2,565 research outputs found
Fluid dynamic aspects of jet noise generation
Jet engine noise generation and noise propagation was investigated by studying supersonic nozzle flow of various nozzle configurations in an experimental test facility. The experimental facility was constructed to provide a coaxial axisymmetric jet flow of unheated air. In the test setup, an inner primary flow exhausted from a 7 in. exit diameter convergent--divergent nozzle at Mach 2, while a secondary flow had a 10 in. outside diameter and was sonic at the exit. The large dimensions of the jets permitted probes to be placed inside the jet core without significantly disturbing the flow. Static pressure fluctuations were measured for the flows. The nozzles were designed for shock free (balanced) flow at Mach 2. Data processing techniques and experimental procedures were developed in order to study induced disturbances at the edge of the supersonic flows, and the propagation of those disturbances throughout the flows. Equipment used (specifications are given) to record acoustic levels (far field noise) is described. Results and conclusions are presented and discussed. Diagrams of the jet flow fields are included along with photographs of the test stand
Nonequlibrium particle and energy currents in quantum chains connected to mesoscopic Fermi reservoirs
We propose a model of nonequilibrium quantum transport of particles and
energy in a system connected to mesoscopic Fermi reservoirs (meso-reservoir).
The meso-reservoirs are in turn thermalized to prescribed temperatures and
chemical potentials by a simple dissipative mechanism described by the Lindblad
equation. As an example, we study transport in monoatomic and diatomic chains
of non-interacting spinless fermions. We show numerically the breakdown of the
Onsager reciprocity relation due to the dissipative terms of the model.Comment: 5pages, 4 figure
Quasi-Static Brittle Fracture in Inhomogeneous Media and Iterated Conformal Maps: Modes I, II and III
The method of iterated conformal maps is developed for quasi-static fracture
of brittle materials, for all modes of fracture. Previous theory, that was
relevant for mode III only, is extended here to mode I and II. The latter
require solution of the bi-Laplace rather than the Laplace equation. For all
cases we can consider quenched randomness in the brittle material itself, as
well as randomness in the succession of fracture events. While mode III calls
for the advance (in time) of one analytic function, mode I and II call for the
advance of two analytic functions. This fundamental difference creates
different stress distribution around the cracks. As a result the geometric
characteristics of the cracks differ, putting mode III in a different class
compared to modes I and II.Comment: submitted to PRE For a version with qualitatively better figures see:
http://www.weizmann.ac.il/chemphys/ander
A Hebbian approach to complex network generation
Through a redefinition of patterns in an Hopfield-like model, we introduce
and develop an approach to model discrete systems made up of many, interacting
components with inner degrees of freedom. Our approach clarifies the intrinsic
connection between the kind of interactions among components and the emergent
topology describing the system itself; also, it allows to effectively address
the statistical mechanics on the resulting networks. Indeed, a wide class of
analytically treatable, weighted random graphs with a tunable level of
correlation can be recovered and controlled. We especially focus on the case of
imitative couplings among components endowed with similar patterns (i.e.
attributes), which, as we show, naturally and without any a-priori assumption,
gives rise to small-world effects. We also solve the thermodynamics (at a
replica symmetric level) by extending the double stochastic stability
technique: free energy, self consistency relations and fluctuation analysis for
a picture of criticality are obtained
Irreducible free energy expansion and overlaps locking in mean field spin glasses
We introduce a diagrammatic formulation for a cavity field expansion around
the critical temperature. This approach allows us to obtain a theory for the
overlap's fluctuations and, in particular, the linear part of the
Ghirlanda-Guerra relationships (GG) (often called Aizenman-Contucci polynomials
(AC)) in a very simple way. We show moreover how these constraints are
"superimposed" by the symmetry of the model with respect to the restriction
required by thermodynamic stability. Within this framework it is possible to
expand the free energy in terms of these irreducible overlaps fluctuations and
in a form that simply put in evidence how the complexity of the solution is
related to the complexity of the entropy.Comment: 19 page
Morphological variation of the newly confirmed population of the javelin sand boa, Eryx jaculus (Linnaeus, 1758) (Serpentes, erycidae) in Sicily, Italy
The presence of the Javelin sand boa in Sicily has recently been confirmed. Here the morphological characters and sexual dimorphism of the Sicilian population of Eryx jaculus are presented. Seven meristic and six metric characters in 96 specimens from Sicily were examined. The results show that tail length, snout-vent length, the distance between nostrils and the number of ventral and subcaudal scales are different between sexes. The characters found in the Sicilian population of the Javelin sand boa resemble those of the African population (ssp. jaculus) rather than the Eurasian population (ssp. turcicus), but biomolecular studies are necessary to understand its taxonomic identity
Current in coherent quantum systems connected to mesoscopic Fermi reservoirs
We study particle current in a recently proposed model for coherent quantum transport. In this model, a system connected to mesoscopic Fermi reservoirs (meso-reservoir) is driven out of equilibrium by the action of super-reservoirs thermalized to prescribed temperatures and chemical potentials by a simple dissipative mechanism described by the Lindblad equation. We compare exact (numerical) results with theoretical expectations based on the Landauer formula
Criticality in diluted ferromagnet
We perform a detailed study of the critical behavior of the mean field
diluted Ising ferromagnet by analytical and numerical tools. We obtain
self-averaging for the magnetization and write down an expansion for the free
energy close to the critical line. The scaling of the magnetization is also
rigorously obtained and compared with extensive Monte Carlo simulations. We
explain the transition from an ergodic region to a non trivial phase by
commutativity breaking of the infinite volume limit and a suitable vanishing
field. We find full agreement among theory, simulations and previous results.Comment: 23 pages, 3 figure
Equilibrium statistical mechanics on correlated random graphs
Biological and social networks have recently attracted enormous attention
between physicists. Among several, two main aspects may be stressed: A non
trivial topology of the graph describing the mutual interactions between agents
exists and/or, typically, such interactions are essentially (weighted)
imitative. Despite such aspects are widely accepted and empirically confirmed,
the schemes currently exploited in order to generate the expected topology are
based on a-priori assumptions and in most cases still implement constant
intensities for links. Here we propose a simple shift in the definition of
patterns in an Hopfield model to convert frustration into dilution: By varying
the bias of the pattern distribution, the network topology -which is generated
by the reciprocal affinities among agents - crosses various well known regimes
(fully connected, linearly diverging connectivity, extreme dilution scenario,
no network), coupled with small world properties, which, in this context, are
emergent and no longer imposed a-priori. The model is investigated at first
focusing on these topological properties of the emergent network, then its
thermodynamics is analytically solved (at a replica symmetric level) by
extending the double stochastic stability technique, and presented together
with its fluctuation theory for a picture of criticality. At least at
equilibrium, dilution simply decreases the strength of the coupling felt by the
spins, but leaves the paramagnetic/ferromagnetic flavors unchanged. The main
difference with respect to previous investigations and a naive picture is that
within our approach replicas do not appear: instead of (multi)-overlaps as
order parameters, we introduce a class of magnetizations on all the possible
sub-graphs belonging to the main one investigated: As a consequence, for these
objects a closure for a self-consistent relation is achieved.Comment: 30 pages, 4 figure
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