Search results for " time"
showing 10 items of 3005 documents
Sensitivity analysis for time optimal orbit transfer
2001
The minimum time transfer of a satellite around the Earth is studied. In order to deal numerically with low thrusts, a new method is introduced: Based on a so-called noncontrollability function, the technique treats the ha1 time as a parameter. The properties of the method arc studied by means of an infinite dimensional sensitivity analysis. The numerical results obtained by this approach for very low thrusts are given
On the optimal control of the circular restricted three body problem
2011
The context of this work is space mechanics. More precisely, we aim at computing low thrust transfers in the Earth-Moon system modeled by the circular restricted three-body problem. The goal is to calculate the optimal steering of the spacecraft engine with respect to two optimization criteria: Final time and fuel consumption. The contributions of this thesis are of two kinds. Geometric, first, as we study the controllability of the system together with the geometry of the transfers (structure of the command) by means of geometric control tools. Numerical, then, different homotopic methods being developed. A two-three body continuation is used to compute minimum time trajectories, and then …
Le Quaternaire franc-comtois. Essai chronologique et paléoclimatique.
1982
Le but de ce travail est d'établir le cadre chronologique et paléoclimatique du Quaternaire en Franche-Comté à partir de l'étude des formations édifiées au cours de cette période.Deux grands types de dépôts ont été plus particulièrement étudiés: ·- Les remplissages karstiques d'avens, de porches de grottes et d'abris sous roche, donnant un éclairage intense, mais ponctuel, de l'environnement physique et biologique, grâce aux faunes abondantes et aux indices climatiques d'ordre sédimentologique.- Les dépôts glaciaires et paraglaciaires (moraines, fluvioglaciaires, glacio-lacustres) qui permettent de situer des coupures climatiques au cours du temps dans l'espace géographique étudié.L'étude d…
An alternative space-time meshless method for solving transient heat transfer problems with high discontinuous moving sources
2016
International audience; The aim of this work is the development of a space-time diffuse approximation meshless method (DAM) to solve heat equations containing discontinuous sources. This work is devoted to transient heat transfer problems with static and moving heat sources applied on a metallic plate and whose power presents temporal discontinuities. The space-time DAM using classical weight function is convenient for continuous transient heat transfer. Nevertheless, for problems including discontinuities, some spurious oscillations for the temperature field occur. A new weight function, respecting the principle of causality, is used to eradicate the physically unexpected oscillations.
Statistics of transitions for Markov chains with periodic forcing
2013
The influence of a time-periodic forcing on stochastic processes can essentially be emphasized in the large time behaviour of their paths. The statistics of transition in a simple Markov chain model permits to quantify this influence. In particular the first Floquet multiplier of the associated generating function can be explicitly computed and related to the equilibrium probability measure of an associated process in higher dimension. An application to the stochastic resonance is presented.
Exact simulation of diffusion first exit times: algorithm acceleration
2020
In order to describe or estimate different quantities related to a specific random variable, it is of prime interest to numerically generate such a variate. In specific situations, the exact generation of random variables might be either momentarily unavailable or too expensive in terms of computation time. It therefore needs to be replaced by an approximation procedure. As was previously the case, the ambitious exact simulation of exit times for diffusion processes was unreachable though it concerns many applications in different fields like mathematical finance, neuroscience or reliability. The usual way to describe exit times was to use discretization schemes, that are of course approxim…
Nonlinear spectral compression in optical fiber: A new tool for processing degraded signals
2017
International audience; We propose two new applications of the spectral focusing by self-phase modulation that occurs in a nonlinear optical fiber. We numerically show the possibility of using nonlinear spectral compression to improve the optical signal to noise ratio and mitigate the amplitude jitter of the signal pulses. We also demonstrate experimentally that use of spectral focusing in a combination with an external sinusoidal phase modulation achieves efficient suppression of coherent spectral background.
Femtosecond Raman time-resolved molecular spectroscopy
2004
International audience; The applicability of several femtosecond time resolved non-linear coherent techniques such as Raman induced polarization spectroscopy (RIPS), degenerate four-wave mixing (DFWM) and coherent anti-Stokes Raman spectroscopy (CARS) for molecular spectroscopy is presented. All methods rely on the initial coherent excitation of molecular states producing wavepackets, whose time evolution is then measured. In the case of RIPS and DFWM only pure rotational transitions are involved, whereas in CARS vibrational states can be excited. First the methodology of concentration and temperature measurements using RIPS in gas mixtures involving N2, CO2, O2, and N2O is shown. In additi…
Alien flora of Europe: Species diversity, temporal trends, geographical patterns and research needs
2008
International audience; The paper provides the first estimate of the composition and structure of alien plants occurring in the wild in the European continent, based on the results of the DAISIE project (2004–2008), funded by the 6th Framework Programme of the European Union and aimed at “creating an inventory of invasive species that threaten European terrestrial, freshwater and marine environments”. The plant section of the DAISIE database is based on national checklists from 48 European countries/regions and Israel; for many of them the data were compiled during the project and for some countries DAISIE collected the first comprehensive checklists of alien species, based on primary data …
Developing fall-sown pea cultivars as an answer to the challenges of climatic changes
2012
Chapitre 4; International audience; Pea is considered rather well adapted to wide temperature ranges, with seedlings able to survive even -20 °C. From a physiological viewpoint, pea becomes tolerant to frost if first exposed to low non-freezing temperatures, causing the so-called cold acclimation. Delayed floral initiation helps some forage pea genotypes to escape the main winter freezing periods, as susceptibility to frost increases during the transition to the reproductive state. The oldest winter pea cultivars carry the dominant allele, Hr, although some bear hr. They are generally characterized by prominent winter hardiness and a long growing season, from sowing in early October until e…