33 research outputs found
Particle Finite Element Method for simulations of Selective Laser Melting with vaporization
editorial reviewedThe purpose of this work is the simulation of selective laser melting processes.
Such processes involve multiple physical phenomena that need to be taken into
account altogether such as thermo-mechanical coupling, solid-liquid-solid
phase change, surface tension and vaporization. The variety of different physical
phenomena, as well as the presence of a highly deformed fluid free surface,
implies multiple constraints on the required numerical procedure. Notably, the
need to compute the free surface position and curvature leads to complex
interface tracking algorithms in the widely-used Eulerian-based models.
The Particle Finite Element Method (PFEM), a Lagrangian method with fast
triangulation and boundary identification algorithms, has been chosen to
overcome some of the difficulties mentioned previously. A new version of the
2D/3D PFEM code presented in (S. Février, “Development of a 3D
Compressible Flow Solver for PFEM Fluid Simulations”, ULiège Master
Thesis, 2020) has been developed to take into account the aforementioned
physical phenomena, notably Marangoni forces and recoil pressure, and the
interactions with a laser.
Alongside the presentation of the mathematical formulation and the description
of its numerical implementation, some simulations involving a moving laser
melting a block of material are presented and discusse
Particle Finite Element Method for simulations of Selective Laser Melting with vaporization
editorial reviewedThe purpose of this work is the simulation of selective laser melting processes. Such pro-
cesses involve multiple physical phenomena that need to be taken into account altogether
such as thermo-mechanical coupling, solid-liquid-solid phase change, surface tension and
vaporization [Cook et al., 2020]. The variety of different physical phenomena, as well as the presence of a
highly deformed fluid free surface, implies multiple constraints on the required numerical
procedure. Notably, the need to compute the free surface position and curvature leads to
complex interface tracking algorithms in the widely-used Eulerian-based models [Chen, 2018]. The
Particle Finite Element Method (PFEM), a Lagrangian method with fast triangulation
and boundary identification algorithms, has been chosen to overcome some of the diffi-
culties mentioned previously [FĂ©vrier, 2020]. A new version of the 2D/3D PFEM code presented in
[FĂ©vrier, 2020 ; Cerquaglia 2019] has been developed to take into account the aforementioned physical phenomena,
notably Marangoni forces and recoil pressure, and the interactions with a laser. Alongside
the presentation of the mathematical formulation and the description of its numerical im-
plementation, some simulations involving a moving laser melting a block of material are
presented and discussed
Identification of Cryptic MHC I–restricted Epitopes Encoded by HIV-1 Alternative Reading Frames
Human immunodeficiency virus (HIV) 1 major histocompatibility complex (MHC) I–restricted epitopes are widely believed to be derived from viral proteins encoded by primary open reading frames. However, the HIV-1 genome contains alternative reading frames (ARFs) potentially encoding small polypeptides. We have identified a panel of epitopes encoded by ARFs within the gag, pol, and env genes. The corresponding epitopic peptides were immunogenic in mice humanized for MHC-I molecules. In addition, cytotoxic T lymphocytes recognizing these epitopes were found in HIV-infected patients. These results reveal the existence of atypical mechanisms of HIV-1 epitope generation. They indicate that the repertoire of epitopes recognized by the cellular anti–HIV-1 immune response is broader than initially thought. This should be taken into account when designing vaccine strategies aimed at activating these responses
Particle Finite Element Method for 2D/3D Fluid-Structure Interactions, Including Contact Interactions
editorial reviewedThe present work focuses on the simulation of 2D and 3D Fluid-Structure Interaction (FSI)
problems involving fluid free surfaces, large deformations of solids including plasticity
and solid-solid contact mechanics, incompressible and weakly compressible flows. The
considered approach is the partitioned coupling between two independent solvers, one for
the fluid and one for the solid. The two solvers are precompiled and wrapped in Python
objects that are called by the coupling algorithm using the Dirichlet-Neumann paradigm. In particular, the new fluid solver uses a Particle Finite Element Method (PFEM), an
adaptive mesh-based Lagrangian algorithm allowing a straightforward tracking of the free
surface and deformation of the fluid domain. The solid solver uses a nonlinear finite
element algorithm with an updated Lagrangian formalism. In this work, the codes
previously developed in have been extended for performing 3D FSI simulations.
In addition, we enable a faster resolution of existing 2D problems as well as new test
cases featuring an additional complexity. Notably, one of our main challenges is the
numerical efficiency of the algorithms, where both the quality of remeshing procedure
and the execution time of the codes are key factors for the reliability and accuracy of the
solution. In addition to a discussion about the concepts presented above, this presentation
will include, but will not be limited to, some examples and comparisons between 2D/3D
FSI simulations such as the displacement and deformation of solid walls due an incident fluid flow or solid-solid contacts between debris in a pipe.Particle Finite Element Method for Fluid-Structure Interaction
Particle Finite Element Method (PFEM) for 2D/3D Fluid-Structure Interactions, Including Contact Interactions
editorial reviewedFluid-Structure Interaction (FSI) is a multiphysics coupling that aims to describe both fluid dynamics and structural mechanics. The present work is an extension of prior developments where a 2D PFEM solver has been developed and coupled with the solid solver Metafor using the Dirichlet-Neumann paradigm for simulating FSI problems involving fluid free surfaces, large deformations of solids and plasticity. During this conference, we present the recent advances in FSI coupling by the introduction of a new in-house PFEM solver extended to 3D incompressible and weakly compressible flows, allowing the simulation of a larger range of complex problems. Notably, the presence of structural contacts within the fluid domain as well as frequent fluid detachments require a particular attention to be paid to the quality of the remeshing and the conservation of mass, which are two crucial elements governing the reliability and accuracy of the solution. In addition to further increasing the importance of the previous factors, the transition to 3D brings new challenges in terms of performance. Indeed, the computation time also becomes a critical element of the problem. The presentation starts with a brief introduction to the basic principles of the PFEM and the Dirichlet-Neumann partitioned coupling, followed by a discussion about the remeshing procedure and the performances of the codes. Finally, this work will include, but will not be limited to, some examples and comparisons between 2D/3D FSI simulations such as the displacement and deformation of solid walls due an incident fluid flow, frictional contact of solids within the fluid domain or a flow-driven motion of debris in a pipe.Particle Finite Element Method for Fluid-Structure Interaction
Generalized-alpha scheme in the PFEM for velocity-pressure and displacement-pressure formulations of the incompressible Navier-Stokes equations
peer reviewedDespite the increasing use of the Particle Finite Element Method (PFEM) in fluid flow simulation and the outstanding success of the Generalized-alpha time integration method, very little discussion has been devoted to their combined performance. This work aims to contribute in this regard by addressing three main aspects. Firstly, it includes a detailed implementation analysis of the Generalized-alpha method in PFEM. The work recognizes and compares different implementation approaches from the literature, which differ mainly in the terms that are alpha-interpolated (state variables or forces of momentum equation) and the type of treatment for the pressure in the time integration scheme. Secondly, the work compares the performance of the Generalized-alpha method against the Backward Euler and Newmark schemes for the solution of the incompressible Navier-Stokes equations. Thirdly, the study is enriched by considering not only the classical velocity-pressure formulation but also the displacement-pressure formulation that is gaining interest in the fluid-structure interaction field. The work is carried out using various 2D and 3D benchmark problems such as the fluid sloshing, the solitary wave propagation, the flow around a cylinder, and the collapse of a cylindrical water column.ALFEWEL
Comparison of Interpolation Algorithms on Non-Matching Meshes for Partitioned Thermo-Mechanical Fluid-Structure Interactions
editorial reviewedFluid-Structure Interaction (FSI) aims to describe multiphysics problems where both fluid dynamics and structural mechanics are involved. The present work focuses on the partitioned coupling between a structural solver based on the Finite Element Method (FEM) and a fluid solver based on the Particle Finite Element Method (PFEM) in order to simulate thermo-mechanical FSI involving free surface flows and large deformations of the domain. The coupling is performed by transferring nodal information (such as the heat flux, the mechanical load, the nodal temperature and the nodal displacement) between the two solvers, under the form of Neumann-Dirichlet boundary conditions imposed at the fluid-structure interface. In many applications, this partitioned scheme also implies non-conforming meshes. For instance, when the fluid and the solid meshes contain elements of different characteristic sizes at their common interface. Consequently, a mesh-interpolation technique is required for the transmission of nodal information. In this work, the so-called Radial Basis Functions (RBF) and K-Nearest Neighbours (KNN) interpolation techniques are compared on 2D and 3D test cases. Both are fast and flexible techniques requiring no topological information other than relative distances between nodes, allowing for a straightforward interpolation between non-conforming meshes. Moreover, Element Transfer Methods (ETM) are also considered. The presentation starts with a brief introduction to the basic principles of the PFEM and the Neumann-Dirichlet partitioned coupling, followed by a discussion regarding the mesh-interpolation techniques. Finally, this work includes, but is not limited to, some examples and comparisons of results with respect to the literature.Particle Finite Element Method for Fluid-Structure Interaction
A new remeshing strategy relying on level-set functions for the Particle Finite Element Method
peer reviewedSince the seminal work of Idelsohn, Oñate and del-Pin (2004), the remeshing process of the Particle Finite Element Method (PFEM) has relied on a Delaunay triangulation (DT) followed by the Alpha--Shape (AS) algorithm. This DT+AS procedure guarantees a good quality of the Lagrangian mesh and allows modelling the merging and splitting of bodies, as in the simulation of free-surface flows. However, the remeshing procedure creates and removes elements during the merging or splitting of bodies, which modifies the mass of the system. In the literature, this issue has been addressed by mesh refinement strategies or by adjusting the parameter ruling the AS algorithm.
The AS algorithm computes, for each element in the DT, a parameter that is representative of the size and distortion of the element, and compares it to a user-defined value. If the parameter is greater than the imposed threshold, then the element is removed from the DT. Differently, in this work we propose a new DT filtering criterion that resorts to a Level-Set (LS) function instead of the Alpha--Shape algorithm. The proposal maps the topology of the domain before the remeshing process using a LS function, where its sign indicates the inner or outer zone of the discretised body, while its magnitude gives an approximation of the distance to the body boundaries. The proposed criterion accepts the elements of the DT if they are inside the body, or very close to the body boundaries. Therefore, the criterion is information-enriched since it considers not only a geometrical aspect but also a topological feature.
The new meshing strategy proposed for PFEM is assessed using benchmark problems for the simulation of free--surface flows, fluid--structure interactions, and phase change, both in 2D and 3D. The results indicate that, at the expense of increased computational time, LS allows a substantial decrease in the mass variation during the remeshing process. In addition, it preserves the smoothness of the free surface and avoids numerical artifacts that are inherent to the AS-based procedure
Overview of the TCV tokamak experimental programme
The tokamak a configuration variable (TCV) continues to leverage its unique shaping capabilities, flexible heating systems and modern control system to address critical issues in preparation for ITER and a fusion power plant. For the 2019-20 campaign its configurational flexibility has been enhanced with the installation of removable divertor gas baffles, its diagnostic capabilities with an extensive set of upgrades and its heating systems with new dual frequency gyrotrons. The gas baffles reduce coupling between the divertor and the main chamber and allow for detailed investigations on the role of fuelling in general and, together with upgraded boundary diagnostics, test divertor and edge models in particular. The increased heating capabilities broaden the operational regime to include T (e)/T (i) similar to 1 and have stimulated refocussing studies from L-mode to H-mode across a range of research topics. ITER baseline parameters were reached in type-I ELMy H-modes and alternative regimes with \u27small\u27 (or no) ELMs explored. Most prominently, negative triangularity was investigated in detail and confirmed as an attractive scenario with H-mode level core confinement but an L-mode edge. Emphasis was also placed on control, where an increased number of observers, actuators and control solutions became available and are now integrated into a generic control framework as will be needed in future devices. The quantity and quality of results of the 2019-20 TCV campaign are a testament to its successful integration within the European research effort alongside a vibrant domestic programme and international collaborations
Application of the Generalized-alpha time integration scheme in PFEM for solving the incompressible Navier-Stokes equations
peer reviewedDespite the increasing use of the Particle Finite Element Method (PFEM) in fluid flow simulation and the outstanding success of the Generalized-alpha time integration method, very little discussion has been devoted to the performance of both methods. This work contributes in this regard by presenting a detailed implementation that combines these methods. In addition, the Generalized-alpha is compared against the conventional time integration schemes of the PFEM literature, i.e., the Backward Euler and Newmark.
The implementation is developed for the incompressible Navier-Stokes equations and a monolithic PFEM formulation stabilized with the Pressure-Stabilizing Petrov-Galerkin (PSPG) method. To extend the analysis, equations are developed for velocity-pressure and displacement-pressure based formulations. The study is carried out using four benchmarks, the flow around a rigid cylinder, sloshing of water in a tank, single wave propagation and dam break problems.
The work shows that different implementation approaches are possible for the Generalized-alpha method, which differ mainly in the terms that are alpha-interpolated (state variables or equilibrium forces) and in whether or not the pressure is considered in the time integration scheme . Whatever the approach, the Generalized-alpha scheme demonstrates superiority in agreement with the observations of the literature, i.e., it does not suffer from excessive numerical dumping for large time steps and exhibits less spurious oscillations that the compared schemes