Nonequilibrium fluctuations in a resistor

In small systems where relevant energies are comparable to thermal agitation, fluctuations are of the order of average values. In systems in thermodynamical equilibrium, the variance of these fluctuations can be related to the dissipation constant in the system, exploiting the Fluctuation-Dissipation Theorem (FDT). In non-equilibrium steady systems, Fluctuations Theorems (FT) additionally describe symmetry properties of the probability density functions (PDFs) of the fluctuations of injected and dissipated energies. We experimentally probe a model system: an electrical dipole driven out of equilibrium by a small constant current $I$, and show that FT are experimentally accessible and valid. Furthermore, we stress that FT can be used to measure the dissipated power $\bar{\cal P}=RI^2$ in the system by just studying the PDFs symmetries.Comment: Juillet 200

G-300: The first French Getaway Special microgravity measurements of fluid thermal conductivity

Thermal conductivity measurements on liquids are difficult to perform on Earth because of thermal motions due to convection. In microgravity, the convection due to buoyancy is evanescent, and a strong reduction of Rayleigh and Nusselt numbers can be expected. Three low viscosity liquids are selected to carry out the measurements; distilled water (standard) and two silicone oils. A modified hot plate method with a simplified guard ring is used; the reduction of convective motions permitted the use in the experimental cells of larger interplate distances and/or temperature differences than in Earth measurements, improving the accuracy. Comparisons between Earth and orbit results may help to understand the convection occurrence in the cells. Thermal, vibrational, and EMI tests have proved that the design satisfies the NASA requirements

Effects of curvature on hydrothermal waves instability of radial thermocapillary flows

URL: http://www-spht.cea.fr/articles/s01/006 Effets de courbure sur l'instabilité en ondes hydrothermales d'un écoulement thermocapillaire radialInternational audienceThe stability of a thermocapillary flow in an extended cylindrical geometry is analysed. This flow occurs in a thin liquid layer with a disk shape when a radial temperature gradient is applied along the horizontal free surface. Besides the aspect ratio, a second parameter related to the local curvature is introduced to describe completely the geometrical effects. We recover classical hydrothermal waves as predicted by Smith and Davis [1] but the properties of these waves are shown to evolve with the curvature parameter, thus leading to a non uniform pattern over the cell. Moreover, it is shown that the problem is not invariant with respect to the exchange of the hot and cold sides.Nous étudions la stabilité linéaire d'un écoulement thermocapillaire en géométrie cylindrique étendue. Un tel écoulement est produit dans un disque de fluide dont la surface libre horizontale est soumise à un gradient de température purement radial. Outre le rapport d'aspect, un second paramètre lié à la courbure locale est introduit pour caractériser la géométrie du problème. L'instabilité en ondes hydrothermales prédite par [1] est retrouvée mais les propriétés des ondes sont altérées par la courbure locale ce qui explique l'existence de structures non uniformes. La dissymétrie du problème vis-à-vis d'une inversion du gradient de température est aussi mise en évidence

Simulating microbial degradation of organic matter in a simple porous system using the 3-D diffusion-based model MOSAIC

This paper deals with the simulation of microbial degradation of organic matter in soil within the pore space at a microscopic scale. Pore space was analysed with micro-computed tomography and described using a sphere network coming from a geometrical modelling algorithm. The biological model was improved regarding previous work in order to include the transformation of dissolved organic compounds and diffusion processes. We tested our model using experimental results of a simple substrate decomposition experiment (fructose) within a simple medium (sand) in the presence of different bacterial strains. Separate incubations were carried out in microcosms using five different bacterial communities at two different water potentials of −10 and −100 cm of water. We calibrated the biological parameters by means of experimental data obtained at high water content, and we tested the model without changing any parameters at low water content. Same as for the experimental data, our simulation results showed that the decrease in water content caused a decrease of mineralization rate. The model was able to simulate the decrease of connectivity between substrate and microorganism due the decrease of water content

Robust seismic velocity change estimation using ambient noise recordings

We consider the problem of seismic velocity change estimation using ambient noise recordings. Motivated by [23] we study how the velocity change estimation is affected by seasonal fluctuations in the noise sources. More precisely, we consider a numerical model and introduce spatio-temporal seasonal fluctuations in the noise sources. We show that indeed, as pointed out in [23], the stretching method is affected by these fluctuations and produces misleading apparent velocity variations which reduce dramatically the signal to noise ratio of the method. We also show that these apparent velocity variations can be eliminated by an adequate normalization of the cross-correlation functions. Theoretically we expect our approach to work as long as the seasonal fluctuations in the noise sources are uniform, an assumption which holds for closely located seismic stations. We illustrate with numerical simulations and real measurements that the proposed normalization significantly improves the accuracy of the velocity change estimation

A programmable 10 Gigabit injector for the LHCb DAQ and its upgrade

The LHCb High Level Trigger and Data Acquisition system selects about 2 kHz of events out of the 1 MHz of events, which have been selected previously by the first-level hardware trigger. The selected events are consolidated into files and then sent to permanent storage for subsequent analysis on the Grid. The goal of the upgrade of the LHCb readout is to lift the limitation to 1 MHz. This means speeding up the DAQ to 40 MHz. Such a DAQ system will certainly employ 10 Gigabit or technologies and might also need new networking protocols: a customized TCP or proprietary solutions. A test module is being presented, which integrates in the existing LHCb infrastructure. It is a 10-Gigabit traffic generator, flexible enough to generate LHCb’s raw data packets using dummy data or simulated data. These data are seen as real data coming from sub-detectors by the DAQ. The implementation is based on an FPGA using 10 Gigabit Ethernet interface. This module is integrated in the experiment control system. The architecture, implementation, and performance results of the solution will be presented

Work fluctuation theorems for harmonic oscillators

The work fluctuations of an oscillator in contact with a thermostat and driven out of equilibrium by an external force are studied experimentally and theoretically within the context of Fluctuation Theorems (FTs). The oscillator dynamics is modeled by a second order Langevin equation. Both the transient and stationary state fluctuation theorems hold and the finite time corrections are very different from those of a first order Langevin equation. The periodic forcing of the oscillator is also studied; it presents new and unexpected short time convergences. Analytical expressions are given in all cases

On spectral scaling laws for incompressible anisotropic MHD turbulence

A heuristic model is given for anisotropic magnetohydrodynamics (MHD) turbulence in the presence of a uniform external magnetic field B_0 {\bf {\hat e}_{\pa}}. The model is valid for both moderate and strong $B_0$ and is able to describe both the strong and weak wave turbulence regimes as well as the transition between them. The main ingredient of the model is the assumption of constant ratio at all scales between \add{the} linear wave period and \add{the} nonlinear turnover timescale. Contrary to the model of critical balance introduced by Goldreich and Sridhar [P. Goldreich and S. Sridhar, ApJ {\bf 438}, 763 (1995)], it is not assumed in addition that this ratio be equal to unity at all scales which allows us to use the Iroshnikov-Kraichnan phenomenology. It is then possible to recover the widely observed anisotropic scaling law \kpa \propto \kpe^{2/3} between parallel and perpendicular wavenumbers (with reference to B_0 {\bf {\hat e}_{\pa}}) and to obtain the universal prediction, $3\alpha + 2\beta = 7$, for the total energy spectrum E(\kpe,\kpa) \sim \kpe^{-\alpha} \kpa^{-\beta}. In particular, with such a prediction the weak Alfv\'en wave turbulence constant-flux solution is recovered and, for the first time, a possible explanation to its precursor found numerically by Galtier et al [S. Galtier et al., J. Plasma Phys. {\bf 63}, 447 (2000)] is given

Convective and absolute Eckhaus instability leading to modulated waves in a finite box

We report experimental study of the secondary modulational instability of a one-dimensional non-linear traveling wave in a long bounded channel. Two qualitatively different instability regimes involving fronts of spatio-temporal defects are linked to the convective and absolute nature of the instability. Both transitions appear to be subcritical. The spatio-temporal defects control the global mode structure.Comment: 5 pages, 7 figures (ReVTeX 4 and amsmath.sty), final versio

Ondes hydrothermales non-linéaires dans un disque et un anneau

URL: http://www-spht.cea.fr/articles/S98/109Nous nous intéressons à la convection thermocapillaire, produite par l'imposition d'un gradient {\it horizontal} de température sur une mince couche de fluide avec surface libre. En géométrie bidimensionnelle nous observons deux modes différents en compétition, tandis qu'en géométrie bidimensionnelle avec conditions limites périodiques, nous étudions la transition à la turbulence d'une onde propagative homogène