28,188 research outputs found
Conserved- and zero-mean quadratic quantities in oscillatory systems
We study quadratic functionals of the variables of a linear oscillatory system and their derivatives. We show that such functionals are partitioned in conserved quantities and in trivially- and intrinsic zero-mean quantities. We also state an equipartition of energy principle for oscillatory systems
Unified Schemes for Radio-Loud Active Galactic Nuclei
The appearance of active galactic nuclei (AGN) depends so strongly on
orientation that our current classification schemes are dominated by random
pointing directions instead of more interesting physical properties. Light from
the centers of many AGN is obscured by optically thick circumnuclear matter and
in radio-loud AGN, bipolar jets emanating from the nucleus emit light that is
relativistically beamed along the jet axes. Understanding the origin and
magnitude of radiation anisotropies in AGN allows us to unify different classes
of AGN; that is, to identify each single, underlying AGN type that gives rise
to different classes through different orientations.
This review describes the unification of radio-loud AGN, which include radio
galaxies, quasars, and blazars. We describe the classification and properties
of AGN and summarize the evidence for anisotropic emission. We outline the two
most plausible unified schemes for radio-loud AGN, one linking quasars and
luminous radio galaxies and another linking BL~Lac objects and less luminous
radio galaxies. Using the formalism appropriate to samples biased by
relativistic beaming, we show the population statistics for two schemes are in
accordance with available data. We analyze the possible connections between
low- and high-luminosity radio-loud AGN. We review potential difficulties with
unification and conclude that none currently constitutes a serious problem. We
discuss likely complications to unified schemes that are suggested by realistic
physical considerations; these will be important to consider when more
comprehensive data for larger complete samples become available. We conclude
with a list of the ten questions we believe are the most pressing in this
field.Comment: 88 pages, latex file, uses aaspp.sty macro (available via ftp from
ftp://aas.org/pubs/aastex/). Accompanying 22 figures and 3 tables available
at http://itovf2.roma2.infn.it/padovani/review.html. (Abstract is abridged.)
The only change is that the revised version indicates this paper is an
invited review for PASP, in press, September 1995 issu
Tribochemistry of graphene on iron and its possible role in lubrication of steel
Recent tribological experiments revealed that graphene is able to lubricate
macroscale steel-on-steel sliding contacts very effectively both in dry and
humid conditions. This effect has been attributed to a mechanical action of
graphene related to its load-carrying capacity. Here we provide further insight
into the functionality of graphene as lubricant by analysing its tribochemical
action. By means of first principles calculations we show that graphene binds
strongly to native iron surfaces highly reducing their surface energy. Thanks
to a passivating effect, the metal surfaces coated by graphene become almost
inert and present very low adhesion and shear strength when mated in a sliding
contact. We generalize the result by establishing a connection between the
tribological and the electronic properties of interfaces, which is relevant to
understand the fundamental nature of frictional forces.Comment: 19 pages, 6 figure
State maps for linear systems
Modeling of physical systems consists of writing the equations describing a phenomenon and yields as a result a set of differential-algebraic equations. As such, state-space models are not a natural starting point for modeling, while they have utmost importance in the simulation and control phase. The paper addresses the problem of computing state variables for systems of linear differential-algebraic equations of various forms. The point of view from which the problem is considered is the behavioral one, as put forward in [J. C. Willems, Automatica J. IFAC, 22 (1986), pp. 561â580; DynamicsReported,2(1989),pp.171â269;IEEETrans.Automat.Control,36(1991),pp. 259â294]
Evolution of reconnection along an arcade of magnetic loops
RHESSI observations of a solar flare showing continuous motions of double
hard X-ray sources interpreted as footpoints of magnetic loops are presented.
The temporal evolution shows many distinct emission peaks of duration of some
tens of seconds ('elementary flare bursts'). Elementary flare bursts have been
interpreted as instabilities or oscillations of the reconnection process
leading to an unsteady release of magnetic energy. These interpretations based
on two-dimensional concepts cannot explain these observations, showing that the
flare elements are displaced in a third dimension along the arcade. Therefore,
the observed flare elements are not a modulation of the reconnection process,
but originate as this process progresses along an arcade of magnetic loops.
Contrary to previous reports, we find no correlation between footpoint motion
and hard X-ray flux. This flare apparently contradicts the predictions of the
standard translation invariant 2.5D reconnection models.Comment: 4 pages, 3 figures, to be published in Astrophysical Journal Letter
Effect of in-mould inoculant composition on microstructure and fatigue behaviour of heavy section ductile iron castings
In this paper, the influence of the in-mould inoculant composition on microstructure and fatigue behaviour of heavy section ductile iron (EN GJS 700-2) castings has been investigated.
Axial fatigue tests under nominal load ratio R=0 have been performed on specimens taken from the core of large casting components. Metallographic analyses have been carried out by means of optical microscopy and important microstructural parameters that affect the mechanical properties of the alloy, such as nodule count, nodularity and graphite shape, were measured. Furthermore, Scanning Electron Microscopy was used to investigate the fracture surfaces of the samples in order to identify crack initiation and propagation zones.
Cracks initiation sites have been found to be microshrinkages close to specimens\u2019 surface in most cases. It was found that in-mould inoculant composition strongly influences the alloy microstructure, such as nodule count and shrinkage porosities size, as well as the fatigue resistance of heavy section ductile iron castings
Practical Algorithms for Multicast Support in Input Queues Switches
Abstract â This paper deals with multicast flow support in N Ă N Input Queued switch architectures. A practical approach to support multicast traffic is presented, assuming that O(N) queues are available at each input port. The focus is on dynamic queueing policies, where, at each input port, multicast flows are assigned to one among the available queues when flows become active: flows are assigned to queues according to switch queue status and, possibly, to flow information. We discuss queueing assignments, scheduling algorithms and flow activity definition models. We explain why dynamic queueing disciplines may outperform static policies, and we show that, even in the most favorable conditions for static policies, they provide comparable performance. I
Exploiting Homology Information in Nontemplate Based Prediction of Protein Structures
In this paper we describe a novel strategy for exploring the conformational space of proteins and show that this leads to better models for proteins the structure of which is not amenable to template based methods. Our strategy is based on the assumption that the energy global minimum of homologous proteins must correspond to similar conformations, while the precise profiles of their energy landscape, and consequently the positions of the local minima, are likely to be different. In line with this hypothesis, we apply a replica exchange Monte Carlo simulation protocol that, rather than using different parameters for each parallel simulation, uses the sequences of homologous proteins. We show that our results are competitive with respect to alternative methods, including those producing the best model for each of the analyzed targets in the CASP10 (10th Critical Assessment of techniques for protein Structure Prediction) experiment free modeling category
- âŠ