Ondes acoustiques dans un guide aleatoire

Harmonic longitudinal acoustical waves are propagated through elastic thin rods whose elastic constant, density and crosssectional area are random functions of distance along the bar from a point of excitation. Partial reflection on successive inhomogeneities reduce the mean amplitude of waves, according to an exponential law, from the point of excitation. A method for calculating the mean damping coefficient is given, and the result is checked against numerical simulations.     

Use of Time Resolved PIV for Validating LES/DNS of the Turbulent Flow within a PCB Enclosure Model

In this paper an approach to the validation of transient numerical simulations of turbulent flows through the use of time resolved PIV data is presented and applied to the case of turbulent flow within a simplified model of printed circuit board (PCB) enclosure. The comparison between numerical and experimental data is not limited here to the time averaged fields, but transient flow structures are also addressed. With this purpose a set of conditionally sampled averages, or cluster averages, are obtained through a Fuzzy Clustering technique. Further, the time history of the flow is analyzed on the base of this set of cluster averages, yielding time sequence relations among them that simplify their interpretation in terms of flow dynamics. In this way, events of alternate vortex shedding have been identified in a selected portion of the flow, and showed to exist in both numerical and experimental sets of data.Supported by FPI/DPI2003-06725-C02-01 from DGI, Ministerio de Educación y Cultura y Fondos FEDER, and grant S/B/CE/33/07 from PDT Uruguay.     

Sizing effects on main relaxation process of cyanate ester glass fiber composites using dynamic mechanical and microthermal analyses

The influence of sizing/polymer interaction or interphase on dynamic mechanical relaxation properties of cyanate ester composites was investigated by means of dynamic mechanical analysis. The dynamic mechanical behavior of different samples (three types of composites with different sizing and neat resin) was analyzed by using two phenomenological models (TFV and WLF). Related coefficients such as C₁, C₂, and T_∞ (Vogel temperature) were evaluated. Results have shown that these parameters were strongly dependent on sizing state, therefore, on interphase. In addition, the frequency dependence of the molecular relaxation process was well described by the Cole‐Cole plot. These results were confirmed by the sizing extract/resin blend study. Microthermal analysis has shown that partial miscibility existed between resin and sizing extract. Local thermal analyses were carried out by positioning the probe over selected regions: bulk resin and sizing/resin blend. A decrease in resin glass‐transition temperature was observed in the sizing resin blend. The different results have shown that a local plasticization of resin by sizing occurred with crosslink density modification. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 205–214, 2006     

UV effects on marine planktonic food webs: A synthesis of results from mesocosm studies

UV irradiance has a broad range of effects on marine planktonic organisms. Direct and indirect effects on individual organisms have complex impacts on food-web structure and dynamics, with implications for carbon and nutrient cycling. Mesocosm experiments are well suited for the study of such complex interrelationships. Mesocosms offer the possibility to conduct well-controlled experiments with intact planktonic communities in physical, chemical and light conditions mimicking those of the natural environment. In allowing the manipulation of UV intensities and light spectral composition, the experimental mesocosm approach has proven to be especially useful in assessing the impacts at the community level. This review of mesocosm studies shows that, although a UV increase even well above natural intensities often has subtle effects on bulk biomass (carbon and chlorophyll), it can significantly impact the food-web structure because of different sensitivity to UV among planktonic organisms. Given the complexity of UV impacts, as evidenced by results of mesocosm studies, interactions between UV and changing environmental conditions (e.g. eutrophication and climate change) are likely to have significant effects on the function of marine ecosystems.     

Vertical mixing and ecological effects of ultraviolet radiation in planktonic communities

We present a mathematical model for a phytoplankton-zooplankton system, based on a predator-prey scheme. The model considers the effects of sinking in the phytoplankton, vertical mixing and attenuation of photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) in the water column. In a first approach, the model was studied under conditions of average PAR irradiance and shows fluctuations and stable equilibrium points. Secondly, we introduced the effects of photoperiod and photoinhibition by UVR and vertical mixing. Under these conditions, the phytoplankton biomass oscillates depending on the combined effects of UVR and mixing. Higher inhibition by UVR and longer mixing periods can induce strong fluctuations in the system but can also produce higher plankton peaks.     

Simulation of ozone depletion using ambient irradiance supplemented with UV lamps

In studies of the biological effects of UV radiation, ozone depletion can be mimicked by performing the study under ambient conditions and adding radiation with UV-B lamps. We evaluated this methodology at three different locations along a latitudinal gradient: Rimouski (Canada), Ubatuba (Brazil) and Ushuaia (Argentina). Experiments of the effect of potential ozone depletion on marine ecosystems were carried out in large outdoor enclosures (mesocosms). In all locations we simulated irradiances corresponding to 60% ozone depletion, which may produce a 130-1900% increase in 305 nm irradiance at noon, depending on site and season. Supplementation with a fixed percentage of ambient irradiance provides a better simulation of irradiance increase due to ozone depletion than supplementation with a fixed irradiance value, particularly near sunrise and sunset or under cloudy skies. Calculations performed for Ushuaia showed that, on very cloudy days, supplementation by the square-wave method may produce unrealistic irradiances. Differences between the spectra of the calculated supplementing irradiance and the lamp for a given site and date will be a function of the time of day and may become more or less pronounced according to the biological weighting function of the effect under study.     

Two methods for the generation of chordal graphs

In this paper two methods for automatic generation of connected chordal graphs are proposed: the first one is based on new results concerning the dynamic maintenance of chordality under edge insertions; the second is based on expansion/merging of maximal cliques. Theoretical and experimental results are presented. In both methods, chordality is preserved along the whole generation process. L. Markenzon’s research is partially supported by grant 301068/2003-8, CNPq, Brazil.