2,851 research outputs found
Motion of buoyant particles and coarsening of solid-liquid mixtures in a random acceleration field
Flow induced by a random acceleration field (g-jitter) is considered in two
related situations that are of interest for microgravity fluid experiments: the
random motion of an isolated buoyant particle and coarsening of a solid-liquid
mixture. We start by analyzing in detail actual accelerometer data gathered
during a recent microgravity mission, and obtain the values of the parameters
defining a previously introduced stochastic model of this acceleration field.
We then study the motion of a solid particle suspended in an incompressible
fluid that is subjected to such random accelerations. The displacement of the
particle is shown to have a diffusive component if the correlation time of the
stochastic acceleration is finite or zero, and mean squared velocities and
effective diffusion coefficients are obtained explicitly. Finally, the effect
of g-jitter on coarsening of a solid-liquid mixture is considered. Corrections
due to the induced fluid motion are calculated, and estimates are given for
coarsening of Sn-rich particles in a Sn-Pb eutectic fluid, experiment to be
conducted in microgravity in the near future.Comment: 25 pages, 4 figures (included). Also at
http://www.scri.fsu.edu/~vinals/ross2.p
Temperley-Lieb Words as Valence-Bond Ground States
Based on the Temperley--Lieb algebra we define a class of one-dimensional
Hamiltonians with nearest and next-nearest neighbour interactions. Using the
regular representation we give ground states of this model as words of the
algebra. Two point correlation functions can be computed employing the
Temperley--Lieb relations. Choosing a spin-1/2 representation of the algebra we
obtain a generalization of the (q-deformed) Majumdar--Ghosh model. The ground
states become valence-bond states.Comment: 9 Pages, LaTeX (with included style files
A meta-analysis of variables that predict significant intracranial injury in minor head trauma.
BACKGROUND: Previous studies have presented conflicting results regarding the predictive effect of various clinical symptoms, signs, and plain imaging for intracranial pathology in children with minor head injury. AIMS: To perform a meta-analysis of the literature in order to assess the significance of these factors and intracranial haemorrhage (ICH) in the paediatric population. METHODS: The literature was searched using Medline, Embase, Experts, and the grey literature. Reference lists of major guidelines were crosschecked. Control or nested case-control studies of children with head injury who had skull radiography, recording of common symptoms and signs, and head computed tomography (CT) were selected. OUTCOME VARIABLE: CT presence or absence of ICH. RESULTS: Sixteen papers were identified as satisfying criteria for inclusion in the meta-analysis, although not every paper contained data on every correlate. Available evidence gave pooled patient numbers from 1136 to 22 420. Skull fracture gave a relative risk ratio of 6.13 (95% CI 3.35 to 11.2), headache 1.02 (95% CI 0.62 to 1.69), vomiting 0.88 (95% CI 0.67 to 1.15), focal neurology 9.43 (2.89 to 30.8), seizures 2.82 (95% CI 0.89 to 9.00), LOC 2.23 (95% CI 1.20 to 4.16), and Glasgow Coma Scale (GCS) <15 of 5.51 (95% CI 1.59 to 19.0). CONCLUSIONS: There was a statistically significant correlation between intracranial haemorrhage and skull fracture, focal neurology, loss of consciousness, and GCS abnormality. Headache and vomiting were not found to be predictive and there was great variability in the predictive ability of seizures. More information is required about the current predictor variables so that more refined guidelines can be developed. Further research is currently underway by three large study groups
Desmin forms toxic, seeding-competent amyloid aggregates that persist in muscle fibers
Desmin-associated myofibrillar myopathy (MFM) has pathologic similarities to neurodegeneration-associated protein aggregate diseases. Desmin is an abundant muscle-specific intermediate filament, and disease mutations lead to its aggregation in cells, animals, and patients. We reasoned that similar to neurodegeneration-associated proteins, desmin itself may form amyloid. Desmin peptides corresponding to putative amyloidogenic regions formed seeding-competent amyloid fibrils. Amyloid formation was increased when disease-associated mutations were made within the peptide, and this conversion was inhibited by the anti-amyloid compound epigallocatechin-gallate. Moreover, a purified desmin fragment (aa 117 to 348) containing both amyloidogenic regions formed amyloid fibrils under physiologic conditions. Desmin fragment-derived amyloid coaggregated with full-length desmin and was able to template its conversion into fibrils in vitro. Desmin amyloids were cytotoxic to myotubes and disrupted their myofibril organization compared with desmin monomer or other nondesmin amyloids. Finally, desmin fragment amyloid persisted when introduced into mouse skeletal muscle. These data suggest that desmin forms seeding-competent amyloid that is toxic to myofibers. Moreover, small molecules known to interfere with amyloid formation and propagation may have therapeutic potential in MFM
Turbulent Pair Diffusion
Kinematic Simulations of turbulent pair diffusion in planar turbulence with a
-5/3 energy spectrum reproduce the results of the laboratory measurements of
Jullien Phys. Rev. Lett. 82, 2872 (1999), in particular the stretched
exponential form of the PDF of pair separations and their correlation
functions. The root mean square separation is found to be strongly dependent on
initial conditions for very long stretches of times. This dependence is
consistent with the topological picture of turbulent pair diffusion where pairs
initially close enough travel together for long stretches of time and separate
violently when they meet straining regions around hyperbolic points. A new
argument based on the divergence of accelerations is given to support this
picture
Microscale swimming: The molecular dynamics approach
The self-propelled motion of microscopic bodies immersed in a fluid medium is
studied using molecular dynamics simulation. The advantage of the atomistic
approach is that the detailed level of description allows complete freedom in
specifying the swimmer design and its coupling with the surrounding fluid. A
series of two-dimensional swimming bodies employing a variety of propulsion
mechanisms -- motivated by biological and microrobotic designs -- is
investigated, including the use of moving limbs, changing body shapes and fluid
jets. The swimming efficiency and the nature of the induced, time-dependent
flow fields are found to differ widely among body designs and propulsion
mechanisms.Comment: 5 pages, 3 figures (minor changes to text
Internal relaxation time in immersed particulate materials
We study the dynamics of the solid to liquid transition for a model material
made of elastic particles immersed in a viscous fluid. The interaction between
particle surfaces includes their viscous lubrication, a sharp repulsion when
they get closer than a tuned steric length and their elastic deflection induced
by those two forces. We use Soft Dynamics to simulate the dynamics of this
material when it experiences a step increase in the shear stress and a constant
normal stress. We observe a long creep phase before a substantial flow
eventually establishes. We find that the typical creep time relies on an
internal relaxation process, namely the separation of two particles driven by
the applied stress and resisted by the viscous friction. This mechanism should
be relevant for granular pastes, living cells, emulsions and wet foams
Enstrophy dissipation in two-dimensional turbulence
Insight into the problem of two-dimensional turbulence can be obtained by an
analogy with a heat conduction network. It allows the identification of an
entropy function associated to the enstrophy dissipation and that fluctuates
around a positive (mean) value. While the corresponding enstrophy network is
highly nonlocal, the direction of the enstrophy current follows from the Second
Law of Thermodynamics. An essential parameter is the ratio of the intensity of driving as a function of
wavenumber , to the dissipation strength , where is the
viscosity. The enstrophy current flows from higher to lower values of ,
similar to a heat current from higher to lower temperature. Our probabilistic
analysis of the enstrophy dissipation and the analogy with heat conduction thus
complements and visualizes the more traditional spectral arguments for the
direct enstrophy cascade. We also show a fluctuation symmetry in the
distribution of the total entropy production which relates the probabilities of
direct and inverse enstrophy cascades.Comment: 8 pages, revtex
Phase diagram of the su(8) quantum spin tube
We calculate the phase diagram of an integrable anisotropic 3-leg quantum
spin tube connected to the su(8) algebra. We find several quantum phase
transitions for antiferromagnetic rung couplings. Their locations are
calculated exactly from the Bethe Ansatz solution and we discuss the nature of
each of the different phases.Comment: 10 pages, RevTeX, 1 postscript figur
Exocrine pancreatic insufficiency with concurrent pancreatitis, inflammatory bowel disease and cholangiohepatitis in a cat
- …