85 research outputs found
Credibility and Monetary Policy
This paper revisits the ability of central banks to manage private sector's expectations depending on its credibility and how this affects the use of interest rate rules and pegs to achieve monetary policy objectives. When private agents can only provide limited incentives for the central bank to follow a policy, we show that resulting limited credibility allows a central bank to prevents the inflation from diverging by defaulting on past promises if necessary. As a result, the Taylor rule, when expected, anchors inflation expectations on a unique equilibrium path as long as the Taylor principle is satisfied. Finally, we also show that limited credibility restricts the impact of long-term interest rate pegs, so as to make current conditions less dependent on future policy changes
Experimental evidence of a hydrodynamic soliton gas
We report on an experimental realization of a bi-directional soliton gas in a
34~m-long wave flume in shallow water regime. We take advantage of the fission
of a sinusoidal wave to inject continuously solitons that propagate along the
tank, back and forth. Despite the unavoidable damping, solitons retain
adiabatically their profile, while decaying. The outcome is the formation of a
stationary state characterized by a dense soliton gas whose statistical
properties are well described by a pure integrable dynamics. The basic
ingredient in the gas, i.e. the two-soliton interaction, is studied in details
and compared favourably with the analytical solutions of the Kaup-Boussinesq
integrable equation. High resolution space-time measurements of the surface
elevation in the wave flume provide a unique tool for studying experimentally
the whole spectrum of excitations.Comment: accepted for publication in Physical Review Letter
A fourth-order compact finite volume scheme for fully nonlinear and weakly dispersive Boussinesq-type equations. Part I: Model development and analysis
International audienceA highâorder finite volume scheme is developed to numerically integrate a fully nonlinear and weakly dispersive set of Boussinesqâtype equations (the soâcalled Serre equations) (J. Fluid Mech. 1987; 176:117â134; Surveys Geophys. 2004; 25(3â4):315â337). The choice of this discretization strategy is motivated by the fact that this particular set of equations is recasted in a convenient quasiâconservative form. Cell face values are reconstructed using implicit compact schemes (J. Comput. Phys. 1999; 156:137â180; J. Comput. Phys. 2004; 198:535â566) and time integration is performed with the help of a fourâstage RungeâKutta method. Numerical properties of the proposed scheme are investigated both, analytically using linear spectral analysis, and numerically for highly nonlinear cases. The numerical analysis indicates that the newly developed scheme has wider stability regions and better spectral resolution than most of the previously published numerical methods used to handle equivalent set of equations. Moreover, it was also noticed that the use of mixedâorder strategies to discretize convective and dispersive terms may result in an important overall reduction of the spectral resolution of the scheme. Additionally, there is some numerical evidence, which seems to indicate that the incorporation of a highâorder dispersion correction term as given by Madsen et al. (Coastal Eng. 1991; 15:371â388) may introduce instability in the syste
Experimental study of integrable turbulence in shallow water
We analyze a set of bidirectional wave experiments in a linear wave flume of
which some are conducive to integrable turbulence. In all experiments the
wavemaker forcing is sinusoidal and the wave motion is recorded by seven
high-resolution side-looking cameras. The periodic scattering transform is
implemented and power spectral densities computed to discriminate linear wave
motion states from integrable turbulence and soliton gas. Values of the
wavemaker forcing Ursell number and relative amplitude are required to be above
some threshold values for the integral turbulence to occur. Despite the
unavoidable slow damping, soliton gases achieve stationary states because of
the continuous energy input by the wavemaker. The statistical properties are
given in terms of probability density distribution, skewness and kurtosis. The
route to integrable turbulence, by the disorganization of the wavemaker induced
sinusoidal wave motion, depends on the non-linearity of the waves but equally
on the amplitude amplification and reduction due to the wavemaker feedback on
the wave field
Physical and numerical modelling of sand liquefaction in waves interacting with a vertical wall
International audienceWave induced liquefaction at a coastal structure is studied. Experiments in a glass-wall flume filled with a partially saturated bed of light-weight sediment are presented. Periodic waves and single wave loadings are simulated. For large enough wave conditions an excess pore pressure is recorded within the soil and a liquefaction threshold is reached. Velocity fields obtained from video recordings display large zones of the bed that behave as a fluid. Phases of soil compaction and dilatation are identified. Moreover, a Discrete Element Method - Pore-scale Finite Volume model is used to simulate the wave-sediment interactions. The computation of the coupling between the flow and the motion of the particles enables to reproduce the excess pore pressure that lead to liquefaction and the progressive compaction of the bed
Etude de couches limites oscillantes par vélocimétrie laser Doppler
International audienceLe transport sĂ©dimentaire induit par les vagues Ă l'approche de la cĂŽte est pilotĂ© par des processus non linĂ©aires et turbulents. Les non-linĂ©aritĂ©s des vagues se caractĂ©risent par une dissymĂ©trie de vitesse (les crĂȘtes hautes des vagues sont de courte durĂ©e et les creux peu profonds de longue durĂ©e) et une asymĂ©trie de vitesse (ou dissymĂ©trie d'accĂ©lĂ©ration, caractĂ©risant la raideur des fronts). Des Ă©tudes rĂ©centes indiquent que des fronts raides (vagues asymĂ©triques) produisent des vitesses dissymĂ©triques dans la couche limite. Ainsi, pour dĂ©velopper des formules de prĂ©diction de transport des sĂ©diments, la comprĂ©hension dĂ©taillĂ©e de la dynamique de la couche limite de fond et des contraintes de cisaillement sous les ondes de surface apparaĂźt essentielle. Ceci justifie de chercher Ă rĂ©aliser des mesures de vitesse dans les tous premiers millimĂštres au-dessus du lit.Par ailleurs, la caractĂ©risation de la turbulence sous les vagues dĂ©ferlantes reste une question ouverte, en particulier pour chercher Ă Ă©valuer la part provenant de la vague dĂ©ferlĂ©e de celle produite par frottement au fond.Actuellement, lâessentiel de notre connaissance des couches limites oscillantes est issu de mesures rĂ©alisĂ©es sur des fonds fixes horizontaux. En laboratoire, lâĂ©volution des non-linĂ©aritĂ©s des vagues, lors de leur propagation et de leur dĂ©ferlement, a Ă©tĂ© principalement Ă©tudiĂ©e pour des plages de pentes relativement fortes (> 1:40). Cependant, des Ă©tudes de terrain rĂ©centes sur des plages rĂ©elles de pentes moins raides (1:80) ont montrĂ© que certains processus non-linĂ©aires sont diffĂ©rents par rapport aux cas des plages de pentes relativement raides (> 1:40). Cette constatation a motivĂ© le lancement dâune sĂ©rie d'expĂ©riences de laboratoire dans le cadre du projet europĂ©en GLOBEX sur une plage Ă fond fixe de pente 1:80
Large Scale Cross-Correlations in Internet Traffic
The Internet is a complex network of interconnected routers and the existence
of collective behavior such as congestion suggests that the correlations
between different connections play a crucial role. It is thus critical to
measure and quantify these correlations. We use methods of random matrix theory
(RMT) to analyze the cross-correlation matrix C of information flow changes of
650 connections between 26 routers of the French scientific network `Renater'.
We find that C has the universal properties of the Gaussian orthogonal ensemble
of random matrices: The distribution of eigenvalues--up to a rescaling which
exhibits a typical correlation time of the order 10 minutes--and the spacing
distribution follow the predictions of RMT. There are some deviations for large
eigenvalues which contain network-specific information and which identify
genuine correlations between connections. The study of the most correlated
connections reveals the existence of `active centers' which are exchanging
information with a large number of routers thereby inducing correlations
between the corresponding connections. These strong correlations could be a
reason for the observed self-similarity in the WWW traffic.Comment: 7 pages, 6 figures, final versio
Clinical and Experimental Factors Influencing the Efficacy of Neurofeedback in ADHD: A Meta-Analysis
Meta-analyses have been extensively used to evaluate the efficacy of neurofeedback (NFB) treatment for Attention Deficit/Hyperactivity Disorder (ADHD) in children and adolescents. However, each meta-analysis published in the past decade has contradicted the methods and results from the previous one, thus making it difficult to determine a consensus of opinion on the effectiveness of NFB. This works brings continuity to the field by extending and discussing the last and much controversial meta-analysis by Cortese et al. (1). The extension comprises an update of that work including the latest control trials, which have since been published and, most importantly, offers a novel methodology. Specifically, NFB literature is characterized by a high technical and methodological heterogeneity, which partly explains the current lack of consensus on the efficacy of NFB. This work takes advantage of this by performing a Systematic Analysis of Biases (SAOB) in studies included in the previous meta-analysis. Our extended meta-analysis (k = 16 studies) confirmed the previously obtained results of effect sizes in favor of NFB efficacy as being significant when clinical scales of ADHD are rated by parents (non-blind, p-value = 0.0014), but not when they are rated by teachers (probably blind, p-value = 0.27). The effect size is significant according to both raters for the subset of studies meeting the definition of âstandard NFB protocolsâ (parents' p-value = 0.0054; teachers' p-value = 0.043, k = 4). Following this, the SAOB performed on k = 33 trials identified three main factors that have an impact on NFB efficacy: first, a more intensive treatment, but not treatment duration, is associated with higher efficacy; second, teachers report a lower improvement compared to parents; third, using high-quality EEG equipment improves the effectiveness of the NFB treatment. The identification of biases relating to an appropriate technical implementation of NFB certainly supports the efficacy of NFB as an intervention. The data presented also suggest that the probably blind assessment of teachers may not be considered a good proxy for blind assessments, therefore stressing the need for studies with placebo-controlled intervention as well as carefully reported neuromarker changes in relation to clinical response
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