Modeling The Hyperloop With COMSOL Multiphysics® : On The Design Of The EPFLoop Pressurized Systems

The EPFLoop team from Ecole Polytechnique Fédérale de Lausanne has developed a capsule thanks to which it won the 3rd place in SpaceX's Hyperloop Pod Competition in 2018. COMSOL Multiphysics was used to analyze and study the pressurized systems of the pod. Three pressure vessels (PVs) of different shape and structure are used to store electrical components in a pressurized environment at 1 bar, meanwhile the external environment is at 8 mbar. The PVs' failure under load was studied using a stationary simulation and shell finite elements in order to represent the plies of carbon fiber-epoxy and foam. The load conditions were the maximum deceleration (2.6 g), the weight of the internal components and the internal pressure of 1 bar. The aim was to design the plies layering with a minimum Tsai-Wu safety factor of 2 everywhere. A parametric sweep was then performed to estimate the maximum allowable working pressure (MAWP, corresponding to a safety factor equal to 2) and the BURST pressure (pressure for which the safety factor is less equal than 1 and failure is imminent). To ensure the normal functioning of electronic components, analyses were done to ensure that the temperature inside the PVs wouldn't be greater than 50°C due to internal electronic heat loads. This has been done by coupling the Heat Transfer in Solid Module with the Laminar Flow Module in order to take into account convection effects. The simulations were validated by measurements during experimental tests. Experimental results confirmed the design and analyses carried out using COMSOL Multiphysics®.

Propriétés hors-équilibre d'une paroi adiabatique

Séverine Pache

We study the evolution of a system composed of N non-interacting particles of mass m distributed in a cylinder of length L. The cylinder is separated into two parts by an adiabatic piston of a mass M ≫ m. The length of the cylinder is a fix parameter and can be finite or infinite (in this case N is infinite). For the infinite case we carry out a perturbative analysis using Boltzmann's equation based on a development of the velocity distribution of the piston in function of a small dimensionless parameter ε = √(m/M). The non-stationary case is solved up to the order ε ;; our analysis shows that the system tends exponentially fast towards a stationary state where the piston has an average velocity V. The characteristic time scale for this relaxation is proportional to the mass of the piston (τ0 = M/A where A is the cross-section of the piston). We show that for equal pressures the collisions of the particles induce asymmetric fluctuations of the velocity of the piston which leads to a macroscopic movement of the piston in the direction of the higher temperature. In the case of the finite model a perturbative approach based on Liouville's equation (using the parameter α = 2m/(M + m)) shows that the evolution towards thermal equilibrium happens on two well separated time scales. The first relaxation step is a fast, deterministic and adiabatic evolution towards a state of mechanical equilibrium with approximately equal pressures but different temperatures. The movement of the piston is more or less damped. This damping qualitatively depends on whether the ratio R = Mgas/M between the total mass of the gas and the mass of the piston is small (R < 2) or large (R > 4). The second part of the evolution is much slower ; the typical time scales are proportional to the mass of the piston. There is a stochastic evolution including heat transfer leading to thermal equilibrium. A microscopic analysis yields the relation XM(t) = L(1/2 - ξ(at)) where the function ξ is independent of M. Using the hypothesis of homogeneity (i.e. the values of the densities, pressures and temperatures at the surface of the piston can be replaced by their respective average values) introduced in the previous analysis the observed damping does not show up. This can be explained by shock waves propagating between the piston and the walls at the extremities of the cylinder. In order to study the behaviour of the system there is hence a need to adequately describe the non-equilibrium fluids around the piston. We carry out an analysis of the infinite case, based on the perturbative approach introduced earlier. In this case the initial conditions are chosen in such a manner that the piston on average stays at the origin. It is shown that it is possible to describe the evolution of the fluids in such a way that it is coherent with the two laws of thermodynamics and the phenomenological relationships. Finally we study the case of a constant velocity of the piston in a finite cylinder. Such a condition and elastic collisions allow us to derive an explicit expression for the distribution of the fluids and hence for the hydrodynamics fields. This expression reveals the presence of shock waves between the piston and the extremities of the cylinder.

EPFL2004

Geological heterogeneity in landslides

Boris Matti

Significant progress has been made these last decades in the development of hydrogeological numerical flow modelling for describing the hydrodynamic behaviour of landslides. However, these new sophisticated methods are still very seldom used in the problems of slope instability in particular because of the hydrogeological complexity which characterizes them; thin aquifers, discontinuous media, succession of saturated and unsaturated zones, low permeabilities, high hydraulic gradients, lithological heterogeneity, strong contrasts of permeabilities and heterogeneous infiltration. Predictive models of flow in the subsurface, which are often based on homogeneous porous media types of representation, are badly adapted to natural systems that are characterized by highly heterogeneous media such as landslides. These models are good and reliable on a landslide scale (regional scale), but their quality may be affected on a local scale by strong geological heterogeneities. Geological heterogeneities of the subsurface take part in determining the hydrodynamical and geomechanical behaviour of landslides. However, their spatial distribution is partially unknown. Thus, the principal objectives of this PhD thesis are: (i) To carry out an integrated multidisciplinary characterization study on the internal structure of landslides in flysch and Quaternary environments, in order to clarify the organisation of the geological heterogeneities and to identify the hydrodynamic implications. (ii) To propose a conceptual model representing the geological architecture and the hydrogeological functioning. (iii) To examine the effects of heterogeneity and anisotropy on flow systems. (iv) To better understand the influence of geological heterogeneities on the mechanical behaviour of large landslides by performing numerical sensitivity analyses, by means of different heterogeneity scenarios on the field parameters. (v) Finally, to test the incidences on slope stabilization techniques; evaluation of the efficiency of a drainage gallery work. The main test site of la Frasse landslide (VD, Switzerland) was chosen, and completed with additional landslide cases. The main results are the following: In most of the case studies, the landslide mass is composed of an old prehistoric stabilized mass, pinched between the active sliding mass and the bedrock, and playing an important hydrologic role. The stabilized mass and the bedrock form the substratum of the landslide. Landslides occurring in these types of media are defined by an organized heterogeneous environment with "fracture" flows and discontinuity porosity. The overall hydraulic conductivity is low, and locally high permeable zones exist. Regional groundwater circulations are limited and form local interconnected aquicludes organised in thin aquifers, and presenting saturated and unsaturated zones. The hydrogeological analyses showed that the system presents a bimodal permeability; (i) Low hydraulic conductivities characterizing the global matrix and defining the capacitive fraction, and (ii) high permeable features, with high hydraulic conductivities defining the conductive fraction, and favouring strong channelling effects. Besides, the observation shows that the aquifer system is generally very reactive with important magnitudes. Often, there is a straight correlation between water level variation and climatic conditions (rainy events). Landslides are characterized by two important inflows namely effective infiltration from the surface and lateral inflows from the neighbouring units. Water transfer between the stabilized mass and the active mass may be important and thus have to be considered. The existence of water transfer between the bedrock and the landslide mass (stabilized and active) is not well established. The bedrock and the landslide mass present a hydrological behavioural independence. Theoretical two- and three-dimensional flow models are used to investigate the effects of the spatial variability of the hydraulic conductivity on the underground flows. The role of the connectivity in generating flow channelling is examined thanks to the observation of close relations between the permeability and the hydraulic pressures. The sensitivity analysis shows clearly that the relation between local permeability and hydraulic pressures is not straight, and that the organization of the flows depends on the heterogeneity of the hydraulic properties and their spatial correlation. Strong channelling effects are observed in highly heterogeneous porous media. The development of flow channelling as a function of the variance of the natural log permeability values and the correlation lengths is demonstrated. The integrated multi-disciplinary geological characterization at the La Frasse test site combined with the hydrogeological and lithological data of several additional case studies led to the proposal of a global conceptual model. The following assumptions are considered to enable a subsequent quantification of flow components: The flow occurs under confined to leaky conditions, with leakage varying in space; The flow framework is controlled by a complex multi-layer system, isolated lenses or perched aquifer; The aquifer system is divided into interconnected hydrological zones presenting various degrees of saturation; Each hydrological zone may function individually from the others; Horizontally and vertically, the flow direction in the porous matrix is affected by prevailing structural patterns generating channeling effects; The flow is multidirectional, free and channelized, and is affected by temporal and spatial changes; The aquifer is under an unsteady flow regime due to seasonal variation of natural gradients; A conceptual model based on a simple reservoir approach is proposed. It allows the representation of most of the field observations and the main characteristics, namely the organized heterogeneity and the duality of the aquifers. The system is represented by various reservoirs more or less connected and saturated. Complex storage capacities and plug-flow effects may record past events and reactive sliding processes several months after the last important rainy event. The analysis shows that function of the capacity and the degree of saturation of the system, an important hydrological event is not necessarily associated to a reactivation. And, according to the degree of complexity of the system (saturation, connectivity...) a localized geological modification (variation of permeability, reservoir burst...) may produce a chain reaction, and generate failures in unexpected places. The conductive fraction favours the drainage of the system, whereas the capacitive fraction controls the distribution of the hydraulic heads. The role of the phreatic nappe, through the conductive fraction, is to drain and control the hydrologic equilibrium of the system. Therefore landslide remediation with the help of a deep drainage gallery is obviously the most valuable method for this type of landslide. It supports and enhances the natural effects of the conductive fraction in draining the system. Finally, in this context the efficiency of civil engineering works was evaluated according to the heterogeneity of the medium. This study describes transient hydrogeological and geomechanical models realized jointly in 2006 by the EPFL and GeoMod SA within the framework of the stabilization work of the La Frasse landslide. These models evaluate the impact of a deep drainage gallery with subvertical pipes towards the surface in terms of reduction of the deformation velocities and increase of the factor of safety of the landslide. Three variants consisting of different inter pipe spacings are tested. Considering the local heterogeneities, the results show that a mean spacing between the pipes of the order of 10 m is able to control the temporal head fluctuations between the wells within a range of some meters. Moreover, this solution induces a strong diminution of the predicted displacements during a specific crisis, from 101cm for the model without drainage to around 14 cm for the drained model, and a significant gain of security (from 1.05 to 1.30).

EPFL2008

Biogeography, Evolution and Diversity of Free-Living Protists

Thierry Jean Heger

Although widely recognized as essential actors of ecosystem processes and representing a significant part of the Earth's biodiversity, free-living protists are poorly known. Major questions concerning their diversity, ecology, and evolution remain completely unresolved, partly because our knowledge of their taxonomy, at each different levels, is still extremely limited. In this thesis, I used testate amoebae as model organisms to get insights into the diversity and biogeography of protists. Testate amoebae are a polyphyletic group of free-living protists that are characterized by a shell composed of proteinaceous, calcareous, or siliceous material. The shells are differentiated by their shapes and ultrastructures, allowing for the identification of morphospecies more easily than in the vast majority of other protists. However, the taxonomic validity of the characters used to define morphospecies and the true diversity of testate amoebae are largely unknown. We studied the taxonomy of cyphoderiid testate amoebae (Rhizaria: Euglyphida), by combining morphological and molecular genetic approaches (chapters 2 and 3). We used light- and scanning electron microscopy to analyze the shell ultrastructure and biometry of several Cyphoderia morphospecies and sequenced gene portions of the Cytochrome Oxidase Subunit 1 (COI) and nuclear small subunit rDNA (SSU). These analyses revealed that while several described species are morphologically distinct and well-supported by the genetic data, other species, such as Cyphoderia ampulla, represent a complex of cryptic or pseudo-cryptic species. The presence of cryptic or pseudo-cryptic Cyphoderia species illustrates the limits of a morphology-based approach for assessing protist diversity using only characters visible by light microscopy. DNA sequence analyses may allow for a better resolution for both species identification and diversity estimates, provided that appropriate genetic markers can be developed. Genetic marker development in protists is particularly challenging because their representation in the international genomic databases is still very limited. The SSU marker also allowed us to build an extensive phylogeny of eulgyhid testate amoebae. We then used this phylogeny as a framework to analyze macro-evolutionary processes in testate amoebae, in particular, how easily a species can cross an ecological barrier and colonize new habitats. The taxa included in our phylogeny occurred either in marine, marine supralittoral or freshwater habitats, and species of each habitat segregated into distinct sub-clades. This indicates that transitions between these environments occur only rarely in the course of evolution (chapter 4). A reliable taxonomy for microorganisms is not only an essential prerequisite for estimating species diversities, it would also be a key component to resolve the long-lasting debate of whether free-living microorganisms are strictly cosmopolitan, or whether at least some species have limited geographic distributions (chapter 1). We analyzed Arcellinida and Euglyphida testate amoeba communities from Amsterdam Island, one of the most remote islands in the world located in the Indian Ocean, to search for endemic species with limited geographic distributions (chapter 5). In macroorganisms, the degree of endemism is generally high in such remote islands. Our extensive inventory of testate amoeba morphospecies provided no clear evidence for endemism. However, given that the morphology-based species identification gives only a limited taxonomic resolution, we cannot exclude the presence of morphologically non-distinctive endemic species (cryptic species). Thus, such taxonomic uncertainties undermine biogeographical studies of testate amoebae and other protists. To circumvent taxonomical uncertainties in biogeographical studies, we analyzed the distribution of a morphologically highly distinctive testate amoeba, Nebela ansata (chapter 6). We report the presence of this species in Nova Scotia, Canada and at sites in New Jersey where it was initially described. An extensive literature survey further confirmed the absence of this species from both, the remaining North American continent as well as other continents. Together, these data provide an unusually convincing case of a free-living microorganism with a very limited distribution range along the temperate East American coast. Overall, my thesis illustrates how, in most cases, the current morphology-based species identification greatly underestimates testate amoeba diversity and calls for an improvement of the taxonomic resolution and reliability. The extension of the current taxonomy using molecular-genetic tools should greatly contribute to this task and also shed light on the debate on the distribution of protists and other microorganisms. Together, such studies will provide a first step towards a better understanding of the diversity and evolution of microorganisms and improve their utility as model organisms in both fundamental and applied questions.

EPFL2010