On computing the jump condition of the dissipation rate in the two-equation turbulence models for two-phase flow and application to air-water waves

Traditional turbulence models are derived for single-phase flow. Extension of the family of two-equation turbulence models for two-phase flow is obtained via scaling the transport equations by the density. In the special case of two-phase flow with a sharp interface, jump conditions exist. Two types of jump conditions are found: (1) jump in the partial differential equation (PDE) physical quantities such as density and viscosity and (2) jump in the turbulence frequency. We first derive and clarify the jump in the equations. The jump in the turbulence frequency is proportional to the kinematic viscosity ratio, which is approximately $10$ in the case of air-water. Then a new field, the inverse turbulence area, is considered to model the turbulence effects instead of the turbulence frequency. For the system of air and water, the effect of the jump of the kinematic viscosity is always greater than the effect arising from the jump of velocity gradient. This approximation leads to the assumption of a continuous inverse turbulence area scale. Validation versus experimental measurements from the literature is then presented to demonstrate the improvement of the model. In particular, the wave breaking phenomenon is simulated in two conditions: spilling and plunging wave breakers. The proposed model shows its ability to predict the turbulence in the surf zone accurately. Finally, it explains the low values of the time-averaged turbulent kinetic energy in the surf zone which is caused by the increase of the turbulence frequency in the air

Theoretical Analysis of SPH in Simulating Free-surface Viscous flows

A theoretical analysis on the performance, close to a free surface, of the most used SPH formulations for Newtonian viscous terms is carried out in this paper. After an introduction of the SPH formalism, the SPH expressions for the viscous term in the momentum equation are analyzed in their continuous form. Using a Taylor expansion, a reformulation of those expressions is undertaken which allows to characterize the behavior of the viscous term close to the free surface. Under specific flow conditions, we show that the viscous term close to the free surface is singular when the spatial resolution is increased. This problem is in essence related to the incompleteness of the kernel function close to the free surface and appears for all the formulations considered. In order to assess the impact of such singular behavior, an analysis of the global energy dissipation is carried out, which shows that such a free-surface singularity vanishes when the integral quantities are considered. Not with standing that, not all the SPH viscous formulas allow the correct evaluation of the energy dissipation rate and, consequently, they may lead to an inaccurate modelling of viscous free-surface flows

The suction effect during freak wave slamming on a fixed platform deck: Smoothed particle hydrodynamics simulation and experimental study

During the process of wave slamming on a structure with sharp corners, the wave receding after wave impingement can induce strong negative pressure (relative to the atmospheric pressure) at the bottom of the structure, which is called the suction effect. From the practical point of view, the suction force induced by the negative pressure, coinciding with the gravity force, pulls the structure down and hence increases the risk of structural damage. In this work, the smoothed particle hydrodynamics (SPH) method, more specifically the δ+SPH model, is adopted to simulate the freak wave slamming on a fixed platform with the consideration of the suction effect, i.e., negative pressure, which is a challenging issue because it can cause the so-called tensile instability in SPH simulations. The key to overcome the numerical issue is to use a numerical technique named tensile instability control (TIC). Comparative studies using SPH models with and without TIC will show the importance of this technique in capturing the negative pressure. It is also found that using a two-phase simulation that takes the air phase into account is essential for an SPH model to accurately predict the impact pressure during the initial slamming stage. The freak wave impacts with different water depths are studied. All the multiphase SPH results are validated by our experimental data. The wave kinematics/dynamics and wave impact features in the wave-structure interacting process are discussed, and the mechanism of the suction effect characterized by the negative pressure is carefully analyzed

Stratégie de couplage faible entre les méthodes FEM et SPH pour les simulations d'IFS

Un couplage bi-dimensionnel pour l'Interaction Fluide-Structure est proposé entre la méthode Smoothed Particles Hydrodynamics (SPH) pour le fluide et la méthode Eléments Finis (EF) pour le solide. Avec ce couplage nous prenons des avantages dans les deux méthodes, à savoir la capacité de la méthode SPH de prendre en compte de grandes déformations du domaine fluide et la capacité éprouvée de prédiction du comportement des solides sous chargement instationnaire de pression de la méthode EF. De plus aucun algorithme spécifique n'est requis à l'interface solide-fluide pour éviter l'interpénétration des matériaux. Tout ceci conduit à une implémentation relativement aisée du couplage. Des cas de validation de ce couplage seront présentés. En particulier la conservation totale de l'énergie à travers le couplage sera soigneusement suivie et analysée, démontrant la validité et la précision de ce couplage totalement explicite. Ainsi les différentes énergies seront exprimées et suivies au cours du temps, à la fois pour la méthode SPH pour le fluide et pour la méthode EF pour le solide. La somme de ces différentes énergies devant être constante dans le temps en absence de dissipation. De bonnes propriétés de convergence par rapport à la conservation de cette énergie totale ont été observées et seront présentées. Les validations du couplage SPH-EF seront présentées en détail, en comparaison de résultats analytiques et expérimentaux. Enfin le modèle sera appliqué à un cas réaliste complexe où les effets 3D ne peuvent être négligés

SPH High-Performance Computing simulations of rigid solids impacting the free-surface of water

Numerical simulations of water entries based on a three-dimensional parallelized Smoothed Particle Hydrodynamics (SPH) model developed by Ecole Centrale Nantes are presented. The aim of the paper is to show how such SPH simulations of complex 3D problems involving a free surface can be performed on a super computer like the IBM Blue Gene/L with 8,192 cores of Ecole polytechnique fédérale de Lausanne. The present paper thus presents the different techniques which had to be included into the SPH model to make possible such simulations. Memory handling, in particular, is a quite subtle issue because of constraints due to the use of a variable-h scheme. These improvements made possible the simulation of test cases involving hundreds of million particles computed by using thousands of cores. Speedup and efficiency of these parallel calculations are studied. The model capabilities are illustrated in the paper for two water entry problems, firstly, on a simple test case involving a sphere impacting the free surface at high velocity; and secondly, on a complex 3D geometry involving a ship hull impacting the free surface in forced motion. Sensitivity to spatial resolution is investigated as well in the case of the sphere water entry, and the flow analysis is performed by comparing both experimental and theoretical reference results

High performance computing 3D SPH model: Sphere impacting the free-surface of water

In this work, an analysis based on a three-dimensional parallelized SPH model developed by ECN and applied to free surface impact simulations is presented. The aim of this work is to show that SPH simulations can be performed on huge computer as EPFL IBM Blue Gene/L with 8'192 cores. This paper presents improvements concerning namely the memory consumption, which remains quite subtle because of the variable-H scheme constraints. These improvements have made possible the simulation of test cases involving tens of millions of particles computed by using more than thousand cores. Furthermore, pv-meshless developed by CSCS, is used to show the pressure field and the effect of impact

Theoretical analysis and numerical verification of the consistency of viscous smoothed-particle-hydrodynamics formulations in simulating free-surface flows

The theoretical formulation of the smoothed particle hydrodynamics (SPH) method deserves great care because of some inconsistencies occurring when considering free-surface inviscid flows. Actually, in SPH formulations one usually assumes that (i) surface integral terms on the boundary of the interpolation kernel support are neglected, (ii) free-surface conditions are implicitly verified. These assumptions are studied in detail in the present work for free-surface Newtonian viscous flow. The consistency of classical viscous weakly compressible SPH formulations is investigated. In particular, the principle of virtual work is used to study the verification of the free-surface boundary conditions in a weak sense. The latter can be related to the global energy dissipation induced by the viscous term formulations and their consistency. Numerical verification of this theoretical analysis is provided on three free-surface test cases including a standing wave, with the three viscous term formulations investigated

Simulation numérique d'éolien offshore

Depuis quelques années, la demande en électricité renouvelable a augmenté significativement. Dans ce contexte, les filières de production d'énergies renouvelables se sont rapidement développées. Dans le même temps, l'éolien a atteint un niveau de maturité tel que les parcs éoliens onshore et offshore posés se sont multipliés. Aujourd'hui, la recherche de vents plus forts et plus constants poussent les acteurs du domaine à se tourner vers le développement de parcs éoliens flottants. Les coûts associés à la réalisation de telles machines sont encore élevés et doivent être optimisés. Un des leviers pour la réduction des coûts est la modélisation numérique. Le développement d'outils numériques permettant une prédiction fine du comportement de ces structures en mer va permettre une meilleure prise en compte des différents chargements mécaniques. L'accès à des résultats précis va tendre à réduire les coefficients de sécurité liés au dimensionnement de ces éoliennes, et ainsi contribuer à la réduction des coûts de CAPEX. Ce travail concerne le développement d'une méthodologie pour la simulation directe de plusieurs éoliennes flottantes, avec une modélisation exacte et précise de ses composantes (par exemple, ses pales). La base logicielle utilisée est la bibliothèque ICI-Tech, développée au sein de l'Institut de Calcul Intensif (ICI) de l'Ecole Centrale de Nantes. Une approche monolithique est utilisée, avec un unique maillage dans la simulation, où les différentes interfaces sont définies par des fonctions de phase. La résolution des équations de Navier-Stokes est alors faite à l'aide d'éléments finis stabilisés, en utilisant le formalisme Variational Multi-Scale (VMS). Pour réduire grandement les coûts de calcul usuellement requis pour modéliser précisément des éoliennes, où des phénomènes d'ordres de grandeurs très diverses sont observés, une procédure d'adaptation de maillage anisotrope permet d'obtenir des mailles de taille variable et adaptées aux phénomènes observés partout dans le domaine de calcul. Finalement, les premiers résultats d'immersion de maillage et d'écoulements autour de l'éolienne sont présentés

Guidelines for the use and interpretation of assays for monitoring autophagy (4th edition)1.

In 2008, we published the first set of guidelines for standardizing research in autophagy. Since then, this topic has received increasing attention, and many scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Thus, it is important to formulate on a regular basis updated guidelines for monitoring autophagy in different organisms. Despite numerous reviews, there continues to be confusion regarding acceptable methods to evaluate autophagy, especially in multicellular eukaryotes. Here, we present a set of guidelines for investigators to select and interpret methods to examine autophagy and related processes, and for reviewers to provide realistic and reasonable critiques of reports that are focused on these processes. These guidelines are not meant to be a dogmatic set of rules, because the appropriateness of any assay largely depends on the question being asked and the system being used. Moreover, no individual assay is perfect for every situation, calling for the use of multiple techniques to properly monitor autophagy in each experimental setting. Finally, several core components of the autophagy machinery have been implicated in distinct autophagic processes (canonical and noncanonical autophagy), implying that genetic approaches to block autophagy should rely on targeting two or more autophagy-related genes that ideally participate in distinct steps of the pathway. Along similar lines, because multiple proteins involved in autophagy also regulate other cellular pathways including apoptosis, not all of them can be used as a specific marker for bona fide autophagic responses. Here, we critically discuss current methods of assessing autophagy and the information they can, or cannot, provide. Our ultimate goal is to encourage intellectual and technical innovation in the field