Contribution à l'étude expérimentale de la turbulence homogène M.H.D. - Première caractérisation de son anisotropie

Although it is theoretically shown that homogeneous MHD turbulence (MHD) with small magnetic Reynolds number has a tendency to anisotropy, experimental studies have so far focused on the measurement of unidirectional properties of this flow. Only the diffusion of a passive scalar was observed in two dimensions indirectly and globally revealing in imperfect conditions of homogeneity, some anisotropy of the velocity field. Therefore, the aim of our work is focused on the direct characterization of the anisotropy. To this end, a numerical signal processing method has been developed for determining the correlations of the velocity at two points and, by Fourier transform, two-dimensional spectrum of studied turbulence. Experimental results demonstrate the progressive encroachment of a spectrum in k-5/3 by a zone in k-3 starting from the high frequencies. A characterization of the anisotropy is then proposed from the chart of the isocorrelations in physical space and isoenergy in Fourier space. These charts, compared to those obtained in the absence of magnetic field, clearly show the quasi-two-dimensionality of the flow for strong numbers of Stuart.Bien qu'il soit théoriquement démontré que la turbulence homogène magnétohydrodynamique (MHD) à petit nombre de Reynolds magnétique présente une tendance à l'anisotropie, les études expérimentales ont porté jusqu'à présent sur la mesure des propriétés monodirectionnelles de cet écoulement. Seule la diffusion d'un scalaire passif a été observée dans deux dimensions révélant de manière indirecte et globale, dans des conditions imparfaites d'homogénéité, une certaine anisotropie du champ de vitesse. Le but de notre travail a donc été centré sur la caractérisation directe de celle-ci. A cet effet, une méthode numérique de traitement du signal a été développée permettant de déterminer les corrélations de vitesse en deux points et, par transformée de Fourier, le spectre bidimensionnel de la turbulence étudiée. Nos résultats expérimentaux mettent en évidence l'envahissement progressif d'un spectre en k-5/3 par une zone en k-3 à partir des hautes fréquences. Une caractérisation de l'anisotropie est ensuite proposée à partir du tracé des cartes d'isocorrélations dans l'espace physique et d'isoénergie dans l'espace de Fourier. Ces cartes comparées à celles obtenues en l'absence de champ magnétique mettent clairement en évidence la quasi-bidimensionnalité de l'écoulement aux forts nombres de Stuart

Hybrid simulations of a multi-span rc viaduct with plain bars and sliding bearings

This paper deals with the seismic response assessment of an old reinforced concrete viaduct and the effectiveness of friction-based retrofitting systems. Emphasis was laid on an old bridge, not properly designed to resist seismic action, consisting of 12 portal piers that support a 13-span bay deck for each independent roadway. On the basis of an OpenSEES finite element frame pier model, calibrated in a previous experimental campaign with cyclic displacement on three 1:4 scale frame piers, a more complex experimental activity using hybrid simulation has been devised. The aim of the simulation was twofold: (i) to increase knowledge of non-linear behavior of reinforced concrete frame piers with plain steel rebars and detailing dating from the late 1950s; and (ii) to study the effectiveness of sliding bearings for seismic response mitigation. Hence, to explore the performance of the as built bridge layout and also of the viaduct retrofitted with friction-based devices, at both serviceability and ultimate limit state conditions, hybrid simulation tests were carried out. In particular, two frame piers were experimentally controlled with eight-actuator channels in the as built case while two frame piers and eight sliding bearings were controlled with 18-actuator channels in the isolated case. The remaining frame piers were part of numerical substructures and were updated offline to accurately track damage evolution

Spatial-Temporal Correlation Analyses of Global Burned Surface Time Series from Remote Sensing Data (1982-1999)

Daily global observations from the Advanced Very High Resolution Radiometers (AVHRR) on the series of meteorological satellites operated by the National Oceanic and Atmospheric Administration (NOAA) between 1982 and 1999 (17 years) were used to generate a new weekly global burnt surface product at a resolution of 8km. Comparison with independently available information on fire locations and timing suggest that whilst the time-series cannot yet be used to make accuracy and quantitative estimates of global burnt area, it does provide a reliable estimate of changes in location, season and interannual variability of burning on the global scale (Carmona-Moreno et al., 2005.a). This paper deals with the connection analysis of this time series and “El Niño” Southern Oscillation (ENSO) events. The spatial-temporal inter-correlation analyses show likely connections between both phenomena at global scale during the period considered even if these results need to be confirmed with longer time series (>40 years) of data.JRC.DDG.H.3-Global environement monitorin

A Wind Turbine Tower Design Based on the Use of Fibre-Reinforced Composites.

Abstract not availableJRC.G-Institute for the Protection and the Security of the Citizen (Ispra

Quasi-static magnetohydrodynamic turbulence at high Reynolds number

We analyse the anisotropy of homogeneous turbulence in an electrically conducting fluid submitted to a uniform magnetic field, for low magnetic Reynolds number, in the quasi- static approximation. We interpret disagreeing previous predictions between linearized theory and simulations: in the linear limit, the kinetic energy of transverse velocity components, normal to the magnetic field, decays faster than the kinetic energy of the axial component, along the magnetic field (Moffatt (1967)); whereas many numerical studies predict a final state characterised by dominant energy of transverse velocity components. We investigate the corresponding nonlinear phenomenon using Direct Numerical Simulations of freely-decaying turbulence, and a two-point statistical spectral closure based on the Eddy Damped Quasi-Normal Markovian model. The transition from the three-dimensional turbulent flow to a "two-and-a-half-dimensional" flow (Montgomery & Turner (1982)) is a result of the combined effects of short-time linear Joule dissipation and longer time nonlinear creation of polarisation anisotropy. It is this combination of linear and nonlinear effects which explains the disagreement between predictions from linearized theory and results from numerical simulations. The transition is characterized by the elongation of turbulent structures along the applied magnetic field, and by the strong anisotropy of directional two-point correlation spectra, in agreement with experimental evidence. Inertial equatorial transfers in both DNS and the model are presented to describe in detail the most important equilibrium dynamics. Spectral scalings are maintained in high Reynolds number turbulence attainable only with the EDQNM model, which also provides simplified modelling of the asymptotic state of quasi-static MHD turbulence.Comment: Journal of Fluid Mechanics, 201

SERFIN Project: Measurements by Optical Methods on the Bottom Part of the Northern In-Filled Wall. Seismic Retrofitting of RC Frames with RC Infilling

Photogrammetry was used to monitor the bottom part of the northern in-filled wall of the SERFIN model. This optical measurement technique is at the developing stage for its application on large structures and the SERFIN project was a testing bench for it. This report gives the first results on the displacement fields and the cracking of the specimen in the monitored zone.JRC.G.4-European laboratory for structural assessmen

A New Tracking Algorithm with Application to a Practical Measurement Case

A new tracking algorithm, based on the C3 thin plate spline interpolation of the template has been tested. The cost function is analytical with respect to the parameters of the linear transformation used in the optimising procedure. When applied to artificial images, this algorithm exhibits a dramatic decrease of the peak-locking. The tests made on practical case show good behaviour and accuracy of the method.JRC.G.5-European laboratory for structural assessmen

Displacement and Strain Field Photogrammetric Measurements of a Reinforced Concrete Slab Submitted to an Earthquakes Loading

These measurements are part of an action aiming to develop and generalise the use of photogrammetry in the European Laboratory for Structural Assessment. ELSA belongs to the Joint Research Centre, a DG of the European Commission. Its experimental facility allows to test full-size buildings with respect to static and cyclic loading, and to simulate earthquake loading through the use of the so-called pseudo-dynamic method â?“ which magnifies the time scale. Structures are usually driven to ruin through series of tests, which implies that fractures opening and closing, and even material removal occur during each cycle of the experiment. The characteristic scales of the tested structures span 2 to 20 meter, and they are made of a wide range of construction materials such as brick masonry, reinforced concrete, steel or composite. The large variety of experimental conditions at ELSA provides a rich field of application for photogrammetric techniques. In the present experiment, the camera was obliquely aiming at the ceiling of the first floor of a building, near the connexion to a pillar, while the structure was under earthquakes loading. In the field of view, a pixel would be of the order of 1 mm. A network of 147 circular targets was disposed on a rectangular mesh (of step 10 cm) on the ceiling, and a series of 1250 photos were sampled at regular time steps in synchronisation with the various sensors and the control loop of the loading. The camera has a resolution of 1536x1024 pixel, with 12 bit sampling and Peltier cooling. This cooling stage provoked the formation of a condensation pattern on the sensor, a parasitic effect that could be fortunately removed through the use of a condensation mask computed by ad-hoc technique applied to the set of images. The camera was calibrated using the method of Bouguet. In first approximation, the ceiling was considered to move quasi-parallel to itself. The targets were monitored along time by various techniques, like fitting of an ellipse to their border, or optimising on the current photo the projection of the perimeter of the target, with respect to translation. The Heidenhain reference sensor of the control loop was placed on the external border of the slab, I meter far from the nearest target. However, the longitudinal displacement are in a very good agreement, showing a difference of 1 mm at maximum, while the amplitude of the movement was 50 mm. On both extremities of the signal, when the building is at rest, a noise of 0.04 mm amplitude could be observed on the difference between Heidenhain and optical method. At the end of the experiment, the difference between the most distant targets (along loading) shows an elongation of approximately 2 mm, corresponding to the remnant opening of the fractures. The quality of the measured displacement field, coupled with the fracture segmentation via morphological image analysis, permitted to compute longitudinal and lateral strain maps as a function of time, with the fracture pattern. A film will be shown, illustrating this quantitative analysis.JRC.G.5-European laboratory for structural assessmen

Interaction between Different Sized Particles during Acoustic Agglomeration of Oil Droplets

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Acoustic Agglomeration of Redispersed Flyash

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe