22,161 research outputs found

    A Feature Tracking velocimetry technique applied to inclined negatively buoyant jets

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    We have applied a Feature Tracking Velocimetry (FTV) technique to measure displacements of particles on inclined negatively buoyant jets (INBJs), issuing from a circular sharp-edged orifice, in order to investigate, among the others, the symmetry properties of the velocity field on this phenomenon. Feature Tracking Velocimetry is less sensitive to the appearance and disappearance of particles and to high velocity gradients than classical Particle Image Velocimetry (PIV). The basic idea of Feature Tracking Velocimetry is to compare windows only where the motion detection may be successful, that is where there are high luminosity gradients. The Feature Tracking Velocimetry algorithm presented here is suitable in presence of different seeding densities, where other techniques produce significant errors, due to the non-homogeneous seeding at the boundary of a flow. The Feature Tracking Velocimetry algorithm has been tested on laboratory experiments regarding simple jets (SJs) and inclined negatively buoyant jets released from a sharp-edged orifice. We present here velocity statistics, from the first to the fourth order, to study, among the others, the differences between simple jets and inclined negatively buoyant jets, and to investigate how the increase in buoyancy affects the inclined negatively buoyant jet behavior. We remark that, to the best of authors’ knowledge, this is the first attempt to investigate velocity statistics of an order higher than the second on Inclined Negatively Buoyant Jets. Among the others quantities, the mean streamwise velocity decay and the integral Turbulent Kinetic Energy have been measured and analyzed, both along the jet axis and in the upper and lower region of the simple jets and inclined negatively buoyant jets, as well as the streamwise and spanwise velocity skewness and kurtosis evolution along the axis. Results show the role of buoyancy in modifying the inclined negatively buoyant jet features; moreover, it is highlighted that the asymmetry of inclined negatively buoyant jets cannot be considered only a far field feature of this phenomenon, as it arises very close to the release point

    Model checking usage policies

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    We study usage automata, a formal model for specifying policies on the usage of resources. Usage automata extend finite state automata with some additional features, parameters and guards, that improve their expressivity. We show that usage automata are expressive enough to model policies of real-world applications. We discuss their expressive power, and we prove that the problem of telling whether a computation complies with a usage policy is decidable. The main contribution of this paper is a model checking technique for usage automata. The model is that of usages, i.e. basic processes that describe the possible patterns of resource access and creation. In spite of the model having infinite states, because of recursion and resource creation, we devise a polynomial-time model checking technique for deciding when a usage complies with a usage policy

    Two-dimensional Poisson Trees converge to the Brownian web

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    The Brownian web can be roughly described as a family of coalescing one-dimensional Brownian motions starting at all times in R\R and at all points of R\R. It was introduced by Arratia; a variant was then studied by Toth and Werner; another variant was analyzed recently by Fontes, Isopi, Newman and Ravishankar. The two-dimensional \emph{Poisson tree} is a family of continuous time one-dimensional random walks with uniform jumps in a bounded interval. The walks start at the space-time points of a homogeneous Poisson process in R2\R^2 and are in fact constructed as a function of the point process. This tree was introduced by Ferrari, Landim and Thorisson. By verifying criteria derived by Fontes, Isopi, Newman and Ravishankar, we show that, when properly rescaled, and under the topology introduced by those authors, Poisson trees converge weakly to the Brownian web.Comment: 22 pages, 1 figure. This version corrects an error in the previous proof. The results are the sam

    From interacting particle systems to random matrices

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    In this contribution we consider stochastic growth models in the Kardar-Parisi-Zhang universality class in 1+1 dimension. We discuss the large time distribution and processes and their dependence on the class on initial condition. This means that the scaling exponents do not uniquely determine the large time surface statistics, but one has to further divide into subclasses. Some of the fluctuation laws were first discovered in random matrix models. Moreover, the limit process for curved limit shape turned out to show up in a dynamical version of hermitian random matrices, but this analogy does not extend to the case of symmetric matrices. Therefore the connections between growth models and random matrices is only partial.Comment: 18 pages, 8 figures; Contribution to StatPhys24 special issue; minor corrections in scaling of section 2.

    Modelization of Thermal Fluctuations in G Protein-Coupled Receptors

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    We simulate the electrical properties of a device realized by a G protein coupled receptor (GPCR), embedded in its membrane and in contact with two metallic electrodes through which an external voltage is applied. To this purpose, recently, we have proposed a model based on a coarse graining description, which describes the protein as a network of elementary impedances. The network is built from the knowledge of the positions of the C-alpha atoms of the amino acids, which represent the nodes of the network. Since the elementary impedances are taken depending of the inter-nodes distance, the conformational change of the receptor induced by the capture of the ligand results in a variation of the network impedance. On the other hand, the fluctuations of the atomic positions due to thermal motion imply an impedance noise, whose level is crucial to the purpose of an electrical detection of the ligand capture by the GPCR. Here, in particular, we address this issue by presenting a computational study of the impedance noise due to thermal fluctuations of the atomic positions within a rhodopsin molecule. In our model, the C-alpha atoms are treated as independent, isotropic, harmonic oscillators, with amplitude depending on the temperature and on the position within the protein (alpha-helix or loop). The relative fluctuation of the impedance is then calculated for different temperatures.Comment: 5 pages, 2 figures, Proceeding of the 18-th International Conference on Fluctuations and Noise, 19-23 September 2005, Salamanca, Spain -minor proofreadings

    A quasi-pure Bose-Einstein condensate immersed in a Fermi sea

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    We report the observation of co-existing Bose-Einstein condensate and Fermi gas in a magnetic trap. With a very small fraction of thermal atoms, the 7Li condensate is quasi-pure and in thermal contact with a 6Li Fermi gas. The lowest common temperature is 0.28 muK = 0.2(1) T_C = 0.2(1) T_F where T_C is the BEC critical temperature and T_F the Fermi temperature. Behaving as an ideal gas in the radial trap dimension, the condensate is one-dimensional.Comment: 4 pages, 5 figure

    Bosonic Field Propagators on Algebraic Curves

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    In this paper we investigate massless scalar field theory on non-degenerate algebraic curves. The propagator is written in terms of the parameters appearing in the polynomial defining the curve. This provides an alternative to the language of theta functions. The main result is a derivation of the third kind differential normalized in such a way that its periods around the homology cycles are purely imaginary. All the physical correlation functions of the scalar fields can be expressed in terms of this object. This paper contains a detailed analysis of the techniques necessary to study field theories on algebraic curves. A simple expression of the scalar field propagator is found in a particular case in which the algebraic curves have ZnZ_n internal symmetry and one of the fields is located at a branch point.Comment: 26 pages, TeX + harvma
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