22,161 research outputs found

### A Feature Tracking velocimetry technique applied to inclined negatively buoyant jets

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 authorsâ€™
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

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

The Brownian web can be roughly described as a family of coalescing
one-dimensional Brownian motions starting at all times in $\R$ and at all
points of $\R$. 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 $\R^2$ 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

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

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

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

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 $Z_n$ internal symmetry
and one of the fields is located at a branch point.Comment: 26 pages, TeX + harvma

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