961 research outputs found
Non-scattering wavenumbers and far field invisibility for a finite set of incident/scattering directions
We investigate a time harmonic acoustic scattering problem by a penetrable
inclusion with compact support embedded in the free space. We consider cases
where an observer can produce incident plane waves and measure the far field
pattern of the resulting scattered field only in a finite set of directions. In
this context, we say that a wavenumber is a non-scattering wavenumber if the
associated relative scattering matrix has a non trivial kernel. Under certain
assumptions on the physical coefficients of the inclusion, we show that the
non-scattering wavenumbers form a (possibly empty) discrete set. Then, in a
second step, for a given real wavenumber and a given domain D, we present a
constructive technique to prove that there exist inclusions supported in D for
which the corresponding relative scattering matrix is null. These inclusions
have the important property to be impossible to detect from far field
measurements. The approach leads to a numerical algorithm which is described at
the end of the paper and which allows to provide examples of (approximated)
invisible inclusions.Comment: 20 pages, 7 figure
Development and mechanical characterization of porous titanium bone substitutes
The authors wish to thank Dr J.-M. Hiver from Institut Jean Lamour, Ecole des Mines de Nancy for his participation in the computed tomography analysis of the porous samplesCommercially Pure Porous Titanium (CPPTi) can be used for surgical implants to avoid the stress shielding effect due to the mismatch between the mechanical properties of titanium and bone. Most researchers in this area deal with randomly distributed pores or simple architectures in titanium alloys. The control of porosity, pore size and distribution is necessary to obtain implants with mechanical properties close to those of bone and to ensure their osseointegration. The aim of the present work was therefore to develop and characterize such a specific porous structure. First of all, the properties of titanium made by Selective Laser Melting (SLM) were characterized through experimental testing on bulk specimens. An elementary pattern of the porous structure was then designed to mimic the orthotropic properties of the human bone following several mechanical and geometrical criteria. Finite Element Analysis (FEA) was used to optimize the pattern. A porosity of 53% and pore sizes in the range of 860 to 1500 ÎŒm were finally adopted. Tensile tests on porous samples were then carried out to validate the properties obtained numerically and identif the failure modes of the samples. Finally, FE elastoplastic analyses were performed on the porous samples in order to propose a failure criterion for the design of porous substitutes
Dissolved iron distribution in the tropical and sub tropical south eastern Pacific
International audienceDissolved iron (DFe) distributions (Fe:P (PO43-) we show that DFe was in deficit compared to PO43- resulting from the remineralisation of organic matter. This suggests that the Marquesas islands and the surrounding plateau are not a significant source of DFe. In the gyre, DFe concentrations in the upper 350 m water column were around 0.1 nM and the ferricline was located well below the nitracline. These low concentrations reflect the low input of DFe from the atmosphere, from the ventilation of the upper thermocline with water containing low DFe, and from the low biological activity within in this ultra oligotrophic gyre
Variabilité de la température de surface et du couvert de glace de mer dans le détroit de Fram au cours des deux derniers millénaires
Une carotte sĂ©dimentaire, couvrant les derniers 2500 ans, a Ă©tĂ© Ă©chantillonnĂ©e sur la marge continentale Ouest du Spitzberg dans le dĂ©troit de Fram (78,92°N 6,77°E, profondeur d'eau: 1497 m) avec pour objectif de reconstituer les variations hydroclimatiques Ă partir de l'analyse des assemblages de dinokystes. L'abondance relative des taxons de dinokystes ainsi qu'une analyse en composantes principales permet de distinguer une pĂ©riode de transition majeure Ă 300 ans cal. BP. Celle-ci est marquĂ©e par la disparition simultanĂ©e des taxons thermophiles Spiniferites mirabilis-hyperacanthus et Impagidinium sphaericum ainsi qu'une augmentation des taxons polaires-subpolaires Impagidinium pallidum et Pentapharsodinium dalei. Les tempĂ©ratures de surface estimĂ©es suggĂšrent des conditions plus chaudes que celles de l'actuel (moyenne de 7°C en ĂtĂ©, soit une anomalie d'environ +2°C) jusqu'Ă 300 ans cal. BP malgrĂ© plusieurs phases de refroidissement vers 1700, 1500, 1200 et 800 ans cal. BP. Les derniers 300 ans se caractĂ©risent par une tendance au refroidissement, avec des tempĂ©ratures d'ĂtĂ© chutant de 7,6 Ă 3,5°C et un couvert de glace de mer atteignant 7 mois/an. Les rĂ©sultats obtenus montrent que le couvert de glace de mer et la tempĂ©rature de surface ocĂ©anique dans la rĂ©gion du dĂ©troit de Fram sont sensibles aux variations hydroclimatiques, principalement liĂ©es au dĂ©bit relatif des eaux chaudes et salĂ©es du courant Nord Atlantique pĂ©nĂ©trant Ă l'Est et des eaux froides et dĂ©ssalĂ©es sortant de l'Arctique, Ă l'Ouest. Nos donnĂ©es mettent Ă©galement en Ă©vidence un optimum thermique vers 1320 ans cal. BP qui reprĂ©senterait le seul intervalle des derniers 2500 ans pouvant ĂȘtre un analogue des conditions modernes post-AD 2000 et se singularisant par une absence du couvert de glace de mer. ______________________________________________________________________________ MOTS-CLĂS DE LâAUTEUR : HolocĂšne, Mers nordiques, DĂ©troit de Fram, Conditions ocĂ©aniques de surface, Dinokyste
The Gender Gap in Pensions: Differences Between Generations
Over successive generations, womens participation in the labour market has risen and their career breaks have become less frequent. The gender wage gap has narrowed. The time spent in education has increased. These socio-economic factors largely explain the reduction for future generations of the current gender differentials in pension levels and retirement ages. Nonetheless, the gender pension gap should remain significant. Men of the 1965-1974 generations retiring from the private sector should receive a pension that is more than 50% higher than womens. Although women are expected to participate more in the labour market, they often work part time and the gender wage gap is expected to remain significant. Changes in retirement system rules will influence future trends in pension levels. The gap between womens and mens pensions would have narrowed much more in future if the 1993 and 2003 reforms had not been implemented.Pensions, Gender Pension Gap, Microsimulation, Pension Reforms
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Intraspecific variation in environmental and geographic space use: insights from individual movement data
Speciesâ ranges arise from the interplay between environmental preferences, biotic and abiotic environmental conditions, and accessibility. Understanding of â and predictive models on â species distributions often build from the assumption that these factors apply homogenously within each species, but there is growing evidence for individual variation. Here, I use movement data to investigate individual-level decisions and compromises regarding the different costs and benefits influencing individualsâ geographic locations, and the species-level spatial patterns that emerge from these.
I first developed a new method that uses tracking data to quantify individual specialisation in geographic space (site fidelity) or in environmental space (environmental specialisation). Applying it to two species of albatrosses, I found evidence of site fidelity but weak environmental specialisation. My results have implications for how limited research efforts are best-targeted: if animals are generalists, effort are best spent by understanding in depth individual patterns, i.e., better to track fewer individuals for long periods of time; whereas if animals tend to be specialists, efforts should be dedicated to tracking as many individuals as possible, even if for shorter periods.
I then investigated individual migratory strategies and their drivers in nine North American bird species, using ringing/recovery data. I found latitudinal redistribution of individuals within the breeding and non-breeding ranges that generally did not follow textbook patterns (âchain migrationâ or âleapfrog migrationâ). Migratory individuals tend to trade off the benefits of migration (better tracking of climatic niche; better access to resources) and its costs (increasing with migratory distance). I found that birds are more likely to remain as residents in areas with warmer winter temperatures, higher summer resource surpluses and higher human population densities (presumably because of a buffering effect of urban areas).
Overall, my results highlight the importance of considering individual variation to understanding the ecological processes underpinning speciesâ spatial patterns.St John's College Benefactors' Scholarship
Cambridge Philosophical Societ
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