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See detailNumerical simulation of the fluid-structure interaction between air blast waves and free-standing plates
Kambouchev, Nayden; Noels, Ludovic ULg; Radovitzky, Raúl

in Computers & Structures (2007), 85(nov-14 Sp. Iss. SI), 923931

A numerical method is used to compute the flow field corresponding to blast waves of different incident profiles propagating in air and impinging on free-standing plates. The method is suitable for the ... [more ▼]

A numerical method is used to compute the flow field corresponding to blast waves of different incident profiles propagating in air and impinging on free-standing plates. The method is suitable for the consideration of compressibility effects in the fluid and their influence on the plate dynamics. The history of the pressure experienced by the plate is extracted from numerical simulations for arbitrary blast strengths and plate masses and used to infer the impulse per unit area transmitted to the plate. The numerical results complement some recent analytical solutions in the intermediate range of plate masses and arbitrary blast intensities where exact solutions are not available. The resulting beneficial effect of the fluid-structure interaction (FSI) in reducing transmitted impulse in the presence of compressibility effects is discussed. In particular, it is shown that in order to take advantage of the impulse reduction provided by the FSI effect, large plate displacements are required which, in effect, may limit the practical applicability of exploiting FSI effects in the design of blast-mitigating systems. (C) 2006 Elsevier Ltd. All rights reserved. [less ▲]

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See detailFluid-structure interaction effects in the dynamic response of free-standing plates to uniform shock loading
Kambouchev, Nayden; Radovitzky, Raúl; Noels, Ludovic ULg

in Journal of Applied Mechanics (2007), 74

The problem of uniform shocks interacting with free-standing plates is studied analytically and numerically for arbitrary shock intensity and plate mass. The analysis is of interest in the design and ... [more ▼]

The problem of uniform shocks interacting with free-standing plates is studied analytically and numerically for arbitrary shock intensity and plate mass. The analysis is of interest in the design and interpretation of fluid–structure interaction (FSI) experiments in shock tubes. In contrast to previous work corresponding to the case of incident blast profiles of exponential distribution, all asymptotic limits obtained here are exact. The contributions include the extension of Taylor’s FSI analysis for acoustic waves, the exact analysis of the asymptotic limits of very heavy and very light plates for arbitrary shock intensity, and a general formula for the transmitted impulse in the intermediate plate mass range. One of the implications is that the impulse transmitted to the plate can be expressed univocally in terms of a single nondimensional compressible FSI parameter. [less ▲]

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See detailIntroduction of Numerical Dissipation in the Variational Updates Framework for Elasto-Plastic Constitutive Models
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

(2007)

The most widely used time integration algorithms for finite-element, based on the Newmark family, are numerically stable only for linear models. Using a variational formulation of elasto-plastic updates ... [more ▼]

The most widely used time integration algorithms for finite-element, based on the Newmark family, are numerically stable only for linear models. Using a variational formulation of elasto-plastic updates for which the stress tensor always derives from an incremental potential, even when plastic behavior is considered, a new Energy-Dissipative Momentum Conserving time integration algorithms can easily be developed. Indeed, the incremental potential guarantees the verification of the numerical dissipation of energy for non-linear elasto-plastic dynamics. [less ▲]

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See detailA New Energy-Dissipative Momentum-Conserving Time Integration Scheme Using the Variational Updates Framework.
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

(2007)

Although they are the most widely used time integration algorithms for finite-element discretizations, algorithms based on the Newmark family are numerically stable only for linear models. To overcome ... [more ▼]

Although they are the most widely used time integration algorithms for finite-element discretizations, algorithms based on the Newmark family are numerically stable only for linear models. To overcome that drawback, researches have recently focused on time integration algorithms stable in the non-linear range, leading to the development of Energy Momentum Conserving Algorithms or EMCA. A new Energy Momentum Conserving Algorithm was recently presented by the authors for elasto-plastic material models. This algorithm is based on a variational formulation of elasto-plastic updates for which the stress tensor always derives from an incremental potential, even when plastic behavior is considered. In this work this formalism is used to develop Energy-Dissipative Momentum-Conserving time integration algorithms, which have property of being stable in non-linear range, but also dissipate high-frequency numerical modes to improve the convergence properties. Using the potential existence of the variational updates framework, the verification of the numerical dissipation of energy in the non-linear range can easily be demonstrated. [less ▲]

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See detailAlternative approaches for the derivation of discontinuous Galerkin methods for nonlinear mechanics
Noels, Ludovic ULg; Radovitzky, Raúl

in Journal of Applied Mechanics (2007), 74

Discontinuous Galerkin methods are commonly derived by seeking a weak statement of the governing differential equations via a weighted-average approach allowing for discontinuous fields at the element ... [more ▼]

Discontinuous Galerkin methods are commonly derived by seeking a weak statement of the governing differential equations via a weighted-average approach allowing for discontinuous fields at the element interfaces of the discretization. In order to ensure consistency and stability of the formulation, this approach requires the definition of a numerical flux and a stabilization term. Discontinuous Galerkin methods may also be formulated from a linear combination of the governing and compatibility equations weighted by suitable operators. A third approach based on a variational statement of a generalized energy functional has been proposed recently for finite elasticity. This alternative approach naturally leads to an expression of the numerical flux and the stabilization terms in the context of large deformation mechanics problems. This paper compares these three approaches and establishes the conditions under which identical formulations are obtained. [less ▲]

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See detailLEODIUM project and student space activities at the University of Liege
Vandenrijt, Jean-François ULg; Rochus, Pierre ULg; Kerschen, Gaëtan ULg et al

Conference (2007)

the origin of the LEODIUM project which stands for Low Earth Orbit Demonstration of Innovations in University Mode. This project aims to create student pico- or nano-satellites. There is a strong ... [more ▼]

the origin of the LEODIUM project which stands for Low Earth Orbit Demonstration of Innovations in University Mode. This project aims to create student pico- or nano-satellites. There is a strong motivation coming from the students and from the academic members to promote space. Being unexperienced in complete satellite design, as a first step in this direction, we started working with the ESA funded SSETI association. At the moment, two teams of students are working on SSETI satellites. One is working as the MECH team which is in charge of the design of the sun-pointing solar panels of the European Student Earth Orbiter (ESEO) satellite mechanisms, the other team is working on the phase-A studies of the Narrow Angle Camera (NAC) which will be the core payload of the European Student Moon Orbiter (ESMO) satellite, both in collaboration with local industrials. The students are mainly working on ESEO and ESMO as part of their master thesis. In addition to that, two PhD students are in charge of the coordination of the teams (one for the ESEO team and another for the ESMO team). They will also be the link between previous and new students every times an overlap will occur and ensure that information is not lost during the transition. In this paper, we will describe the current design and the achievements on the ESEO solar panels and the ESMO NAC camera. Explanation will also be given on how the collaboration between the local student and the SSETI association is managed. Finally, we will briefly introduce the prospective for the LEODIUM project. [less ▲]

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See detailPARTICIPATION OF ULG STUDENTS TO THE SSETI MICROSATELLITE PROGRAMME USING SAMCEF
Bolland, M.; COLLIGNON, G.; VANDENPLAS, X. et al

Conference (2007)

The objective of this presentation is to discuss the participation of students at the University of Liège to the educational spacecraft project SSETI-ESEO proposed by the European Space Agency. SSETI ... [more ▼]

The objective of this presentation is to discuss the participation of students at the University of Liège to the educational spacecraft project SSETI-ESEO proposed by the European Space Agency. SSETI students are now mainly involved in the ESEO satellite for which the University of Liège is in charge of the MECH work package. The students have to propose a design for the structure of the solar panels (including the geometry and materials) and their deployment and pointing mechanisms (including electrical circuits, motors, hinges and actuators). Through detailed finite element computations in the SAMCEF software, they have to verify the structural integrity of the panels during the launch phase and to guarantee a safe deployment once on-orbit. Another task is to ensure an appropriate orientation of the solar panels to collect the maximum power from the sun. An industrial partner specialized in the development of computer-aided engineering solutions, SAMTECH, and a research center, Liège Space Center (CSL), also bring their expertise to the project. [less ▲]

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See detailAn Energy-Dissipative Momentum-Conserving Time Integration Algorithm for Viscoplastic Hyperelastic Models Based on the Variational Updates Framework
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

(2007)

Traditional time integration algorithms for finite-element discretization are numerically stable only for linear models. To overcome that drawback, a new class of "Energy Momentum Conserving Algorithms ... [more ▼]

Traditional time integration algorithms for finite-element discretization are numerically stable only for linear models. To overcome that drawback, a new class of "Energy Momentum Conserving Algorithms" or EMCA and ``Energy-Dissipative Momentum-Conserving'' or EDMC algorithms, verifying the conservation/dissipation laws in the non-linear range, was recently developed. They consist in a mid-point scheme with an adequate evaluation of the internal forces, which strongly assume the existence of an energy potential. Following the variational visco-plastic constitutive updates formalism, a general energy-dissipative momentum-conserving algorithm for hyperelastic-based formulation is proposed. The main feature of this variational formulation is that the stress tensor always derives from an incremental potential, even if plastic deformations occur, allowing to use it in the above integration algorithms. [less ▲]

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See detailNumerical simulation of blaststructure interaction
Noels, Ludovic ULg; Radovitzky, Raul

Conference (2006)

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See detailA New Discontinuous Galerkin Formulation for Non-Linear Elasticity
Noels, Ludovic ULg; Chandran, Ram B; Radovitzky, Raul

Conference (2006)

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See detailSimulation of crashworthiness problems with improved contact algorithms for implicit time integration
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

in International Journal of Impact Engineering (2006), 32(5), 799-825

When studying crashworthiness problems, contact simulation can be the source of a number of problems. A first one is the discontinuities in the normal evolution for a boundary discretized by finite ... [more ▼]

When studying crashworthiness problems, contact simulation can be the source of a number of problems. A first one is the discontinuities in the normal evolution for a boundary discretized by finite elements. Another problem is the treatment of the contact forces that can introduce numerical energy in the system. In this paper, we propose to combine a method of discontinuity smoothing with the energy-momentum consistent scheme that recently appeared in the literature. (c) 2005 Elsevier Ltd. All rights reserved. [less ▲]

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See detailAn energy momentum conserving algorithm using the variational formulation of visco-plastic updates
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

in International Journal for Numerical Methods in Engineering (2006), 65(6), 904942

In this paper we use the variational formulation of elasto-plastic updates proposed by Ortiz and Stainier (Comput. Methods Appl. Mech. Eng. 1999; 171:419-444) in the context of consistent time integration ... [more ▼]

In this paper we use the variational formulation of elasto-plastic updates proposed by Ortiz and Stainier (Comput. Methods Appl. Mech. Eng. 1999; 171:419-444) in the context of consistent time integration schemes. We show that such a formulation is well suited to obtain a general expression Of energy momentum conserving algorithms. Moreover, we present numerical examples that illustrate the efficiency of our developments. Copyright (c) 2005 John Wiley [less ▲]

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See detailEnergy conserving balance of explicit time steps to combine implicit and explicit algorithms in structural dynamics
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

in Computer Methods in Applied Mechanics & Engineering (2006), 195(19-22), 21692192

Recent developments have proved the advantage of combining both time implicit and time explicit integration algorithms in structural dynamics. A major problem is to define the initial conditions for the ... [more ▼]

Recent developments have proved the advantage of combining both time implicit and time explicit integration algorithms in structural dynamics. A major problem is to define the initial conditions for the implicit simulation on the basis of a solution obtained from an unbalanced explicit resolution. The unbalanced nature of the explicit algorithm leads to oscillations in the fields of interest. Therefore, the values obtained after an explicit computation cannot be used directly as initial conditions for an implicit simulation. In this paper, we develop such initial values that lead to a stable (no numerical creation of energy) and energy-conserving transition. (c) 2005 Elsevier B.V. All rights reserved. [less ▲]

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See detailNonlinear compressibility effects in fluid-structure interaction and their implications on the air-blast loading of structures
Kambouchev, Nayden; Noels, Ludovic ULg; Radovitzky, Raúl

in Journal of Applied Physics (2006), 100(6), 063519

The impulse imparted by a blast wave to a freestanding solid plate is studied analytically and numerically focusing on the case in which nonlinear compressibility effects in the fluid are important, as is ... [more ▼]

The impulse imparted by a blast wave to a freestanding solid plate is studied analytically and numerically focusing on the case in which nonlinear compressibility effects in the fluid are important, as is the case for explosions in air. The analysis furnishes, in effect, an extension of Taylor's pioneering contribution to the understanding of the influence of fluid-structure interaction (FSI) on the blast loading of structures [The Scientific Papers of Sir Geoffrey Ingram Taylor, edited by G. K. Batchelor (Cambridge University Press, Cambridge, 1963), Vol. III, pp. 287-303] to the nonlinear range. The limiting cases of extremely heavy and extremely light plates are explored analytically for arbitrary blast intensity, from where it is concluded that a modified nondimensional parameter representing the mass of compressed fluid relative to the mass of the plate governs the FSI. The intermediate asymptotic FSI regime is studied using a numerical method based on a Lagrangian formulation of the Euler equations of compressible flow and conventional shock-capturing techniques. Based on the analytical and numerical results, an approximate formula describing the entire range of relevant FSI conditions is proposed. The main conclusion of this work is that nonlinear fluid compressibility further enhances the beneficial effects of FSI in reducing the impulse transmitted to the structure. More specifically, it is found that transmitted impulse reductions due to FSI when compared to those obtained ignoring FSI effects are more significant than in the acoustic limit. This result can be advantageously exploited in the design and optimization of structures with increased blast resistance. (c) 2006 American Institute of Physics. [less ▲]

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See detailA general discontinuous Galerkin method for finite hyperelasticity. Formulation and numerical applications
Noels, Ludovic ULg; Radovitzky, Raúl

in International Journal for Numerical Methods in Engineering (2006), 68(1), 6497

A discontinuous Galerkin formulation of the boundary value problem of finite-deformation elasticity is presented. The primary purpose is to establish a discontinuous Galerkin framework for large ... [more ▼]

A discontinuous Galerkin formulation of the boundary value problem of finite-deformation elasticity is presented. The primary purpose is to establish a discontinuous Galerkin framework for large deformations of solids in the context of statics and simple material behaviour with a view toward further developments involving behaviour or models where the DG concept can show its superiority compared to the continuous formulation. The method is based on a general Hu-Washizu-de Veubeke functional allowing for displacement and stress discontinuities in the domain interior. It is shown that this approach naturally leads to the formulation of average stress fluxes at interelement boundaries in a finite element implementation. The consistency and linearized stability of the method in the non-linear range as well as its convergence rate are proven. An implementation in three dimensions is developed, showing that the proposed method can be integrated into conventional finite element codes in a straightforward manner. In order to demonstrate the versatility, accuracy and robustness of the method examples of application and convergence studies in three dimensions are provided. Copyright (c) 2006 John Wiley [less ▲]

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See detailA virtual test facility for the efficient simulation of solid material response under strong shock and detonation wave loading
Deiterding, Ralf; Radovitzky, Raúl; Mauch, Sean et al

in Engineering With Computers (2006), 22(3-4), 325347

A virtual test facility (VTF) for studying the three-dimensional dynamic response of solid materials subject to strong shock and detonation waves has been constructed as part of the research program of ... [more ▼]

A virtual test facility (VTF) for studying the three-dimensional dynamic response of solid materials subject to strong shock and detonation waves has been constructed as part of the research program of the Center for Simulating the Dynamic Response of Materials at the California Institute of Technology. The compressible fluid flow is simulated with a Cartesian finite volume method and treating the solid as an embedded moving body, while a Lagrangian finite element scheme is employed to describe the structural response to the hydrodynamic pressure loading. A temporal splitting method is applied to update the position and velocity of the boundary between time steps. The boundary is represented implicitly in the fluid solver with a level set function that is constructed on-the-fly from the unstructured solid surface mesh. Block-structured mesh adaptation with time step refinement in the fluid allows for the efficient consideration of disparate fluid and solid time scales. We detail the design of the employed object-oriented mesh refinement framework AMROC and outline its effective extension for fluid-structure interaction problems. Further, we describe the parallelization of the most important algorithmic components for distributed memory machines and discuss the applied partitioning strategies. As computational examples for typical VTF applications, we present the dynamic deformation of a tantalum cylinder due to the detonation of an interior solid explosive and the impact of an explosion-induced shock wave on a multi-material soft tissue body. [less ▲]

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See detailLagrangian simulations of penetration environments via mesh healing and adaptative optimisation
Mauch, Sean; Noels, Ludovic ULg; Zhao, Zizu et al

(2006)

In this work, we demonstrate the feasibility of fully-Lagrangian finite element simulations of the mechanics of three-dimensional penetration environments. The key enabling component is a robust library ... [more ▼]

In this work, we demonstrate the feasibility of fully-Lagrangian finite element simulations of the mechanics of three-dimensional penetration environments. The key enabling component is a robust library informed with state-of-the-art algorithms for mesh healing and optimization, which is repeatedly used during the simulations to eliminate deformation-induced mesh distortion and to maintain the quality of the numerical solution. The computational strategy effectively avoids the need to resort to artifacts such as element deletion or conversion to meshless particles which have been proposed to eliminate the issue of mesh distortion. The effectiveness of the computational strategy is demonstrated in a simulation of deep oblique penetration of a spherical-nosed steel rod on an aluminum target. [less ▲]

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See detailA new discontinuous Galerkin method for non-linear Mechanics
Noels, Ludovic ULg; Radovitzky, Raúl

(2006)

A discontinuous Galerkin framework for large deformations of solids is established. The method is based on a general Hu-Washizu-de Veubeke functional allowing for displacement and stress discontinuities ... [more ▼]

A discontinuous Galerkin framework for large deformations of solids is established. The method is based on a general Hu-Washizu-de Veubeke functional allowing for displacement and stress discontinuities in the domain interior. The focus of this paper is on the treatment of large elasto-plastic deformations. The method is shown to possess the required numerical properties: consistency in the non-linear range and linearized stability. In order to demonstrate the versatility, accuracy and robustness of the method examples of application and convergence studies in three dimensions are provided. Further developments of the method will lead to applications where physical discontinuities must be modeled like fragmentation or failure. [less ▲]

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See detailSimulation of complex impact problems with implicit time algorithms: Application to a turbo-engine blade loss problem
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

in International Journal of Impact Engineering (2005), 32(1-4), 358386

Recent developments, in non-linear structural dynamics, have led to a new kind of implicit algorithms: the energy-momentum conserving algorithm (EMCA) and the energy-dissipative, momentum-conserving ... [more ▼]

Recent developments, in non-linear structural dynamics, have led to a new kind of implicit algorithms: the energy-momentum conserving algorithm (EMCA) and the energy-dissipative, momentum-conserving algorithm. Contrarily to commonly used algorithms, such as the explicit central difference or the alpha-generalized method, the stability of those algorithms is always ensured in the non-linear range. Thanks to this unconditional stability the only requirement on the time step size is that it must be small enough to capture the physics. This requirement is less restrictive than a conditional stability. In previous works, we have developed a new formulation of the internal forces for a hypoelastic model, that leads to an EMCA. In this paper, we will extend this formulation to an energy-dissipative, momentum-conserving algorithm. We will prove with an academic example, that our algorithm is more accurate than the alpha-generalized method in the non-linear range. Then we will simulate a blade loss problem to demonstrate the efficiency of our developments on complex dynamics simulations. (c) 2005 Elsevier Ltd. All rights reserved. [less ▲]

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