5,558 research outputs found
Geometrical statistics of the vorticity vector and the strain rate tensor in rotating turbulence
We report results on the geometrical statistics of the vorticity vector
obtained from experiments in electromagnetically forced rotating turbulence. A
range of rotation rates is considered, from non-rotating to rapidly
rotating turbulence with a maximum background rotation rate of rad/s
(with Rossby number much smaller than unity). Typically, in our experiments
. The measurement volume is located in the
centre of the fluid container above the bottom boundary layer, where the
turbulent flow can be considered locally statistically isotropic and
horizontally homogeneous for the non-rotating case, see van Bokhoven et al.,
Phys. Fluids 21, 096601 (2009). Based on the full set of velocity derivatives,
measured in a Lagrangian way by 3D Particle Tracking Velocimetry, we have been
able to quantify statistically the effect of system rotation on several flow
properties. The experimental results show how the turbulence evolves from
almost isotropic 3D turbulence ( rad/s) to quasi-2D
turbulence ( rad/s) and how this is reflected by several
statistical quantities. In particular, we have studied the orientation of the
vorticity vector with respect to the three eigenvectors of the local strain
rate tensor and with respect to the vortex stretching vector. Additionally, we
have quantified the role of system rotation on the self-amplification terms of
the enstrophy and strain rate equations and the direct contribution of the
background rotation on these evolution equations. The main effect is the strong
reduction of extreme events and related (strong) reduction of the skewness of
PDFs of several quantities such as, for example, the intermediate eigenvalue of
the strain rate tensor and the enstrophy self-amplification term.Comment: 17 pages, 6 figures, 3 table
Area Quantization in Quasi-Extreme Black Holes
We consider quasi-extreme Kerr and quasi-extreme Schwarzschild-de Sitter
black holes. From the known analytical expressions obtained for their
quasi-normal modes frequencies, we suggest an area quantization prescription
for those objects.Comment: Final version to appear in Mod. Phys. Lett.
Lipid Regulation as a Critical Factor in the Development of Alzheimer\u27s Disease
Alzheimer’s disease (AD) is the most common form of dementia in the United States, representing around eighty percent of all cases. For more than two decades, researchers have been led by the amyloid cascade hypothesis, which assumes that accumulation of the amyloid peptide Aβ, derived by proteolytic processing from the amyloid precursor protein (APP), is the key pathogenic trigger in AD. To date, therapies have largely focused on removing Aβ from the brain, an approach that has produced disappointing clinical outcomes. I present an alternative hypothesis in which Aβ production and aggregation is a symptom of a larger, systemic disease affecting the regulation of lipids, including cholesterol. In addition to assigning a physiological function for APP and Aβ generation, my hypothesis suggests that lipid dysregulation would likely occur early in the disease process, making it an ideal target for identification of disease risk or even intervention. Using a mouse model, I show that expression of APP is involved in the regulation of cholesterol synthesis, endocytosis, and myelination pathways. Using human cell culture models, I demonstrate that fibroblasts and peripheral blood mononuclear cells taken from AD patients show signs of lipid dysregulation, and that neuron-like cells develop this dysregulation when exposed to oxysterols. Finally, I developed and characterized a method of quantifying these detrimental changes using a fluorescence compound, filipin, which could form the basis of a commercial test to evaluate the potential risk of conversion from mild cognitive impairment to AD
A Study of the Hexakis Icosahedron Vacuum Lighter Than Air Vehicle and the Effects of Air Evacuation on the Structural Integrity
The research this paper focuses on is comparing the structural differences between the icosahedron and hexakis icosahedron frame and skin for use as a vacuum lighter than air vehicle (VLTAV), analyzing the stress concentrations in the hexakis icosahedron both with and without the skin, and finding the optimal location and size of the air evacuation method for creating the internal vacuum. Previous research to date has identified dynamic loading on the structure and optimization of the structure, but this will be the first research to analyze the manufacturing of the structure through the development of the air evacuation design. Findings demonstrated that the hexakis icosahedron was significantly stiffer when compared to the icosahedron, and the W=B ratio of the hexakis icosahedron was smaller for the same structural and material characterizations. The hexakis-icosahedron came the closest to the yield stress of the material in the frame by its self, consisting of the Carbon Nano-Tube (CNT)/Bismaleimide (BMI) composite at 3.38 GPa, with the yield stress being 3.8 GPa. The near zero stress in the frame considering individual beams was found to lie at approximately 20% and 66% of the beam length. Placing small holes for air evacuation resulted in minimal stress changes for the entrance to the evacuation system, but created failure points at the positions of the structure where the exit to the air evacuation system was located. Adding material to the exit system solved the failure point and kept the stress levels below yielding. Further research into the geometry of the vertex revealed interactions between the 10 beams conjoining on a single location and led to the development of a second air evacuation design and analysis. Analyzing the simplified model of the air evacuation resulted in a maximum stress of 3.484 GPa, which again is below the yield limit of the material and gave the structure an overall safety factor of 1.09
Ontología y gnoseología en la Logica ingredientibus de Pedro Abelardo
El trabajo se ocupa de la sección de las Glosas a la Isagogé de Porfirio de Pedro Abelardo dedicada a las tres cuestiones sobre los universales. La parte “destructiva”, en la que Abelardo somete a crítica las doctrinas realistas de Guillermo de Champeaux no tiene un sentido meramente negativo, sino que busca llegar al punto de partida de la propia posición de Abelardo: las cosas no sólo difieren por sus formas (accidentes), sino también por sus materias (esencias). Al hablar de la imagen correspondiente al nombre universal, Abelardo no se detiene a explicar la formación del concepto universal a partir de la cosa, sino que más bien se refiere a la significación intelectual de los términos, en tanto “generan intelecciones”. Esta omisión puede explicarse por el hecho de que, para Abelardo, la inteligencia humana rara vez o nunca llega a captar las esencias de las cosas.The paper deals with the section of Peter Abaelard’s Glossae super Porphyrium concerning the three questions about the universals. The pars destruens, in which Abaelard criticizes the realistic doctrines of William of Champeaux, does not have a merely negative function, but it tries to reach the starting-point of Abaelard’s own position, namely, that things differ not only in their forms or accidents, but also in their matters or essences. When he speaks of the image of the universal term, he does not explain the process of the elaboration of the universal concept starting from the thing, but he rather refers to the intellectual signification of terms, in so far as they “produce intellections”. This default could be explained because of the fact that, in Abaelard’s view, the human intelligence hardly ever or never grasps the essences of things
Finite-size scaling as a way to probe near-criticality in natural swarms
Collective behaviour in biological systems is often accompanied by strong
correlations. The question has therefore arisen of whether correlation is
amplified by the vicinity to some critical point in the parameters space.
Biological systems, though, are typically quite far from the thermodynamic
limit, so that the value of the control parameter at which correlation and
susceptibility peak depend on size. Hence, a system would need to readjust its
control parameter according to its size in order to be maximally correlated.
This readjustment, though, has never been observed experimentally. By gathering
three-dimensional data on swarms of midges in the field we find that swarms
tune their control parameter and size so as to maintain a scaling behaviour of
the correlation function. As a consequence, correlation length and
susceptibility scale with the system's size and swarms exhibit a near-maximal
degree of correlation at all sizes.Comment: Selected for Viewpoint in Physics; PRL Editor's Suggestio
Collective behaviour without collective order in wild swarms of midges
Collective behaviour is a widespread phenomenon in biology, cutting through a
huge span of scales, from cell colonies up to bird flocks and fish schools. The
most prominent trait of collective behaviour is the emergence of global order:
individuals synchronize their states, giving the stunning impression that the
group behaves as one. In many biological systems, though, it is unclear whether
global order is present. A paradigmatic case is that of insect swarms, whose
erratic movements seem to suggest that group formation is a mere epiphenomenon
of the independent interaction of each individual with an external landmark. In
these cases, whether or not the group behaves truly collectively is debated.
Here, we experimentally study swarms of midges in the field and measure how
much the change of direction of one midge affects that of other individuals. We
discover that, despite the lack of collective order, swarms display very strong
correlations, totally incompatible with models of noninteracting particles. We
find that correlation increases sharply with the swarm's density, indicating
that the interaction between midges is based on a metric perception mechanism.
By means of numerical simulations we demonstrate that such growing correlation
is typical of a system close to an ordering transition. Our findings suggest
that correlation, rather than order, is the true hallmark of collective
behaviour in biological systems.Comment: The original version has been split into two parts. This first part
focuses on order vs. correlation. The second part, about finite-size scaling,
will be included in a separate paper. 15 pages, 6 figures, 1 table, 5 video
High overtones of Dirac perturbations of a Schwarzschild black hole
Using the Frobenius method, we find high overtones of the Dirac quasinormal
spectrum for the Schwarzschild black hole. At high overtones, the spacing for
imaginary part of is equidistant and equals to
, ( is the black hole mass), which
is twice less than that for fields of integer spin. At high overtones, the real
part of goes to zero. This supports the suggestion that the
expected correspondence between quasinormal modes and Barbero-Immirzi parameter
in Loop Quantum Gravity is just a numerical coincidence.Comment: 5 pages, Latex, 3 figures, Physical Review D.,at pres
Study of the bone pathology in early mucolipidosis II (I-cell disease).
Histological examination of the bones obtained on autopsy of a 5-month-old child with mucolipidosis II (I-cell disease) revealed inhibition of the growth plate calcification with defective vascular invasion and signs of hyperparathyroidism. These findings are the chondro-osseous basis of the early radiological ricket-like appearance of bones in the neonatal period or soon thereafter. Whether the early skeletal abnormalities of mucolipidosis II result from a primary enzymatic defect of cartilage and bone cells or from factors controlling bone metabolism deserves further study
Ozone fumigation successfully controlled and eradicated multidrug-resistant Acinetobacter baumanii from an intensive care unit
Cette contribution porte sur la construction de savoirs professionnels dans l'alternance en formation initiale à l'enseignement. Elle postule que cette construction de savoirs se relie à l'émergence de l'identité professionnelle: le sujet en formation se construit comme acteur social en même temps des savoirs propices à son agir. Plus précisément, notre préoccupation de recherche concerne les indicateurs de cette co-construction soi/savoirs professionnels tels qu'on peut les inférer à partir de l'analyse de discours écrits ou oraux produits par les étudiants
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