1,811 research outputs found
M/M/ queues in semi-Markovian random environment
In this paper we investigate an M/M/ queue whose parameters depend on
an external random environment that we assume to be a semi-Markovian process
with finite state space. For this model we show a recursive formula that allows
to compute all the factorial moments for the number of customers in the system
in steady state. The used technique is based on the calculation of the raw
moments of the measure of a bidimensional random set. Finally the case when the
random environment has only two states is deeper analyzed. We obtain an
explicit formula to compute the above mentioned factorial moments when at least
one of the two states has sojourn time exponentially distributed.Comment: 17 pages, 2 figure
Benchmark Analysis of EBR-II Protected Loss-of-Flow Transient
Coordinated Research Project (CRP) on EBR-II Shutdown Heat Removal Tests (SHRT) was established by International Atomic Energy Agency (IAEA). The objective of the project is to support and to improve validation of simulation tools and projects for Sodium-cooled Fast Reactors (SFR). The Experimental Breeder Reactor II (EBR-II) plant was a uranium metal-alloy-fuelled liquid-metal-cooled fast reactor designed and operated by Argonne National Laboratory (ANL) for the U.S. Department of Energy at the Argonne-West site.
In the frame of this project, benchmark analysis of one of the EBR-II shutdown heat removal tests, protected loss-of-flow transient (SHRT-17), has been performed at the Gruppo di Ricerca Nucleare San Piero a Grado (GRNSPG) in Pisa, Italy.
The aim of this paper is to present modeling of EBR-II reactor design using RELAP-3D, and to present results of the transient analysis of SHRT-17. Complete nodalization of the reactor was made from the beginning. Model is divided in primary side that contains core, pumps, reactor pool and, for this kind of reactor specific, Z pipe, and intermediate side that contains Intermediate Heat Exchanger (IHX). Core was modeled with 82 channels that represent all fuel assemblies, and 14 channels for reflector and blanket assemblies.
After achievement of acceptable steady-state results, transient analysis was performed. Starting from full power and flow, both the primary loop and intermediate loop coolant pumps were simultaneously tripped and the reactor was scrammed to simulate a protected loss-of-flow accident. In addition, the primary system auxiliary coolant pump, that normally had an emergency battery power supply, was turned off. Despite early rise of the temperature in the reactor, the natural circulation characteristics managed to keep it at acceptable levels and cooled the reactor down safely at decay heat power levels.
Thermal-hydraulics characteristics and plant behavior was focused on prediction of natural convection cooling by evaluating the reactor core flow and temperatures and their comparison with experimental data that were provided by ANL
The InfraCyrus infrasound sensor
Infrasound sensors are used for a wide range of geophysical applications as the monitoring of volcanic
eruptions, the detection of bolides and the recording of infrasounds generated by earthquakes. In 2006 we
started the development of cheap infrasound sensors, based on commercial electret microphones. They have
been characterized by comparing their response function with existing broadband infrasound sensors. These
sensors, called InfraCyrus, have a good response between 1 and 10 Hz, making their application useful for
various geophysical purposes. Currently, about a dozen of sensors are deployed in the Neapolitan area
showing good performances in the recording of local and regional infrasonic transients
Bianchi Type I Cosmology in N=2, D=5 Supergravity
The dynamics and evolution of Bianchi type I space-times is considered in the
framework of the four-dimensional truncation of a reduced theory obtained from
the N=2,D=5 supergravity. The general solution of the gravitational field
equations can be represented in an exact parametric form. All solutions have a
singular behavior at the initial/final moment, except when the space-time
geometry reduces to the isotropic flat case. Generically the obtained
cosmological models describe an anisotropic, expanding or collapsing, singular
Universe with a non-inflationary evolution for all times.Comment: revised version to appear in PR
Quantum Decoherence of Single-Photon Counters
The interaction of a quantum system with the environment leads to the
so-called quantum decoherence. Beyond its fundamental significance, the
understanding and the possible control of this dynamics in various scenarios is
a key element for mastering quantum information processing. Here we report the
quantitative probing of what can be called the quantum decoherence of
detectors, a process reminiscent of the decoherence of quantum states in the
presence of coupling with a reservoir. We demonstrate how the quantum features
of two single-photon counters vanish under the influence of a noisy
environment. We thereby experimentally witness the transition between the
full-quantum operation of the measurement device to the "semi-classical
regime", described by a positive Wigner function. The exact border between
these two regimes is explicitely determined and measured experimentally
Metastable de Sitter vacua in N=2 to N=1 truncated supergravity
We study the possibility of achieving metastable de Sitter vacua in general
N=2 to N=1 truncated supergravities without vector multiplets, and compare with
the situations arising in N=2 theories with only hypermultiplets and N=1
theories with only chiral multiplets. In N=2 theories based on a quaternionic
manifold and a graviphoton gauging, de Sitter vacua are necessarily unstable,
as a result of the peculiar properties of the geometry. In N=1 theories based
on a Kahler manifold and a superpotential, de Sitter vacua can instead be
metastable provided the geometry satisfies some constraint and the
superpotential can be freely adjusted. In N=2 to N=1 truncations, the crucial
requirement is then that the tachyon of the mother theory be projected out from
the daughter theory, so that the original unstable vacuum is projected to a
metastable vacuum. We study the circumstances under which this may happen and
derive general constraints for metastability on the geometry and the gauging.
We then study in full detail the simplest case of quaternionic manifolds of
dimension four with at least one isometry, for which there exists a general
parametrization, and study two types of truncations defining Kahler
submanifolds of dimension two. As an application, we finally discuss the case
of the universal hypermultiplet of N=2 superstrings and its truncations to the
dilaton chiral multiplet of N=1 superstrings. We argue that de Sitter vacua in
such theories are necessarily unstable in weakly coupled situations, while they
can in principle be metastable in strongly coupled regimes.Comment: 40 pages, no figure
Effects of synchronous music on treadmill running among elite triathletes
This is the post-print version of the final paper published in Journal of Science and Medicine in Sport. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2011 Elsevier B.V.Objectives: Music can provide ergogenic, psychological, and psychophysical benefits during physical activity, especially when movements are performed synchronously with music. The present study developed the train of research on synchronous music and extended it to elite athletes. Design: Repeated-measures laboratory experiment. Method: Elite triathletes (n = 11) ran in time to self-selected motivational music, a neutral equivalent and a no-music control during submaximal and exhaustive treadmill running. Measured variables were time-to-exhaustion, mood responses, feeling states, RPE, blood lactate concentration, oxygen consumption and running economy. Results: Time-to-exhaustion was 18.1% and 19.7% longer, respectively, when running in time to motivational and neutral music, compared to no music. Mood responses and feeling states were more positive with motivational music compared to either neutral music or no music. RPE was lowest for neutral music and highest for the no-music control. Blood lactate concentrations were lowest for motivational music. Oxygen consumption was lower with music by 1.0%â2.7%. Both music conditions were associated with better running economy than the no-music control. Conclusions: Although neutral music did not produce the same level of psychological benefits as motivational music, it proved equally beneficial in terms of time-to-exhaustion and oxygen consumption. In functional terms, the motivational qualities of music may be less important than the prominence of its beat and the degree to which participants are able to synchronise their movements to its tempo. Music provided ergogenic, psychological and physiological benefits in a laboratory study and its judicious use during triathlon training should be considered.QAS Centre of Excellence for Applied Sport
Science Researc
Optical response of a misaligned and suspended Fabry-Perot cavity
The response to a probe laser beam of a suspended, misaligned and detuned
optical cavity is examined. A five degree of freedom model of the fluctuations
of the longitudinal and transverse mirror coordinates is presented. Classical
and quantum mechanical effects of radiation pressure are studied with the help
of the optical stiffness coefficients and the signals provided by an FM
sideband technique and a quadrant detector, for generic values of the product
of the fluctuation frequency times the cavity round trip. A
simplified version is presented for the case of small misalignments. Mechanical
stability, mirror position entanglement and ponderomotive squeezing are
accommodated in this model. Numerical plots refer to cavities under test at the
so-called Pisa LF facility.Comment: 14 pages (4 figures) submitted to Phys. Rev.
The Relativistic Avatars of Giant Magnons and their S-Matrix
The motion of strings on symmetric space target spaces underlies the
integrability of the AdS/CFT correspondence. Although these theories, whose
excitations are giant magnons, are non-relativistic they are classically
equivalent, via the Polhmeyer reduction, to a relativistic integrable field
theory known as a symmetric space sine-Gordon theory. These theories can be
formulated as integrable deformations of gauged WZW models. In this work we
consider the class of symmetric spaces CP^{n+1} and solve the corresponding
generalized sine-Gordon theories at the quantum level by finding the exact
spectrum of topological solitons, or kinks, and their S-matrix. The latter
involves a trignometric solution of the Yang-Baxer equation which exhibits a
quantum group symmetry with a tower of states that is bounded, unlike for
magnons, as a result of the quantum group deformation parameter q being a root
of unity. We test the S-matrix by taking the semi-classical limit and comparing
with the time delays for the scattering of classical solitons. We argue that
the internal CP^{n-1} moduli space of collective coordinates of the solitons in
the classical theory can be interpreted as a q-deformed fuzzy space in the
quantum theory. We analyse the n=1 case separately and provide a further test
of the S-matrix conjecture in this case by calculating the central charge of
the UV CFT using the thermodynamic Bethe Ansatz.Comment: 33 pages, important correction to S-matrix to ensure crossing
symmetr
Alleviating the non-ultralocality of coset sigma models through a generalized Faddeev-Reshetikhin procedure
The Faddeev-Reshetikhin procedure corresponds to a removal of the
non-ultralocality of the classical SU(2) principal chiral model. It is realized
by defining another field theory, which has the same Lax pair and equations of
motion but a different Poisson structure and Hamiltonian. Following earlier
work of M. Semenov-Tian-Shansky and A. Sevostyanov, we show how it is possible
to alleviate in a similar way the non-ultralocality of symmetric space sigma
models. The equivalence of the equations of motion holds only at the level of
the Pohlmeyer reduction of these models, which corresponds to symmetric space
sine-Gordon models. This work therefore shows indirectly that symmetric space
sine-Gordon models, defined by a gauged Wess-Zumino-Witten action with an
integrable potential, have a mild non-ultralocality. The first step needed to
construct an integrable discretization of these models is performed by
determining the discrete analogue of the Poisson algebra of their Lax matrices.Comment: 31 pages; v2: minor change
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