5,284 research outputs found
Reactive scheduling using a multi-agent model: the SCEP framework
Multi-agent systems have been successfully applied to the scheduling problem for some time. However, their use often leads to poorly unsatisfactory disappointing results. A new multi-agent model, called supervisor, customers, environment, producers (SCEP), is suggested in this paper. This model, developed for all types of planning activities, introduces a dialogue between two communities of agents leading to a high level of co-operation. Its two main interests are the following: first it provides a more efficient control of the consequences generated by the local decisions than usual systems to each agent, then the adopted architecture and behaviour permit an easy co-operation between the different SCEP models, which can represent different production functions such as manufacturing, supply management, maintenance or different workshops. As a consequence, the SCEP model can be adapted to a great variety of scheduling/planning problems. This model is applied to the basic scheduling problem of flexible manufacturing systems, andit permits a natural co-habitation between infinite capacity scheduling processes, performedby the manufacturing orders, and finite capacity scheduling processes, performed by the machines. It also provides a framework in order to react to the disturbances occurring at different levels of the workshop
gamma* gamma -> pi pi at large Q^2
The QCD analysis of the process gamma* gamma -> pi pi at large Q^2 and small
center-of-mass energy allows one to access a new hadronic observable describing
the exclusive transition from a q qbar or g g state to a pair of mesons. A
fruitful study may be envisaged at existing machines.Comment: 7 pages, to appear in the Proceedings of PHOTON 2000, Ambleside,
England, August 200
Experimental and numerical study of the accuracy of flame-speed measurements for methane/air combustion in a slot burner
Measuring the velocities of premixed laminar flames with precision remains a controversial issue in the combustion community. This paper studies the accuracy of such measurements in two-dimensional slot burners and shows that while methane/air flame speeds can be measured with reasonable accuracy, the method may lack precision for other mixtures such as hydrogen/air. Curvature at the flame tip, strain on the flame sides and local quenching at the flame base can modify local flame speeds and require correc- tions which are studied using two-dimensional DNS. Numerical simulations also provide stretch, dis- placement and consumption flame speeds along the flame front. For methane/air flames, DNS show that the local stretch remains small so that the local consumption speed is very close to the unstretched premixed flame speed. The only correction needed to correctly predict flame speeds in this case is due to the finite aspect ratio of the slot used to inject the premixed gases which induces a flow acceleration in the measurement region (this correction can be evaluated from velocity measurement in the slot section or from an analytical solution). The method is applied to methane/air flames with and without water addition and results are compared to experimental data found in the literature. The paper then discusses the limitations of the slot-burner method to measure flame speeds for other mixtures and shows that it is not well adapted to mixtures with a Lewis number far from unity, such as hydrogen/air flames
Gluon radiation by heavy quarks at intermediate energies
Employing scalar QCD we study the gluon emission of heavy quarks created by
the interaction with light quarks considered as dynamical scattering centers.
We develop approximation formulas for the high energy limit and study when the
full calculation reaches this high energy limit. For zero quark masses and in
the high energy limit our model reproduces the Gunion-Bertsch results. We
justify why scalar QCD represents a good approximation to the full QCD approach
for the energy loss of heavy quarks. In the regime of accessible phenomenology
we observe that the emission at small transverse momentum (dead cone effect) is
less suppressed than originally suggested. We also investigate the influence of
a finite gluon mass on the discussed results
Universal current fluctuations in the symmetric exclusion process and other diffusive systems
We show, using the macroscopic fluctuation theory of Bertini, De Sole,
Gabrielli, Jona-Lasinio, and Landim, that the statistics of the current of the
symmetric simple exclusion process (SSEP) connected to two reservoirs are the
same on an arbitrary large finite domain in dimension as in the one
dimensional case. Numerical results on squares support this claim while results
on cubes exhibit some discrepancy. We argue that the results of the macroscopic
fluctuation theory should be recovered by increasing the size of the contacts.
The generalization to other diffusive systems is straightforward.Comment: 6 pages, 4 figure
Dielectric properties of CaCu3Ti4O12 based multiphased ceramics
A âsoft chemistryâ method, the coprecipitation, has been used to synthesize the perovskite CaCu3Ti4O12 (CCT). Three main types of materials were obtained for both powders and sintered ceramics: a monophased consisting of the pure CCT phase, a biphased (CCT + CaTiO3), and a three-phased (CCT + CaTiO3 + copper oxide (CuO or Cu2O)). These ceramics, sintered at low temperature, 1050 °C, present original dielectric properties. The relative permittivity determined in the temperature range (â150 < T < 250 °C) is significantly higher than the one reported in the literature. Internal barrier layer capacitor is the probable mechanism to explain the particular behaviour. Moreover, the presence of a copper oxide phase beside the perovksite CCT plays an important role for enhancing the dielectric properties
Radiative energy loss in the absorptive QGP: taming the long formation lengths in coherent emission
In an absorptive plasma, damping of radiation mechanisms can influence the
bremsstrahlung formation in case of large radiation formation lengths. We study
qualitatively the influence of this effect on the gluon bremsstrahlung spectrum
off heavy quarks in the quark-gluon plasma. Independent of the heavy-quark
mass, the spectrum is found to be strongly suppressed in an intermediate gluon
energy region which grows with increasing gluon damping rate and increasing
energy of the heavy quark. Thus, just as polarization effects in the plasma
render the bremsstrahlung spectra independent of the quark mass in the soft
gluon regime, damping effects tend to have a similar impact for larger gluon
energies.Comment: contribution to the International Meeting "Excited QCD", Peniche,
Portugal, 06 - 12 May 201
Effects of hydrogen and steam addition on laminar burning velocity of methaneâair premixed flame: Experimental and numerical analysis
Effects of hydrogen enrichment and steam addition on laminar burning velocity of methaneeair premixed flame were studied both experimentally and numerically. Measurements were carried out using the slot burner method at 1 bar for fresh gases temperatures of 27 °C and 57 °C and for variable equivalence ratios going from 0.8 to 1.2. The hydrogen content in the fuel was varied from 0% to 30% in volume and the steam content in the air was varied from 0 to 112 g/kg (0e100% of relative humidity). Numerical calculations were performed using the COSILAB code with the GRI-Mech 3.0 mechanism for one-dimensional premixed flames. The calculations were implemented first at room temperature and pressure and then extended to higher temperatures (up to 917 K) and pressures (up to 50 bar). Measurements of laminar burning velocities of methanee hydrogeneair and methaneeairesteam agree with the GRI-Mech calculations and previous measurements from literature obtained by different methods. Results show that enrich- ment by hydrogen increases of the laminar burning velocity and the adiabatic flame temperature. The addition of steam to a methaneeair mixture noticeably decreases the burning velocity and the adiabatic flame temperature. Modeling shows that isentropic compression of fresh gases leads to the increase of laminar burning velocity
Low temperature reflectivity study of ZnO/(Zn,Mg)O quantum wells grown on M-plane ZnO substrates
We report growth of high quality ZnO/Zn0.8Mg0.2O quantum well on M-plane
oriented ZnO substrates. The optical properties of these quantum wells are
studied by using reflectance spectroscopy. The optical spectra reveal strong
in-plane optical anisotropies, as predicted by group theory, and marked
reflectance structures, as an evidence of good interface morphologies.
Signatures ofc onfined excitons built from the spin-orbit split-off valence
band, the analog of exciton C in bulk ZnO are detected in normal incidence
reflectivity experiments using a photon polarized along the c axis of the
wurtzite lattice. Experiments performed in the context of an orthogonal photon
polarization, at 90^{\circ}; of this axis, reveal confined states analogs of A
and B bulk excitons. Envelope function calculations which include excitonic
interaction nicely account for the experimental report
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