Fish assemblages and stream hydraulics: consistent relations across spatial scales and regions

General relationships between organisms and their habitat, consistent across spatial scales and regions, suggest the existence of repeatable ecological processes and are useful for the management of stream networks. From published data, we defined four guilds of European fish species with contrasting preferences for microhabitat hydraulics within stream reaches. At the scale of stream reaches and across 139 French sites (590 460 fishes sampled), we analysed how fish guild proportions were related to reach hydraulics (proportion of pools vs. riffles %POOL; median discharge by unit width Q50/W). The strongest correlations were observed between two fish guilds and %POOL (p < 0.001, r² ≥ 0.41) and between one fish guild proportion and Q50/W (p < 0.001, r² = 0.10). These reach–scale relationships were consistent across six large French basins, and consistent with the analyses made at the microhabitat scale. Therefore, microhabitat preferences for hydraulics are strong enough to generate consistent reach‐scale community responses to hydraulics across regions, despite the influence of other filters such as temperature, nutrient levels or history. The distribution of basic geomorphic features (pools, riffles) in streams and their modification (by dams, weirs and dikes) can modify the proportion of fish guilds by up to 80%, probably contributing to the long‐term decline of riffle‐dwelling species in Europe. Copyright © 2006 John Wiley & Sons, Ltd.     

TRANSFERABILITY OF HYDRAULIC PREFERENCE MODELS FOR AQUATIC MACROINVERTEBRATES

Quantifying the transferability across stream reaches and sampling occasions of models relating the density of aquatic taxa to microhabitat hydraulics are important for increasing our understanding of the mechanisms explaining microhabitat selection. We revisited data collected at multiple occasions in two regions, that is, in nine reaches of seven German streams and in four reaches of a large French river, to provide detailed information by taxon on the transferability of hydraulic preferences of aquatic macroinvertebrates. In each region and for each taxon, we tested the statistical significance of a regional preference model relating density to microhabitat hydraulics simultaneously in all surveys (i.e. reach–occasion combinations). The transferability of hydraulic preferences was assessed using cross‐validation procedures within each region. The regional models showed significant associations with microhabitat hydraulics for 56% of the 151 taxa collected in Germany and for 67% of 66 taxa in France. Cross validations indicated that regional models significantly explained density variations within independent surveys for 60% of the cases in Germany and 54% in France. We conclude that both regional and survey‐specific models are useful in specific studies, depending on the taxa considered and the spatial extent and objectives of the application. Graphs of all raw data and models are supplied as supporting information. Copyright © 2012 John Wiley & Sons, Ltd.     

Dual-stage tertiary creep of a ceramic matrix composite

The creep behaviour of an alumina fibre/silicon carbide matrix composite has been studied. The creep curves are characterised by a short primary stage followed by one or two tertiary stages. The secondary regime of this composite is limited to a single point. The occurrence of one or two tertiary stages in the creep behaviour is discussed, and some theoretical considerations are invoked to explain this behaviour. Creep in this composite is controlled by two mechanisms, namely viscoplastic creep of the alumina fibres and damage accumulation within the composite. The two tertiary stages differ in the damage mechanisms occurring, the first one being related to fibre–matrix debonding only, whereas successive fibre failure dominates in the second part. The second tertiary regime occurs only at low creep stresses, for which a non-catastrophic rupture of the composite is observed.     

Nanocomposite piezoelectric films of P(VDF‐TrFE)/LiNbO3

Piezoelectric films were prepared by incorporation of lithium niobate (LiNbO₃) nanoparticles into copolymer of vinylidene difluoride and trifluoroethylene. Nanoparticles of LiNbO₃ with ferroelectric phase were successfully synthesized and dispersed homogenously by ultrasonication in the copolymer matrix without any surfactant or surface functionalization. The nanocomposites were fully characterized by electronic microscopy, X‐ray diffraction, differential scanning calorimetry, dynamical mechanical analysis, and piezometer. Surprisingly, the copolymer matrix crystallinity and morphology were not affected by the incorporation of nanoparticles. Therefore the nanocomposites remained good mechanicals properties and high ferroelectricity coupled to nonlinear optical activity thanks to the noncentro symmetric space group of lithium niobate. This could be a novel approach to develop new multifunctional materials. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Modélisation par méthode de propagation de faisceau à partir d’un coupleur à champ évanescent

We present numerical results obtained by 3D-BPM concerning evanescent field coupling in monomode fibres. At first we study the coupling between a monomode fibre and a mode sink. We show the existence or the absence of guided modes. BPM, is also applied to the study of directional couplers consisting of two fibres separated by an index liquid. The transmission coefficient is computed for various geometries and the electric field intensity is visualised by plotting isovalues.     

Monitoring Density and Temperature in C/C Composites Processing by CVI with Induction Heating

Carbon/carbon composites are processed by chemical vapor infiltration (CVI) with radio-frequency inductive heating, which leads to inside-out temperature gradients, suitable for the production of homogeneously densified pieces if properly controlled throughout the whole processing. We present here a 2D axisymmetrical case where a comprehensive numerical model is tested against experimental runs. The numerical thermal model takes into account induction heating, radiative, conductive, and convective effects, intermediate regime diffusion and densification reactions in the pores, and the evolution of the porous medium. The results are the time evolution of the temperature, concentration, and composite material density field, as well as the input power necessary to ensure a given maximal temperature in the preform. Experimental data are measurements of the temperature and density fields at various infiltration stages. Comparison between experience and simulation, yielding an useful agreement, shows that porosity becomes trapped inside the preform as densification proceeds, because of the progressive lowering of the temperature gradient steepness. The discrepancies between computations and experimental data rely on the only approximate knowledge of some quantities, principally the reaction kinetics, which are currently under investigation.     

Réalisation d’une cavité résonnante à fibre unimodale

A method for the evaluation of the remaining cladding thickness of evanescent field couplers is proposed. Directional couplers with a coupling coefficient near 99,99 % and ring cavity of finesse about 300 were so realized.     

Field Deformation on a Bent Step Index Fibre

Abstract

We consider the problem of propagation of light in a bent step index circular core optical fibre. We first derive the exact equations in toroidal coordinates and give an analytic solution at first order with respect to the parameter λ/(curvature radius), neglecting radiation losses.     

Catalytic Routes Towards Single Wall Carbon Nanotubes

Single wall carbon nanotubes (SWCNT) have become a strategic material in the area of nanotechnologies nowadays, and catalytic chemical vapor deposition seems to be the most promising technique in view of an industrial-scale production. However, the selective catalytic production of single wall carbon nanotubes is still a challenge, since catalytic systems performances both in terms of selectivity and activity are still relatively low. One of the main challenges for the catalytic growth of SWCNT is the control of the catalyst nanoparticles size distribution along the high temperatures required by the process. This article provides a comprehensive overview of the state of the art of the strategies that have been followed to selectively grow single wall carbon nanotubes. It focuses on catalysts preparation and activity/selectivity and on the growth mechanism of these nanostructures. Particular attention is given to the identification of the parameters that control the selectivity of the reaction, such as the choice of the metal/support couple, the particle's sizes, and the chemical vapor deposition conditions.