Ingénierie et technologie spatiale

thumb|upright=1.6|Ingénieurs de la NASA pendant la mission Apollo 13 L'ingénierie et technologie spatiale désigne l'ensemble des fonctions concernant la conception, la construction, l'envoi dans l'espace et le contrôle ultérieur des véhicules spatiaux et des installations terrestres associées. Il s'agit d'un cas particulier d'ingénierie employé dans l'industrie spatiale. Principes généraux Puisqu'ils se déplacent dans l'espace, les véhicules spatiaux doivent subir des conditions éprouvantes : des forts gradients de température et de pression, de fortes contraintes structurales, des vibrations. Afin de permettre la survie de ces véhicules tout en leur permettant de remplir leur mission, les ingénieurs spatiaux doivent prendre en compte un grand nombre de domaines différents, comme la mécanique des structures, la propulsion, la thermique, la mécanique spatiale. La connaissance de l'ensemble de ces domaines et leur coordination afin de concevoir des véhicules spatiaux constit

Fire performance evaluation of GFRP-balsa sandwich bridge decks

Niloufar Vahedi

Bridges are a critical component of national infrastructures and economies. A significant percentage of bridges built all around the world are made of reinforced concrete which according to building codes have a life span between 80-100 years. However, structural inspections demonstrate that deterioration can begin in as little as 10 years due to the limited durability of steel-reinforced concrete. Exposure to humidity, freeze-thaw cycles and de-icing salts during winter result in corrosion of the steel reinforcement and spalling of the surface concrete which subsequently lead to partial or full bridge closure for required repair. Maintenance, upgrading and replacement of bridges may have significant economic and social impact due to disturbances in traffic flow. Mentioned reasons motivated scientists and engineers to develop and use more durable alternative materials with lower life-cycle cost. Composite sandwich bridge decks composed of glass fiber-reinforced (GFRP) face sheets and a balsa wood core, which are subject of this projects, provide the possibility of a more durable, lightweight and easily installed alternative. Balsa wood is a lightweight wood species exhibiting high strength- and stiffness-to-weight ratios, and therefore it is a favorable core material in bridge decks. However, this new type of sandwich bridge decks has to meet the same requirements as traditional concrete decks do. In particular, their behavior during a fire incident has to be known and predictable. Although fire is an accidental action according to Eurocodes, it has to be taken into account in the design of bridges and should not lead to a premature collapse. The knowledge about the combined thermo-mechanical behavior during a fire of these new sandwich decks is therefore crucial, in particular since the polymer matrix of the GFRP material and the balsa core are sensitive to high temperatures and combustible.GFRP sandwich structures are also used in other high-risk fire applications, e.g. in the automotive, naval and aerospace fields and numerous works about their thermo-mechanical behavior have been performed. However, these results cannot be simply transferred to GFRP-balsa sandwich bridge decks which are subjected to much more severe loads and their life span is much longer. The aim of this project is thus to investigate the thermo-mechanical behavior of GFRP-Balsa sandwich bridge decks in order to overcome this barrier of insufficient knowledge.Three level of investigations are planned, a) on the constituent material level, b) on the sandwich panel level, and c) on the bridge deck and superstructure level. Experimental investigations and modeling are conducted on each level from small- to full-scale. Since the performance of GFRP laminates under elevated temperatures have been investigated by many researchers, only thermo-physical and thermo-mechanical properties of balsa wood are investigated by proper experiments. At the second level, thermo-mechanical sandwich response models are developed and validated by experiments. The final level is conducted in order to investigate the thermo-mechanical behavior of the bridge deck at the structural system level which includes adhesively-bonded joints between the individual balsa panels of the deck and between the deck and the bridge girders. Furthermore, post-fire recovery models are established in order to assess the remaining structural capacity after a fire incident.

EPFL2022

Efficient algorithms for wave problems

Boris Bonev

Wave phenomena manifest in nature as electromagnetic waves, acoustic waves, and gravitational waves among others.Their descriptions as partial differential equations in electromagnetics, acoustics, and fluid dynamics are ubiquitous in science and engineering. Having numerical methods to solve these problems efficiently is therefore of great importance and value to domains such as aerospace engineering, geophysics, and civil engineering.Wave problems are characterized by the finite speeds at which waves propagate and present a series of challenges for the numerical methods aimed at solving them. This dissertation is concerned with the development and analysis of numerical algorithms for solving wave problems efficiently using a computer. It contains two parts:The first part is concerned with sparse linear systems which stem from discretizations of such problems. An approximate direct solver is developed, which can be computed and applied in quasilinear complexity. As such, it can also be used as a preconditioner to accelerate the computation of solutions using iterative methods. This direct solver is based on structured Gaussian elimination, using a nested dissection reordering and the compression of dense, intermediate matrices using rank structured matrix formats. We motivate the use of these formats and demonstrate their usefulness in our algorithm. The viability of the method is then verified using a variety of numerical experiments. These confirm the quasilinear complexity and the applicability of the method.The second part focuses on the solution of the shallow water equations using the discontinuous Galerkin method. These equations are used to model tsunamis, storm surges, and weather phenomena.We aim to model large-scale tsunami events, as would be required for the development of an early-warning system. This necessitates the development of a well-balanced numerical scheme, which is efficient, flexible, and robust. We analyze the well-balanced property in the context of discontinuous Galerkin methods and how it can be obtained. Another problem that arises with the shallow water equations is the presence of dry areas. We introduce methods to handle these in a well-balanced, and physically consistent manner. The resulting method is validated using tests in one dimension, as well as simulations on the surface of the Earth. The latter are compared to real-world data obtained from buoys and satellites, which demonstrate the applicability and accuracy of our method.

EPFL2021

Object-oriented approach for dynamic system modelling and simulation

Adel Besrour

Software engineering always cares to provide solutions for building applications as close as possible to what they should be, according to the requirements and the final users needs. Systems behavior simulation is a very common application to virtually reproduce and often predict the real-world behavior. Simulation is one of the most operational research tool in a large variety of engineering and scientific domains: Transport, telecommunication, medicine, chemical processes, physics, etc. The complexity of such application is relative to the increasing complexity of the systems. In this context, it is relevant to bring together different tools and formalisms such as markovian chain, Petri nets, etc., to improve the existent approaches and so to answer the simulations performances needs. The principle objective of this thesis is to bring together techniques from software engineering and safety engineering in order to improve the state of the art of modeling and simulation of dynamic systems in the industrial context. In addressing this objective, this work initially involves defining the essential limitations of the used formalisms, methods and tools regarding from one hand the software engineering modeling and simulation techniques and from the other hand the existent risk analysis methodologies. This work is conducted with respect to the problem of danger identification, considering the context of the complex systems behavior and their interaction with the human operator. In software engineering, it is well known that Petri/high-level nets have attractive characteristics to be used in systems simulation and behavior prediction such as the natural graphical representation, and their well-defined semantic. They are well-suited for the description of complex situations with concurrency (interleaving and true concurrency depending on the underlying semantics), conflict and confusion. However, the absence of structuring capabilities has been one of the main criticisms raised against Petri nets/high-level nets. Thus, there have been many attempts to introduce structuring principles in nets of this kind [BCM88] [Kie89] [JR91]. The attractive characteristics of Petri/high-level nets have prompted researchers to enrich these formalisms with object-oriented features. CO-OPN (Concurrent Object-Oriented Petri Net) approach, brings together the power of both Petri/high-level nets and object-orientation techniques, it has been devised so as to offer an adequate framework for the specification and design of large scale concurrent system [BG91]. CO-OPN, as a powerful modeling tool, has been used in a limited way to simulate systems. This work aims to provide a CO-OPN extension to allow a more realistic systems' simulation. Actually, its simulator semantic uses to be a suitable approach for modeling near closed systems and software components, because they need to loose coupling with external world. But, when we model more realistic problems like industrial processes, where human interaction is a relevant event, this approach is not sufficient to catch all system activity attributes. Moreover, the CO-OPN interpretation process does not allow interaction with the object internal states. This work provides a new solution to overcome CO-OPN simulation limitations and a set of prototypes to assist dynamic systems simulations. Furthermore, this work has been conducted in a Risk Analysis (RA) context, a domain where computer-based simulations research are of utmost interest. Actually, classical approaches used to address complex workplace hazard in a limited way, using checklists or sequence models. Moreover, the use of single oriented methods, such as AEA (man-oriented), FMEA (machine oriented) or HAZOP (process oriented), is not satisfactory to overcome the increasing sophistication of industrial processes. The automation of a part of the analysis process as well as the multiple-oriented approach allowed by dynamic modeling may indeed enhance significantly the analysis completeness and reduce the time analyzing time. This work, based on Object Oriented Petri net formalism (CO-OPN), propose an alternative multi-oriented approach where existent methods limitations have been criticized to develop a dynamic model, MORM (Man-machine Occupational Risk Modeling). A real industrial system (metal wire making process) has been specified to implement the different approach steps (system identification, model application, system simulation, system analysis).

EPFL2005