Migration, trapping, and venting of gas in a soft granular material

Gas migration through a soft granular material involves a strong coupling between the motion of the gas and the deformation of the material. This process is relevant to a variety of natural phenomena, such as gas venting from sediments and gas exsolution from magma. Here, we study this process experimentally by injecting air into a quasi-2D packing of soft particles and measuring the morphology of the air as it invades and then rises due to buoyancy. We systematically increase the confining pre-stress in the packing by compressing it with a fluid-permeable piston, leading to a gradual transition in migration regime from fluidization to pathway opening to pore invasion. We find that mixed migration regimes emerge at intermediate confinement due to the spontaneous formation of a compaction layer at the top of the flow cell. By connecting these migration mechanisms with macroscopic invasion, trapping, and venting, we show that mixed regimes enable a sharp increase in the average amount of gas trapped within the packing, as well as much larger venting events. Our results suggest that the relationship between invasion, trapping, and venting could be controlled by modulating the confining stress

Effect of coupling with internal and external fluids on the mechanical behavior of aerostats

International audienceIn the context of the simulation of aerostats in flight, we are interested here in the coupling between a deformable structure, the fluid contained inside and the fluid flow outside. To study the dynamic stability of such systems, the fluid-structure coupled equations are linearized around an equilibrium position and, by assuming that the fluid flow perturbations are potential, the loads exerted by the fluids on the moving structure can be decomposed in terms proportional, respectively, to the displacement, velocity and acceleration fields of the structure, representing what are generally called the added stiffness, damping and mass effects of the fluid on the structure. In this work, a focus is made on the added mass because, for such lightweight structures, its effect is of prime importance. A Boundary Element Method (BEM) is proposed to compute the fluid added mass operators, for external and internal fluids, and for any structure deformation field. Numerical and experimental validations are conducted on an axisymmetric ellipsoid mockup immerged in water and subject to rigid motions. Variations of the imposed movement amplitude and velocity have also helped to evaluate the validity domain of this model

COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study

Background: The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms. Methods: International, prospective observational study of 60 109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms. Results: ‘Typical’ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (≤ 18 years: 69, 48, 23; 85%), older adults (≥ 70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each P < 0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country. Interpretation: This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men

The Gaia mission

Gaia is a cornerstone mission in the science programme of the EuropeanSpace Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page. http://www.cosmos.esa.int/gai

Modélisation des effets de fluides externes et internes sur le comportement dynamique des dirigeables flexibles

This PhD thesis deals with the modeling and simulation of fluid-structure interactions between a flexible airship, an external flow and an internal lifting gas. The fluid is considered as potential due to the large size of the airships. The potential flow is solved on a mesh of the fluid-structure interface using the Boundary Element Method, and the work of the associated forces is obtained using the Finite Element Method. The fluid equations are expressed in an Arbitrary Lagrangian-Eulerian formalism. In order to calculate on an invariant reference mesh, we consider large rigid body motions (rotations and translations) to which are added small rigid body motions and small deformations with respect to which the fluid problem is linearized. This leads in practice to differentiate the operators of the Boundary Element Method to characterize the kinematics of the fluid and the operators of the Finite Element Method to determine the associated forces exerted on the fluid-structure interface, in order to take into account the perturbations in the fluid domain related to the movements of the interface. The linearization allows to express the fluid forces using added mass, gyroscopic and stiffness operators, proportional respectively to the acceleration, velocity and displacements of the interface. In addition, a simplified fin model based on Theodorsen theory is used to include their lift in the model.The structural part of the problem is modeled using the Finite Element Method for a membrane. The membrane is prestressed by the static pressure of the internal fluid and the external flow. The fluid-structure model thus obtained is used to study the stability of such a coupled fluid-structure system on an airship test case. The fluid-structure operators are used to obtain the eigenvalues of the system at various speeds of the airship. The fluid-structure problem is expensive to solve since the matrices of the Boundary Element Method are full and to reduce the size of the problem, the latter is projected on a reduced basis of the structure modes in vacuum. We observe that beyond a critical velocity, a risk of divergence by flutter arises.Finally, the added mass obtained by the potential flow model is compared with experimental results obtained with a rigid model oscillating rotationally in a fluid tank at rest. By varying the oscillation frequency of the model, the density and the viscosity of the fluid, it was possible to characterize the order of magnitude of the viscous forces compared to the inertial forces of the fluid predicted for a potential fluid as a function of the Stokes number.Cette thèse de doctorat porte sur la modélisation et la simulation des interactions fluide-structure entre un dirigeable souple, un écoulement externe et un gaz porteur interne. On considère que le fluide est potentiel du fait de la grande taille des dirigeables. L’écoulement potentiel est résolu sur un maillage de l’interface fluide-structure grâce à la Méthode des Éléments de Frontière, et le travail des efforts associés est obtenu par la Méthode des Eléments Finis. Les équations du fluide sont exprimées dans un formalisme Arbitrairement Lagrangien-Eulérien. Afin de se ramener à des calculs sur un maillage de référence invariant, on considère de grands mouvements d'ensemble (rotations et translations) auxquels s’ajoutent de petits mouvements de corps rigides et de petites déformations par rapport auxquels le problème fluide est linéarisé. Cela conduit en pratique à différencier les opérateurs de la Méthode des Éléments de Frontière pour caractériser la cinématique du fluide ainsi que les opérateurs de la Méthode des Eléments Finis pour déterminer les efforts associés exercés sur l’interface fluide-structure, ceci afin de prendre en compte les perturbations dans le domaine fluide liées aux mouvements de l’interface. La linéarisation permet d'exprimer les efforts du fluide à l’aide d’opérateurs de masse, gyroscopique et de raideur ajoutés, proportionnels respectivement à l’accélération, la vitesse et les déplacements de l’interface. De plus, un modèle simplifié d’ailerons basé sur la théorie de Theodorsen est utilisé afin d'ajouter leur portance au modèle.La partie structure du problème est modélisée à partir de la Méthode des Eléments Finis appliquée à une membrane. Celle-ci est précontrainte par la pression statique du fluide interne et de l’écoulement externe. Le modèle fluide-structure ainsi obtenu est utilisé pour étudier la stabilité d'un tel système couplé fluide-structure sur un cas test de dirigeable. Les opérateurs fluide-structure permettent d’obtenir les valeurs propres du système pour diverses vitesses du dirigeable. Le problème fluide-structure est coûteux à résoudre car matrices de la Méthode des Éléments de Frontière sont pleines, et pour réduire la taille du problème, celui-ci est projeté sur une base réduite des modes de la structure dans le vide. Au-delà d’une vitesse critique, on constate qu’un risque de divergence par flottement apparaît.Enfin, la masse ajoutée obtenue par le modèle d’écoulement potentiel est comparée avec des résultats expérimentaux obtenus à partir d’une maquette rigide oscillant en rotation dans une cuve de fluide au repos. En faisant varier la fréquence d’oscillation de la maquette, la densité et la viscosité du fluide, on a pu caractériser, en fonction du nombre de Stokes, l’ordre de grandeur des efforts visqueux relativement aux efforts inertiels du fluide prédits pour un fluide potentiel

Modélisation des effets des fluides externes et internes sur le comportement dynamique des dirigeables déformables

International audienceDuring an airship flight, a coupling occurs between small deformations of the structure, the inner fluid and the outer flowing fluid, leading to added mass, damping and rigidity phenomena (for high Stokes numbers). An added mass calculation method of the inner and outer fluids is here introduced, and its performance is checked by comparing numerical results with analytic formulas found in the litterature. The range of validity of the potential model is determined experimentaly on a small sized model oscillating in a water tank.Pendant le vol d’un dirigeable, le couplage entre les petites déformations de la structure, le fluide interne et le fluide externe en écoulement conduit à des phénomènes de masse, amortissement et raideur ajoutés (pour des nombres de Stokes élevés). On présente ici une méthode de calcul de la masse ajoutée par les fluides internes et externes dont la performance est testée en comparant les résultats numériques obtenus à des formules analytiques présentes dans la littérature. Le domaine de validité du modèle potentiel supposé ici est déterminé expérimentalement sur une maquette de taille réduite en oscillation dans un bassin d’eau

RESOLUTION OF FLUID-STRUCTURE COUPLED PROBLEMS WITH FLOW USING THE BOUNDARY ELEMENT METHOD

Effects of fluid-structure coupling on the dynamic behaviour of flexible airship can be modelled with a potential, incompressible, inviscid flow. A new formalism to study linear variations of the flow induced on the fluid-structure interface in a time dependent ambient flow is introduced. The features of the Boundary Element Method used to solve this problem numerically are exhibited. Numerical results of the linear model are compared with analytical and non linear numerical results, assessing the validity and the limitations of the approach