HAL-Ecole des Ponts ParisTech
Not a member yet
42743 research outputs found
Sort by
Forme normale alternante des tresses : Régularité et Automaticité
No full textThe main focus of this thesis is the alternating normal form of the standard braid monoid. I first highlight two braid invariants thanks to which I establish a local characterization of words in alternating normal form. Thanks to this new characterization, I construct and study the minimal automaton recognizing words in normal form, and define an efficient algorithm recognizing words in normal form in linear time in term of both their length and their number of strands. I then describe the effect of right-multiplication on the alternating normal form, proving its right-automaticity. Finally, I study the well-ordering induced by the alternating normal form. Thanks to the local characterization, I establish an explicit formula for the rank of a braid, and provide a direct proof of the fact that the order is right-invariant using the tools developed in the proof of the right-automaticity.Cette thèse s'intéresse à la forme normale alternante du monoïde standard de tresse. Dans un premier temps, j'exhibe deux invariants de tresses à l'aide desquels je caractérise la forme normale alternante de manière locale. Grace à cette nouvelle caractérisation, je construis et j'étudie l'automate minimal qui reconnaît les mots en forme normale alternante, et je définis un algorithme qui vérifie qu'un mot est en forme normale en temps linéaire par rapport à sa longueur et son nombre de fils. Je décris ensuite comment la forme normale alternante évolue lors de la multiplication à droite, ce qui me permet d'établir qu'elle est automatique à droite. Pour terminer, j'étudie le bon ordre induit par la forme normale alternante. J'utilise la caractérisation locale pour déduire une formule explicite du rang d'une tresse dans cette ordre, et je montre que cet ordre est invariant à droite grâce aux outils développés dans la preuve de l'automaticité à droite
Déploiement de la sonde Fluocopée® en Seine (France) : Vers un suivi in situ et à haute fréquence des milieux aquatiques grâce à la spectrométrie de fluorescence
No full textInternational audience Over the past two decades, there has been a notable advancement in the development of high-frequency measuring equipment for the monitoring of biophysical and chemical parameters in surface water. Optical probes, especially fluorescence probes, are of particular importance in the integration of high-frequency measurements into environmental monitoring. The joint development of the Fluocopée® probe by LEESU and SIAAP represents a further contribution to this dynamic. This innovative fluorescence probe is capable of monitoring temporal evolution of 25 fluorophores in situ at high frequency (every 15 minutes), thereby enabling the characterization of dissolved organic matter (DOM). The extensive range of fluorophores monitored by the Fluocopée® probe facilitates the monitoring of water quality and the investigation of the biogeochemical processes linked to DOM in aquatic environments. Furthermore, its sensitivity is compatible with the levels of OM concentration observed in continental aquatic environments. Since October 2023, several Fluocopée® probes have been implemented on the river Seine and its two main tributaries (the Marne and Oise rivers) at six sites upstream and downstream of the Paris conurbation (see Figure 1). This allows us to assess the spatial variability of organic matter in the river Seine across the Paris conurbation at a high temporal frequency and provides a valuable opportunity to enhance our comprehension of the organic matter biogeochemical dynamics in the river Seine as well as to assess the impact of urban pressures. The installation of Fluocopée® probes at sites already equipped (as part of the MeSeine monitoring system or drinking water treatment plants intakes) with numerous measuring devices has been shown to facilitate the interpretation of fluorescence data by providing supplementary information from the chronicles of other physicochemical parameters (pH, turbidity, dissolved O2, TSS, Abs254nm, fecal indicator bacteria etc.). Furthermore, proxies for determining dissolved organic carbon (DOC) and its biodegradable fraction on the basis of fluorescence measurements have been developed in our laboratory. The development of these models was achieved by identifying the most suitable existing correlation between these physiochemical parameters and fluorescence measurements using various statistical algorithms (e.g., multilinear regressions, partial least squares regressions, machine learning algorithms, etc.). Used in association with Fluocopée®, these proxies provide estimation of these parameters at high frequency in addition to fluorescence measurements. The fluorescence, DOC and biodegradable DOC concentration measurements acquired at high frequency over a year using our monitoring system will be presented and discussed. The influence of the hydroclimatic situation and the impact of urban pressures on the organic matter dynamics in the Seine across Paris Conurbation will be assessed. Additionally, it will provide a detailed account of the methodology employed to process these data sets, from the initial acquisition of raw data to its subsequent validation.Figure 1 : Implantation of Fluocopée® probe
Load‐Finding: a Form‐Finding Method for Plane‐Faced Funicular Gridshells
No full textInternational audienceGridshells and shells are geometrical objects that must comply with mechanical and manufacturing constraints. Being funicular, and having planar panels (for discretized structures) is a combination of constraints often searched and studied in the literature. The study of funicular structures is the subject of graphic statics, with the panel planarity constraint added in methods using the Airy stress function. It is proposed in this paper to reverse the existing procedures and to present a “load‐finding” method: rather than creating structures with almost flat panels, we design almost funicular grids with perfectly flat panels. We hence use lifting methods with strict control over geometrical properties and we prescribe heights and forces at the boundary to calculate the load under which the structure operates without bending, i.e. the funicular load associated with the structure. We can then optimize the lift to find the load which will be the closest to a target distribution. We hence get a space of planar faced polyhedra in equilibrium under a certain load that projects onto the input mesh, and which is controlled by heights and forces defined on guide curves. The funicular load deviation is a purely static tool for locating and quantifying loads that cause moments in the structure. A finite element analysis of the structures produced opens a discussion on the influence of the parameters and metric chosen in our method, as well as on certain biases introduced by our method and a method using an elastic calculation
Joint evolution of irrigation, the water cycle and water resources under a strong climate change scenario from 1950 to 2100 in the IPSL-CM6
No full textInternational audienceIrrigation, a key activity for food security, uses local water resources to increase evapotranspiration, creating feedback loops with the atmosphere and water resources. With climate change, it is unclear how irrigation will evolve in the future and how it may influence the evolution of water resources and the water cycle. It is also unclear whether irrigation may be constrained by climate change or water resource shortages. Here, we compare two surface-atmosphere simulations performed with the IPSL-CM6 model from 1950-2100: one with irrigation and one without irrigation. In both simulations, the evolutions of atmospheric radiative forcing, land use, and irrigated areas are taken from CMIP6, which uses a historical dataset for the data before 2014 and the SSP5-RCP8.5 dataset for data after 2014. The two simulations reveal strong global warming and precipitation increases between 1950-2000 and 2050-2100 average values (+5.6 °C and +8.1 %, on average, over land with irrigation). Over the same period, our results indicate an increase in irrigation (+76 % increase in irrigation in the 2050-2100 compared to the 1950-2000 period), which is in line with an important expansion of irrigated areas. The influence of irrigation on evapotranspiration in irrigated areas is greater in 2050-2100 than in 1950-2000 (+12 % vs. +8 %, respectively). Evapotranspiration has also been found to increase in non-irrigated areas near irrigated zones owing to an increase in precipitation under historical and future climate conditions. Water depletion due to irrigation is more intense in the future than in the historical period, although climate change increases water storages and river discharge due to more precipitation in the future. We also identified areas where future environmental conditions can limit irrigation or where irrigation can increase tensions over water use (approximately one-third of irrigated areas, including the Mediterranean basin, California, and Southeast Asia). Our results highlight the importance of considering irrigation in climate projections and future water resources assessments
First JWST thermal phase curves of temperate terrestrial exoplanets reveal no thick atmosphere around TRAPPIST-1 b and c
No full textInternational audience72 pages, 4 main text Figures, 20 Extended Data Figures, 6 Supplementary Figures. Accepted for publication in Nature Astronom
Use of the continuous wavelet transform for processing seismic structural responses
No full textInternational audienceThe Continuous Wavelet Transform (CWT) method is now commonly employed for monitoring the health of civil engineering structures subjected to various types of excitation. This article begins with a brief overview of the CWT method’s key features, ensuring it is used effectively and in line with the desired objectives. It then discusses the use of a progressive Morse-class mother wavelet, which has excellent time-frequency localisation properties. The positions and amplitudes of the CWT modulus maxima, known as ‘ridges’ and ‘skeletons’ respectively, are shown to carry significant information about the response signal. Two exampleapplications are presented: an elasto-plastic single-degree-of-freedom oscillator, which is often used in seismic applications, and the well-known Sherman Oaks building, which has been subjected to a sequence of six damaging earthquakes over the last forty years. Despite the non-stationary nature of the examined response signals, the CWT method has proven highly efficient in analysing structural response signals and detecting nonlinearities in the vibration response
Universal families of arcs and curves on surfaces
No full textInternational audienceThe main goal of this paper is to investigate the minimal size of families of curves on surfaces with the following property: a family of simple closed curves Γ on a surface realizes all types of pants decompositions if for any pants decomposition of the surface, there exists a homeomorphism sending it to a subset of the curves in Γ. The study of such universal families of curves is motivated by questions on graph embeddings, joint crossing numbers and finding an elusive center of moduli space. In the case of surfaces without punctures, we provide an exponential upper bound and a superlinear lower bound on the minimal size of a family of curves that realizes all types of pants decompositions. We also provide upper and lower bounds in the case of surfaces with punctures which we can consider labelled or unlabelled, and investigate a similar concept of universality for triangulations of polygons, where we provide bounds which are tight up to logarithmic factors
Transport into the polar stratosphere from the Asian monsoon region
No full textInternational audienceThe Southeast Asian boundary layer has witnessed alarming pollution levels in recent years, which even affects the trace gas composition in the Southern Hemisphere by inter-hemispheric transport. We use SF 6 observations and the Lagrangian chemistry transport model Chemical Lagrangian Model of the Stratosphere (CLaMS), driven by the ERA5 reanalysis data for the period 2010-2014, to assess the impact of the Asian monsoon (AM) region (15-45°N, 30-120°E) as a significant source of pollutants for the stratosphere, in particular in polar regions. We examine the contribution of transport from the AM region to the Northern Hemisphere polar region (NP) (60-90°N) and to the Southern Hemisphere polar region (SP) (60-90°S). Despite the smaller geographical size of the AM region when compared to the Southern Hemisphere subtropics (15-45°S) and the tropics (15°S-15°N), our findings reveal that the air mass fractions from the AM to the polar regions are approximately 1.5 times larger than the corresponding contributions from the Southern Hemisphere subtropics and only about 2 times smaller than those from the tropics. The transport of air masses from the AM boundary layer to the stratospheric polar vortex primarily occurs above an altitude of about 450 K and over timescales exceeding 2 years. In contrast, transport timescales to the polar regions situated below the vortex are shorter, typically less than about 2 years. Furthermore, the transport contribution from the AM region to the polar regions exhibits distinctive inter-annual variability, significantly influencing the distributions of pollutants. Our analysis of detrended SF 6 from an Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) over the polar regions reveals a strong correlation with the fraction of relatively young air (less than 2 years old) originating from the AM, Southern Hemisphere subtropics, and the tropics. Importantly, our reconstructed SF 6 data indicate that approximately 20 % of SF 6 in both the northern and southern polar stratosphere originates from the AM boundary layer. The largest fraction of SF 6 in the polar stratosphere still originates from the tropical boundary layer, contributing about 50 % of SF 6
Influence of atmospheric waves and deep convection on water vapour in the equatorial lower stratosphere seen from long-duration balloon measurements
No full textInternational audienceAbstract. Most atmospheric species enter the stratosphere through the tropical tropopause layer (TTL), a place of interplay between many processes of different scales. Water vapour (H2Ovap) is a key compound in this layer and its entry into the tropical stratosphere is crucial for stratospheric chemistry and climate. We present a methodology based on the calculation of in situ H2Ovap and temperature anomalies to estimate the modulation of H2Ovap due to atmospheric waves and deep convection. H2Ovap data were obtained from in situ measurements of five Pico-Strat Bi Gaz spectrometers that were flown under long-duration balloons during the Strateole 2 campaigns. The calculation of Pearson's correlation coefficients is performed between averaged ERA5 reanalysis temperatures and in situ H2Ovap anomalies. In the case of a monotonic vertical gradient of H2Ovap, the absolute value of the correlation coefficient is high (typically 0.65). For the other flights we highlight lower correlations, due to changes in time of the vertical gradient of stratospheric H2Ovap, and large convective systems overshooting the tropopause. This is the case for one of the flights, which flew over the Raï typhoon (correlation coefficient of 0.31 due to both contributions). Depending on the flights, we also show that for 47 % up to 70 % of the probed nights, H2Ovap anomalies can be explained by atmospheric waves, which highlights the major role played by waves on H2Ovap in the TTL. We also show that long-duration balloon measurements are important in highlighting the overshooting signature of H2Ovap in the upper TTL