Dynamique de la végétation des savanes des Llanos colombiens sous différentes charges et différents temps de repos après feux

La partie orientale de la Colombie, les Llanos, est constituée de vastes formations herbeuses. Ces grandes étendues de savanes représentent un potentiel énorme pour l'élevage extensif de bovins. Ce sont des pâturages naturels qui possèdent malheureusement une faible productivité et une qualité médiocre dues à des sols de basse fertilité, déficients en phosphore et de forte acidité. Afin d'améliorer la productivité des animaux soumis à un élevage extensif dans les savanes des Llanos colombiens, une étude en collaboration avec le CIAT et l'EMVT a été faite sur : - l'évolution des espèces végétales sous l'influence de trois charges animales (forte, moyenne, basse) et de différents temps de repos après feux (0, 2, 4, 8 semaines), - la biomasse (aérienne et racinaire), - la dynamique de la végétation, - et la valeur pastorale des principales espèces fourragères (essentiellement des Poacées) constituant les savanes des Llanos Orientaux. Les résultats de cette étude 'appelée essai Caribey) figurent dans le présent document et sont comparés avec d'autres résultats obtenus par différents auteurs, ceci afin de pouvoir déterminer les raisons de l'évolution de la végétation (résumé d'auteur

The MIRAS demonstrator pilot project. Towards SMOS

[eng] Following several earlier feasibility studies, in 1998 ESA began the MIRAS Demonstrator Pilot Project within the Agency's Technology Research Programme (TRP) and General Support Technology Programme (GSTP) in an attempt to provide a technology solution to the inherent challenges of L-band radiometry. The objective was to build a representative element of the MIRAS instrument, which has subsequently been selected as the main payload for the Soil Moisture and Ocean Salinity (SMOS) mission. A second phase was initiated in April 2001 to demonstrate key end-to-end instrument performances, including antenna deployment and image validation, following approval of the SMOS mission Phase-A at the end of 1999

Evolution of the bi-stable wake of a square-back automotive shape

Square-back shapes are popular in the automotive market for their high level of practicality. These geometries, however, are usually characterised by high drag and their wake dynamics present aspects, such as the coexistence of a long-time bi-stable behaviour and short-time global fluctuating modes that are not fully understood. In the present paper, the unsteady behaviour of the wake of a generic square-back car geometry is characterised with an emphasis on identifying the causal relationship between the different dynamic modes in the wake. The study is experimental, consisting of balance, pressure, and stereoscopic PIV measurements. Applying wavelet and cross-wavelet transforms to the balance data, a quasi-steady correlation is demonstrated between the forces and bi-stable modes. This is investigated by applying proper orthogonal decomposition to the pressure and velocity data sets and a new structure is proposed for each bi-stable state, consisting of a hairpin vortex that originates from one of the two model’s vertical trailing edges and bends towards the opposite side as it merges into a single streamwise vortex downstream. The wake pumping motion is also identified and for the first time linked with the motion of the bi-stable vortical structure in the streamwise direction, resulting in out-of-phase pressure variations between the two vertical halves of the model base. A phase-averaged low-order model is also proposed that provides a comprehensive description of the mechanisms of the switch between the bi-stable states. It is demonstrated that, during the switch, the wake becomes laterally symmetric and, at this point, the level of interaction between the recirculating structures and the base reaches a minimum, yielding, for this geometry, a 7% reduction of the base drag compared to the time-averaged result