Coupled Level set moment of fluid method for simulating multiphase flows

International audienceA coupled level set moment of fluid (CLSMOF) method for numerical simulation of multiphase flows is presented in this paper. This numerical method of liquid/gas interface capture is a hybrid of classical moment of fluid (MOF) method and coupled level set volume of fluid (CLSVOF) method. In this CLSMOF method, MOF interface reconstruction is used only for the under-resolved liquid structures while the level set function is used for the interface reconstruction for the resolved structures. This method combines the advantages of accurate capture of under-resolved liquid strucutres from MOF method and sharp interface representation by the level set function. The results presented in this paper demonstrates the ability and accuracy of the CLSMOF method to be as high as that of the MOF method while incurring relatively less computational expense. Finally, the application of CLSMOF method to simulation of turbulent diesel jet yeilded a very satisfactory volume conservation

A 3D Moment of Fluid method for simulating complex turbulent multiphase flows

International audienceThis paper presents the moment of fluid method as a liquid/gas interface reconstruction method coupled with a mass momentum conservative approach within the context of numerical simulations of incompressible two-phase flows. This method tracks both liquid volume fraction and phase centroid for reconstructing the interface. The interface reconstruction is performed in a volume (and mass) conservative manner and accuracy of orientation of interface is ensured by minimizing the centroid distance between original and reconstructed interface. With two-phase flows, moment of fluid method is able to reconstruct interface without needing phase volume data from neighboring cells. The performance of this method is analyzed through various transport and deformation tests, and through simple two-phase flows tests that encounter changes in the interface topologies. Exhaustive mesh convergence study for the reconstruction error has been performed through various transport and deformation tests involving simple two-phase flows. It is then applied to simulate atomization of turbulent liquid diesel jet injected into a quiescent environment. The volume conservation error for the moment of fluid method remains small for this complex turbulent case

A comparative study of DNS of airblast atomization using CLSMOF and CLSVOF methods

International audienceThe results from direct numerical simulations (DNS) of planar pre-filming airblast atomization are presented in this paper. The configuration of the airblast atomization is inspired from a published experimental configuration of Gepperth et al (2012, "Ligament and Droplet Characteristics in Prefilming Airblast Atomization", ICLASS 2012). The simulations have been performed using our in-house Navier-Stokes solver ARCHER. Two DNS have been performed each respectively using coupled level moment of fluid (CLSMOF) and coupled level set volume of fluid (CLSVOF) methods for liquid/gas interface reconstruction. The operating point investigated in the simulations correspond to aircraft altitude relight conditions. The DNS data are post-processed consistent to that of the experimental data to extract droplet and ligament statistics. The droplet diameter distribution from the simulations is found to be having satisfactory agreement with the experimental data. Two breakup mechanisms of atomization are observed: sheet breakup producing small droplets and ligament breakup producing medium and bulgy droplets. The CLSMOF method is observed to produce more medium and bulgy droplets owing to dominant ligament breakup while CLSVOF method produced more number of small droplets owing to predominant sheet breakup mechanism. A good agreement was found between simulations and experiments for Sauter Mean Diameter (SMD) of the droplets. The droplet diameter distribution from the simulations are found to under-predict the peak of the distribution but displays similar profile as that of the experiments. The droplet velocity distribution from the simulations is found to agree well with that of the experiments. The liquid ligaments formed at the trailing edge of the pre-filmer plate are characterized by their lengths. The breakup length of the ligaments, defined as arithmetic mean of the ligament lengths, computed from the simulations agree satisfactorily with the value computed from the experimental data

An investigation of characteristics of airblast atomization using 3D DNS for altitude relight conditions

International audienceThis paper presents results from direct numerical simulations (DNS) of planar pre-filming airblast atomization. The liquid/gas interface has been captured using coupled level set moment of fluid method. This method is a hybrid between moment of fluid and coupled level set volume of fluid methods. The numerical method has been applied to airblast atomization analyzed experimentally by Gepperth et al. (2012, "Ligament and Droplet Characteristics in Prefilming Air-blast Atomization", ICLASS 2012). The operating point investigated in this work correspond to aircraft altitude relight condition. The post-processing of the DNS data is performed consistently with that for the experimental data. The main mode of breakup observed is torn sheet breakup. A good agreement was found between simulations and experiments for Sauter Mean Diameter and droplet streamwise velocity distribution while satisfactory agreement has been found for the droplet diameter distribution and ligament breakup length

A CONSISTENT MASS AND MOMENTUM FLUX COMPUTATION METHOD USING RUDMAN-TYPE TECHNIQUE WITH A CLSVOF SOLVER

ABSTRACT In this paper, a computational method is presented that addresses the problem of multiphase flow characterized by phases with significant density ratio accompanied by strong shearing. The Coupled Level-Set Volume-of-Fluid (CLSVOF) technique is used for interface tracking, while the momentum transfer is coupled to that of mass by means of momentum fluxes computed using a sub-grid. This is an extended adaptation of Rudman's volume tracking techniqu

DNS and LES of primary atomization of turbulent liquid jet injection into a gaseous crossflow environment

International audienceIn this paper, we study the primary atomization characteristics of liquid jet injected into a gaseous crossow through direct numerical simulations (DNS) and large eddy simulations (LES). The DNS use a coupled level set volume of uid (CLSVOF) sharp interface capturing method resolving all relevant scales to predict the drop size distribution (DSD) for drops larger than the grid spacing. The LES use a volume of uid (VOF) diused interface method modelling the sub grid droplets. The purpose of this paper is to provide a comparison of the results of drop data between DNS and LES. The simulations are performed for a liquid jet injection with liquid-gas momentum ux ratio of 6.6, liquid jet Reynolds number of 14,000 injected into a crossowing air with Reynolds number 570,000 and Weber number of 330 at a liquid-togas density ratio of 10. Two distinct and simultaneous atomization/breakup mechanisms have been observed in the simulations: column/bag breakup and ligament/surface breakup. It was found that the DSDs obtained from the DNS and LES each follow a log-normal distribution based on their respective droplet diameter data. An overlap region exists between the individual DSDs from the DNS and LES when combined. The width of this overlap region decreases along the downstream direction. A log-normal distribution is found to be a good t to the combined DSD incorporating both resolved and sub-grid droplets. This information is relevant for the secondary atomization simulations and modeling

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/)

Application of a level set method for simulation of droplet collisions

Tanguy, S Berlemont, AA level set technique for interface tracking is presented, both for the continuum surface force formulation and the ghost fluid method approach. A projection method is used to solve incompressible Navier-Stokes equations that are coupled to a transport equation for the level set function, defined as the algebraic distance to interface. Results are presented for head-on droplet collisions in coalescence and reflexive regimes with a 2D axi-symmetric code, and for an off-center droplet collision in a separation regime for a large impact parameter with 3D code. Simulations provided realistic and various droplet collision behaviors and they correspond to experimental observations. (c) 2005 Elsevier Ltd. All rights reserved

Eulerian and Lagrangian approaches for predicting the behaviour of discrete particles in turbulent flows

Gouesbet, G Berlemont, AA review of the work carried out during the last two decades by a group in Rouen, on Eulerian and Lagrangian approaches for predicting the behaviour of discrete particles in turbulent flows, is presented. The opportunity of this review is taken to direct the reader to a much larger literature and to point out unsolved problems. (C) 1999 Elsevier Science Ltd. All rights reserved

Développement d une méthode Level Set pour le suivi d interface. Application de la rupture de jet liquide

Ce mémoire présente des développements d une simulation numérique directe pour l étude de l atomisation des jets liquides. Une méthode Level Set, pour décrire l interface, est couplée à la méthode Ghost Fluid, pour prendre en compte les discontinuités à sa traversée. Cette approche est appliquée à l étude des jets liquides dans le régime de Rayleigh. L accord entre les simulations et différents résultats théoriques et expérimentaux montre l intérêt d une telle approche. Cependant, la méthode Level Set présente comme principale contrainte de ne pas être conservative en masse et voit son domaine d application limité à l étude des jets à faible vitesse. Le couplage VOF/Level Set est ici développé et permet de palier ce défaut. Ce couplage est testé dans des configurations académiques afin d être validé et de lui apporter différentes améliorations. Le code calcul est alors appliqué à une étude sur l atomisation d un jet liquide haute vitesse. Des données statistiques sont extraites afin d aider à la modélisation de l atomisation primaire où des études ont été menées en collaboration avec des équipes développant de tels outils. Des simulations sont extraits des séquences d images montrant différents mécanismes de formation de gouttes montrant la diversité et la complexité de ces mécanismes dans les conditions étudiées. Les limitations et les apports d une telle approche sont finalement discutés.This thesis presents new developments of Direct Numerical Simulation for the study of liquid jet atomization. Interface tracking is performed by a Level Set method, and the Ghost Fluid Method is used to capture accurately sharp discontinuties. It is first applied on the study of the destabilisation of liquid jet in the Rayleigh regime. Numerical simulation results show an encouraging agreement with theoretical and experimental results. However, the main drawback of the Level Set method is a possible mass loss, and it restricts the application of the method to low speed jets. A coupled VOF/Level Set is thus developped to ensure mass conservation. This method is applied at test cases to be validated and improved. A specific study is then devoted to the atomization of high speed jets. Simulations are carried out to create a data base of statistical variables and improve closure relations in atomization modelling. Atomization mechanisms and droplet formation are detailed by image analysing of simulation which show the diversity and complexity of these mechanisms in this condition. Finally advantage and drawback of the method are discussed.ROUEN-BU Sciences (764512102) / SudocROUEN-BU Sciences Madrillet (765752101) / SudocSudocFranceF