Perturbations in generalized multi-field inflation

We study the linear perturbations of multi-field inflationary models governed by a Lagrangian which is a general function of the scalar fields and of a global kinetic term combining their spacetime gradients with an arbitrary field space metric. Our analysis includes k-inflation, DBI inflation and its multi-field extensions which have been recently studied. For this general class of models, we calculate the action to second order in the linear perturbations. We decompose the perturbations into an (instantaneous) adiabatic mode, parallel to the background trajectory, and entropy modes. We show that all the entropy modes propagate with the speed of light whereas the adiabatic mode propagates with an effective speed of sound. We also identify the specific combination of entropy modes which sources the curvature perturbation on large scales. We then study in some detail the case of two scalar fields: we write explicitly the equations of motion for the adiabatic and entropy modes in a compact form and discuss their quantum fluctuations and primordial power spectra.Comment: 27 pages, typos corrected, published versio

Full-scale measurement of the response of a CPV tracker structure prototype under wind load

The response of concentrated photovoltaic (CPV) structures under wind loading has to be carefully predicted to optimize the design of these structures. Designers normally rely on wind tunnel tests and equivalent static approaches as recommended by codes for the design of this type of nonconventional structures. This paper aims to evaluate the full scale wind response of a concentrated photovoltaic (CPV) prototype with mirror surface of 8 ∗ 16 m2 and correlate it with wind measurements made near the solar structure. The full-scale wind response of the structure was evaluated with continuous 10 min measurements of deformations on tower leg members during moderate wind events. The utility of this work derives from enriching field measurement data in order to understand the behavior of this type of structure and to validate the previous wind tunnel testing works. Standard wind design process based on the analytical method, which is adopted by ASCE 7, is also revisited in the light of the full scale observations

Building tomorrow’s society / Bâtir la société de demain

Abstract: Cold-formed steel angles could be an advantageous alterative to hot rolled steel angle for the application of light-weight electrical transmission towers. However, standard design equations to evaluate the resistance and failure modes in tension of angles connected with bolts on one leg only were not specifically developed and validated for the application of lipped angles. In particular, the phenomenon of shear lag on this type of angle is difficult to evaluate. In this paper, the behavior of cold-formed steel lipped-angle with typical transmission tower bolted connections is studied. At first, experimental tests on two and three bolts connections are performed. Tensile load is applied on a single angle configuration to investigate the failures mode and capacity of the connected members. Then, three dimension (3D) finite element (EF) models of the bolted members are developed. The accuracy of the FE models is validated by the good agreement with experimental study. Results obtained from these investigations added to other published results are compared to recommended design prediction with the aim to assess their validity and limitation. From this, a new design equation for net-section strength is proposed

Active Control of Laser Wavefronts in Atom Interferometers

Wavefront aberrations are identified as a major limitation in quantum sensors. They are today the main contribution in the uncertainty budget of best cold atom interferometers based on two-photon laser beam splitters, and constitute an important limit for their long-term stability, impeding these instruments from reaching their full potential. Moreover, they will also remain a major obstacle in future experiments based on large momentum beam splitters. In this article, we tackle this issue by using a deformable mirror to control actively the laser wavefronts in atom interferometry. In particular, we demonstrate in an experimental proof of principle the efficient correction of wavefront aberrations in an atomic gravimeter

Modeling multilayered wire strands, a strategy based on 3D finite element beam-to-beam contacts - Part II: Application to wind-induced vibration and fatigue analysis of overhead conductors

Abstract: Wind-induced loads cause electrical transmission line fatigue. Evaluation procedures consider descriptors such as deflection amplitude (Yb) and far-field vibration (fymax), which cannot relate endurance limits and wire loads. The investigation uses the finite element (FE) strategy developed in part I to study Aluminum Conductor Steel Reinforced (ACSR) submitted to wind induced loads. The analysis underlines the Yb and fymax discrepancies. A factorial design leads to a model relating them with precision of 92%. Comparisons with experimental ACSR data indicate that fatigue predictions from the Coffin-Manson relation associated with the FE model provide realistic evaluations of service lives

Détermination de la plus-value de la télédétection pour le suivi des niveaux d’eau et du bilan hydrique : Application au réservoir Eastmain-1

Le nouvel altimètre radar SWOT va permettre de produire des cartographies à haute résolution des niveaux des eaux de surface à partir de 2021. L’objectif du projet est de déterminer en quoi ce satellite à haute-résolution peut être comparé avec les stations de mesures de niveaux d’eau in situ dans le but d’améliorer la couverture spatio-temporelle de l’estimation des niveaux d’eau des réservoirs et lacs jaugés ou non. De plus, il sera question d’évaluer la qualité du bilan hydrique estimée par le satellite. Pour y arriver, le réservoir Eastmain-1 a été modélisé par éléments finis à l’aide du modèle hydrodynamique TELEMAC. Des images synthétiques de SWOT ont été constituées à l’aide des simulateurs SWOT-HR créé par le JPL (Jet Propulsion Laboratory) pour évaluer les erreurs de classification du terrain et SWOT-LS créé par le CNES (Centre National d’Études Spatiales) pour générer des images synthétiques de niveaux d’eaux à partir de ceux calculés par TELEMAC. Il a été démontré que pour un réservoir de la taille d’Eastmain-1, les images SWOT synthétiques cumulées sur une période de 5 mois ont une erreur quadratique moyenne sur l’estimation du niveau moyen de l’eau de l’ordre d’environ 1,5 cm. Sur le long terme, il a été montré que les niveaux d’eau simulés SWOT semblent avoir un léger biais de -3 mm par rapport aux niveaux du modèle TELEMAC. Le volume au réservoir et une estimation des apports calculés à partir d’images synthétiques SWOT donnent des séries temporelles relativement peu bruitées

Study on extraction parameters of flutter derivatives for the development of a time-domain formulation of self-excited forces

Abstract: This paper addresses the evaluation of the dependence of bridge-deck flutter derivatives to the extraction parameters used in wind tunnel tests. The studied parameters are the model scale, model velocity and amplitude of motion. The measurements of flutter derivatives for the Great Belt Bridge were done through forced-vibration tests. This research shows that the flutter derivatives measured for different model scales have similar trends. Some derivatives show a dependence with respect to the amplitude. When studied in their dimensional form, the flutter derivatives H*5, A*5, H*2 and A*2 exhibit nonlinearities for the velocity. Therefore, the study of a new nondimensionalization for the flutter derivatives based on the velocity would be interesting for the development of a new time-domain formulation of self-excited forces

Prédiction des vibrations éoliennes d'un système conducteur-amortisseur avec une méthode temporelle non linéaire

Les vibrations éoliennes sont la cause principale de bris de conducteurs en fatigue des lignes aériennes de transport d'énergie électrique. Ces vibrations sont dues à des détachements tourbillonnaires produits dans le sillage du conducteur. Une méthode commune de réduction des vibrations est l'ajout d'amortisseurs de vibrations près des pinces de suspension. Contrairement aux essais en ligne expérimentale, la modélisation numérique permet d'évaluer rapidement et à faible coût la performance d'un amortisseur de vibration sur une portée de ligne aérienne. La technologie la plus fréquemment utilisée fait appel au principe de balance d'énergie (PBE) en évaluant le niveau de vibrations pour lequel la puissance injectée par le vent est égale à la puissance dissipée par le conducteur et l'amortisseur. Les méthodes actuelles pour la prédiction des vibrations reposent sur des hypothèses simplificatrices quant à la modélisation de l'interaction conducteur-amortisseur. Une approche prometteuse pour la prédiction des vibrations est l'utilisation d'un modèle numérique temporel non linéaire qui permet de mieux représenter la masse, la géométrie, la rigidité et l'amortissement du système. L'objectif principal de ce projet de recherche est de développer un modèle numérique avec intégration temporelle directe d'un conducteur et d'un amortisseur en vibration permettant de reproduire le comportement dynamique du système pour la gamme de fréquence et d'amplitude typique des vibrations éoliennes des conducteurs. Un modèle par éléments finis d'un conducteur seul en vibration résolu par intégration temporelle directe a d'abord été développé en considérant une rigidité de flexion variable. Comme une rigidité de flexion constante et égale à 50% de la rigidité de flexion maximale théorique ( EImax ) est jugée adéquate pour la modélisation du conducteur, c'est cette valeur qui a été utilisée pour la suite du projet. Ensuite, des modèles non-linéaires pour deux types d'amortisseur de vibrations (Stockbridge et Hydro-Québec) ont été développés. Ces modèles reproduisent adéquatement le comportement dynamique des amortisseurs pour une grande gamme de fréquences et d'amplitudes de déplacement. Finalement, les modèles d'amortisseur ont été intégrés au modèle de conducteur et uil nouvel outil de prédiction de vibrations éoliennes basé sur le principe de balance d'énergie (PBE) a été développé. Celui-ci permet de bien reproduire des àmplitudes de vibration mesurées sur une ligne expérimentale, en particulier dans la gamme 8 à 35 Hz. À plus long terme, un tel modèle pourrait servir de point de départ vers une modélisation temporelle complète du phénomène de vibrations éoliennes des conducteurs incluant l'excitation éolienne, l'auto-amortissement des conducteurs et des conditions limites réalistes pour les lignes aériennes

Building tomorrow’s society / Bâtir la société de demain

Abstract: Power transmission lines predominantly involve lattice towers, which are typically composed of steel angle members connected together by means of bolted joints. An effective design of such structures requires the consideration of a range of complex phenomena likely to affect either the carrying capacity or the failure mode. In practice, simple numerical models are combined with standard design equations to consider these effects. A few advanced numerical models reported in the literature deal with eccentricities, stiffness of the connections, and joint slippage. However, the impact of residual stresses on the global behavior of lattice towers is not addressed in prior works. In this work, the influence of residual stresses is studied numerically using the finite element software Code_Aster. The proposed model employs multi-fibre beam elements to model the elastoplastic angle members, and discrete elements to represent the bolted connections. Both the connection eccentricity and the rotational stiffness of connections are modeled. The associated problem is solved in an incremental way, so as to deal with geometric and material nonlinearities, and the results are compared with experimental tests. Considering residual stresses in advanced models is an important step for the numerical evaluation of the failure of lattice towers

STR-929: EXPERIMENTAL TESTING OF A NEW HYSTERETIC DAMPER FOR BASE ISOLATED BRIDGES

An experimental procedure was developed to implement a simplified design method of supplemental dampers for isolated highway bridges. For seismically base-isolated bridges subjected to an earthquake, the principal modes of the structure are in the isolation level which effectively lessens the seismic base shear conveyed from the superstructure to the substructure. However, this also has the effect of increasing superstructure displacement. To control the deformation of the isolators, supplemental energy dissipating devices can be introduced into the isolation system. This may nevertheless increase the total structure base shear and the merit of adding dampers has to be evaluated properly. In this paper a new type of steel hysteretic damper is proposed and tested in combination with an elastomeric isolator using displacement controlled cyclic tests. This very simple and economic damper was designed using a simplified method presented in earlier works. First, having characterized the response of an elastomeric isolation bearing used in a real structure, a simple and economic hysteretic damper was designed in accordance with the simplified method. In the next step, the designed damper in parallel with an elastomeric isolation bearing was tested by a unidirectional displacement-controlled quasi static method. Results show that this new type of damper could be used in combination with isolators to control superstructure displacement and substructure base shear