The enriched space–time finite element method (EST) for simultaneous solution of fluid–structure interaction

International audienceThe paper introduces a weighted residual-based approach for the numerical investigation of the interaction of fluid flow and thin flexible structures. The presented method enables one to treat strongly coupled systems involving large structural motion and deformation of multiple-flow-immersed solid objects. The fluid flow is described by the incompressible Navier–Stokes equations. The current configuration of the thin structure of linear elastic material with non-linear kinematics is mapped to the flow using the zero iso-contour of an updated level set function. The formulation of fluid, structure and coupling conditions uniformly uses velocities as unknowns. The integration of the weak form is performed on a space–time finite element discretization of the domain. Interfacial constraints of the multi-field problem are ensured by distributed Lagrange multipliers. The proposed formulation and discretization techniques lead to a monolithic algebraic system, well suited for strongly coupled fluid–structure systems. Embedding a thin structure into a flow results in non-smooth fields for the fluid. Based on the concept of the extended finite element method, the space–time approximations of fluid pressure and velocity are properly enriched to capture weakly and strongly discontinuous solutions. This leads to the present enriched space–time (EST) method. Numerical examples of fluid–structure interaction show the eligibility of the developed numerical approach in order to describe the behavior of such coupled systems. The test cases demonstrate the application of the proposed technique to problems where mesh moving strategies often fail

Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.

RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Modal reduction of a vibroacoustic problem for a parametric study using XFEM

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Une méthode de calcul efficace pour l’étude paramétrique du flambage non-linéaire de structures tridimensionnelles : application à la fiabilité

The present work was done in partnership with DGA-CTSN who would like to evaluate the reliability of submarines shells. The evaluation of reliability causes two problems: mechanical model and computational time. In a first part, in order to model correctly the geometry and imperfections of a submarine shell, a three- dimensional finite element which is able to compute elasto-plastic bending of thin structures is developed. In a second part, in order to decrease computational time, a novel parametric method is proposed. In this method, information obtained by a reference calculation (usually with parameters set to their mean values), is used to compute the modified structure response. Finally, this work gives solutions for the mechanical-reliability coupling: a finite element that enables a good model of the mechanical problem, and a parametric method which decreases computational time by a factor from 1.5 to 15.À l'initiative de la DGA, ce travail a pour but d'évaluer la fiabilité des coques de bâtiments submersibles. L'évaluation de la fiabilité pose deux problèmes : la modélisation du phénomène mécanique et le temps de calcul. D'une part, afin de pouvoir modéliser correctement la géométrie et les imperfections, un élément tridimensionnel capable de rendre compte de la flexion élasto-plastique des structures minces est développé. D'autre part, afin de diminuer le temps de calcul de l'analyse de fiabilité, nous proposons une méthode de calcul paramétrée originale basée sur l'utilisation des informations d'un calcul de référence (en général les valeurs moyennes des paramètres) pour calculer la réponse d'une structure aux paramètres modifiés. Finalement, le travail apporte des solutions au couplage mécanique fiabilité : l'élément développé est capable de modéliser correctement le problème mécanique et la méthode de calcul paramétrée permet de diminuer le temps de calcul d'un facteur variant de 1,5 à 15

Modelization of immersed structures at arbitrary positions in an acoustic fluid using XFEM

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The extended finite element method combined with a modal synthesis approach for vibro-acoustic problems

International audienceThe optimization of a vibro-acoustic problem is of main importance for passengers' comfort in transportation vehicles in terms of interior noise. Engineers use numerical tools to predict the response of this coupled problem, but it may lead to a prohibitive computational time. Based on FEM, this work aims at reducing the computational time. The first idea is to keep the same mesh of the acoustic cavity for all the structure configurations and to enrich the pressure approximation by using the extended FEM (XFEM). The enrichment is based on a Heaviside function completed at the structure tip by a continuous ramp function. The second idea is to build reduced basis. The structure basis is composed of its eigenmodes, whereas a modal synthesis method with a fixed interface is used to build the fluid basis. The interface DOFs are the enriched DOF of the XFEM, whereas the internal domain corresponds to the acoustic cavity with no structure. These two combined ideas enable to minimize the computational time in the study of the influence of the structure positions in an acoustic cavity. The method is implemented for shell structures embedded in a 3D acoustic domain

Méthode XFEM et modèles réduits en dynamique des structures et en vibro-acoustique

This thesis describes the researches I have done since my PhD defense in 2002. These researches mainly concern simplified and/or reduced models for damped structures and/or vibro-acoustic problems.In a first part, a work on the eXtended Finite Element Method (XFEM) is presented. The method is applied to spectral finite elements and to fluid-structure interaction.In a second part, the reduction of sound level in acoustic cavities by using porous material is investigated. The XFEM is used to model thin flexible structures immersed in the acoustic fluid. Reduced basis of the vibro-acoustic problem are built, they are based on domain decomposition techniques as well as modal synthesis methods.Finally, several simplified models of damped structures are proposed for axisymmetric structures and for sandwich structures with a visco-elastic core. An extension to non-linear rubber devices between two structures is currently investigated.Ce mémoire fait état des recherches effectuées depuis ma thèse en 2002. Ces recherches portent principalement sur des modèles simplifiés et/ou réduits pour des problèmes touchant à la réduction des vibrations acoustiques et/ou structurelles.Dans un premier temps, un travail mené sur la méthode des éléments finis étendus (XFEM) est présenté. La méthode est appliquée à des éléments finis spectraux puis à des problèmes d'interaction fluide-structure.Dans un deuxième temps, on s'intéresse à la réduction du bruit dans les cavités acoustiques par utilisation de matériaux poreux. La présence de structures minces dans la cavité acoustique est modélisée par la méthode XFEM. Le modèle numérique associé est réduit par des méthodes de décomposition de domaine et de synthèse modale dans le domaine fréquentiel.Enfin, plusieurs modèles simplifiés de structures amorties sont développés. Des structures de révolution sont calculées en décomposant le déplacement en séries de Fourier. Des éléments finis d'interface sont développés pour modéliser la couche de matériau amortissant des structures sandwich munies de matériaux visco-élastique à c{\oe}ur. Des bases de projections sont proposées pour réduire la taille de ces problèmes. Des extensions aux liaisons élastomère en grandes transformations sont en cours

An extended finite element method approach for structural-acoustic problems involving immersed structures at arbitrary positions

International audienceNoise reduction for passengers' comfort in transport industry is now an important constraint to be taken into account during the design process. This process involves to study several configurations of the structures immersed in a given acoustic cavity in the context of an optimization, uncertainty, or reliability study for instance. The finite element method may be used to model this coupled fluid–structure problem but needs an interface conforming mesh for each studied configuration that may become time consuming. This work aims at avoiding this remeshing step by using noncompatible meshes between the fluid and the structures. The immersed structures are supposed to be thin shells and are localized in the fluid domain by a signed distance level-set. To take into account the pressure discontinuity from one side of the structures to the other one, the fluid pressure approximation is enriched according to the structures positions by a Heaviside function using a partition of unity strategy (extended finite element method). The same fluid mesh of the empty cavity is then used during the whole parametric study. The method is implemented for a three-dimensional fluid and tested on academic examples before being applied to an industrial-like case

Efficient method to study the influence of immersed structures in an acoustic fluid using XFEM and modal reductions

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