studies of gravity currents having focused on cases of uniform density. New thin-layer equations are developed to describe stably stratified fluid flows in which the density field is materially advected with the flow. Similarity solutions describing both the fixed-volume release of a distributed density stratification and the continuous input of fluid containing a distribution of densities are obtained. The results indicate that the density distribution of the stratification significantly influences the vertical structure of the gravity current. When more mass is distributed into lighter densities, it is found that the shape of the current changes from the convex shape familiar from studies of the uniform-density case to a concave shape in which lighter fluid accumulates primarily vertically above the origin of the current. For a constant-volume release, the density contours stratify horizontally, a simplification which is used to develop analytical solutions. For currents introduced continuously, the horizontal velocity varies with vertical position, a feature which does not apply to uniform-density gravity currents in porous media. Despite significant effects on vertical structure, the density distribution has almost no effect on overall horizontal propagation, for a given total mass. Good agreement with data from a laboratory study confirms the predictions of the model
Retinal nerve fibre layer thinning is associated with drug resistance in epilepsy.
Retinal nerve fibre layer (RNFL) thickness is related to the axonal anterior visual pathway and is considered a marker of overall white matter 'integrity'. We hypothesised that RNFL changes would occur in people with epilepsy, independently of vigabatrin exposure, and be related to clinical characteristics of epilepsy
Global existence for semilinear reaction-diffusion systems on evolving domains
We present global existence results for solutions of reaction-diffusion
systems on evolving domains. Global existence results for a class of
reaction-diffusion systems on fixed domains are extended to the same systems
posed on spatially linear isotropically evolving domains. The results hold
without any assumptions on the sign of the growth rate. The analysis is valid
for many systems that commonly arise in the theory of pattern formation. We
present numerical results illustrating our theoretical findings.Comment: 24 pages, 3 figure
Relativistic suppression of Auger recombination in Weyl semimetals
Auger recombination (AR) being electron-hole annihilation with
energy-momentum transfer to another carrier is believed to speed up in
materials with small band gap. We theoretically show that this rule is violated
in gapless three-dimensional materials with ultra-relativistic electron-hole
dispersion, Weyl semimetals (WSM). Namely, AR is prohibited by energy-momentum
conservation laws in prototypical WSM with a single Weyl node, even in the
presence of anisotropy and tilt. In real multi-node WSM, the geometric
dissimilarity of nodal dispersions enables weak inter-node AR, which is further
suppressed by strong screening due to large number of nodes. While partial AR
rates between the nodes of the same node group are mutually equal, the
inter-group processes are non-reciprocal, so that one of groups is
geometrically protected from AR. Our calculations show that geometrical
protection can help prolonging AR lifetime by the two orders of magnitude, up
to the level of nanoseconds.Comment: 6 pages + 10 pages of supporting informatio
Research disruptions and recovery
With the pandemic of COVIDâ19, the routine work of surgeons has been reduced to release resources to critical care and other functions. This is necessary and right, in order to protect the health of our communities now. As a consequence, many of us who undertake clinical research have seen our work suspended. With a rich portfolio of colorectal research, this includes studies such as ROSSINI2, PITSTOP, & MASH, and has delayed the start of other important studies such as ELF 2 and Damascus]. When we return to something akin to normality, we shall be looking to reâestablish the studies in a timely manner. Funders have generally been flexible with regards to the disruption and have provided guidance on how the disruption might be managed1. There is no doubt that they may be asked to provide additional support for studies in progress, which may impact future funding opportunities
Obesity: A Biobehavioral Point of View
Excerpt: If you ask an overweight person, âWhy are you fat?â, you will, almost invariably, get the answer, âBecause 1 eat too much.â You will get this answer in spite of the fact that of thirteen studies, six find no significant differences in the caloric intake of obese versus nonobese subjects, five report that the obese eat significantly less than the nonobese, and only two report that they eat significantly more
Two-component {CH} system: Inverse Scattering, Peakons and Geometry
An inverse scattering transform method corresponding to a Riemann-Hilbert
problem is formulated for CH2, the two-component generalization of the
Camassa-Holm (CH) equation. As an illustration of the method, the multi -
soliton solutions corresponding to the reflectionless potentials are
constructed in terms of the scattering data for CH2.Comment: 22 pages, 3 figures, draft, please send comment
Increased Alpha-Band Power during the Retention of Shapes and Shape-Location Associations in Visual Short-Term Memory
Studies exploring the role of neural oscillations in cognition have revealed sustained increases in alpha-band (~8â14âHz) power during the delay period of delayed-recognition short-term memory tasks. These increases have been proposed to reflect the inhibition, for example, of cortical areas representing task-irrelevant information, or of potentially interfering representations from previous trials. Another possibility, however, is that elevated delay-period alpha-band power (DPABP) reflects the selection and maintenance of information, rather than, or in addition to, the inhibition of task-irrelevant information. In the present study, we explored these possibilities using a delayed-recognition paradigm in which the presence and task relevance of shape information was systematically manipulated across trial blocks and electroencephalographic was used to measure alpha-band power. In the first trial block, participants remembered locations marked by identical black circles. The second block featured the same instructions, but locations were marked by unique shapes. The third block featured the same stimulus presentation as the second, but with pretrial instructions indicating, on a trial-by-trial basis, whether memory for shape or location was required, the other dimension being irrelevant. In the final block, participants remembered the unique pairing of shape and location for each stimulus. Results revealed minimal DPABP in each of the location-memory conditions, whether locations were marked with identical circles or with unique task-irrelevant shapes. In contrast, alpha-band power increases were observed in both the shape-memory condition, in which location was task irrelevant, and in the critical final condition, in which both shape and location were task relevant. These results provide support for the proposal that alpha-band oscillations reflect the retention of shape information and/or shapeâlocation associations in short-term memory
Shear testing and failure modelling of calcium phosphate coated AZ31 magnesium alloys for orthopaedic applications
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