Local dissipation properties and collision dynamics in a sustained homogeneous turbulent suspension composed of finite size particles

Particulate flows are present in many applications and the effect of particle size is still not well under- stood. The present paper describes three cases of sustained homogeneous turbulence interacting with particles. Simulations correspond to three particle-fluid density ratios and 3% volume fraction in zero gravity field. Fully resolved particle simulations are based on fictitious domain and penalty method. The local dissipation around particles is studied according to the density ratio. Spatial description of the averaged dissipation is provided. Collision statistics are also investigated. The inter collision time and the angle of collision are compared to the kinetic theory. The effect of the inter particle film drainage is highlighted by simulating the same configurations with and without lubrication model

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

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 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

Simulation des écoulements turbulents avec des particules de taille finie en régime dense

Un grand nombre d'écoulements naturels et industriels mettent en jeu des particules (sédimentation,lit fluidisé, sprays...). Les écoulements chargés en particules sont bien décrits numériquement sous l'hypothèse des particules plus petites que toutes les échelles de l'écoulement. Cette thèse consiste à simuler numériquement une turbulence homogène et isotrope soutenue chargée en particules dont la taille est supérieure à l'échelle de Kolmogorov. Pour se faire une méthode de simulation a été développée au sein du code Thétis puis validée. L'originalité de cette méthode consiste en l'utilisation d'une approche de pénalisation associée à la viscosité dans la zone solide. Les particules sont transportées de façon lagrangienne. Les principaux résultats concernent trois simulations faisant varier le rapport de densité entre le fluide et le solide. Chaque simulation simule le mouvement de 512particules avec un diamètre 22 fois plus grand que l'échelle spatiale de Kolmogorov remplissant ainsi3% du volume total. La dispersion des particules est étudiée et montre des comportements comparables à ceux observés pour des particules ponctuelles. Un intérêt particulier est porté sur le régime collisionnel. On observe que la corrélation des vitesses avec le fluide environnant réduit le nombre de chocs frontaux par rapport au cas théorique de particules d'un gaz dense. L'effet de la prise en compte du fluide visqueux entre les particules (couche de lubrification) lors de la collision a été étudiée. L'écoulement moyen à l'échelle des particules est aussi analysé, mettant en évidence l'existence d'une couche de dissipation autour des particules.Many applications and natural environment flows make use of particles (sedimentation, fluidized bed,sprays...). Particle laden flows are described correctly by numerical methods when the particles are smaller than all other spatial scales of the flow. This thesis involves the numerical simulation of a particle laden sustained homogeneous isotropic turbulence whose particle's size is larger than the Kolmogovov spatial scale. A numerical method has been developed and validated in the numerical code Thetis. The novelty of this method is the viscosity penalization approach. The particles are tracked by a Lagrangian way. The main results obtained are related to three simulations where the density ratio between the solid and the fluid varies. Each simulation reproduces the movement of 512particles whose diameter is 22 times the Kolmogorov spatial scale (3% volumetric solid fraction).The dispersion of particles is studied and has similar behavior than those observed with point particles simulations. The collision regime is also investigated. It is shown that he number of frontal collision is lower than its estimate for kinetic theory of gazes because there is a correlation between the particles velocity and the surrounding fluid. The modification of the collision regime when the lubrication film between particles at collision is taken into account is studied. Finally, the averaged flow around particles is analyzed and shows that there is a dissipation layer around particles.TOULOUSE-ISAE (315552318) / SudocSudocFranceF

Droplet oscillations in a turbulent flow

The oscillations of an initially unperturbed spherical droplet immersed in a homogeneous and isotropic turbulent background flow are investigated through spherical harmonic decomposition. As suggested in the literature, the shape oscillations under turbulent conditions are related to the frequency of droplets oscillating in a fluid without background flow. A series of direct numerical simulations (DNS) of droplets with single deformation modes in a fluid at rest are first performed. The frequency and damping rate are compared with weakly viscous linear theory. Then, a database of 220 droplets deformed under turbulent conditions for a single Weber and Reynolds number is generated with an identical numerical set-up. Each spherical harmonic coefficient shows an oscillatory motion with comparable frequency to the single deformation mode simulations. The power spectrum of the coefficients provides the amount of surface of each mode. After a transient regime, the surface area reaches a stationary saturation level. The saturation level of each mode is linked to the turbulence and the energy stored at the interface. Droplets after a high deformation are studied with and without background flow. As expected, the physics of relaxation is driven by capillary forces

Simulation of immiscible two-phase flows based on a kinetic diffuse interface approach

International audienceA direct numerical simulation (DNS) code is developed to simulate immiscible two-phase flows based on the recently developed discrete unified gas-kinetic scheme (DUGKS). This scheme simulates hydrodynamic equations of the quasi-incompressible Cahn-Hilliard-Navier-Stokes system by the use of two mesoscopic distributions and the proper design of their equilibrium distributions and source terms. Several immiscible two-phase flows are used to validate the scheme in both 2D and 3D, including a stationary droplet in 2D and 3D, the Rayleigh-Taylor flows, and two-phase homogeneous isotropic decaying turbulence. The results obtained by DUGKS are compared carefully to these from the literature and the ARCHER code, i.e., a Coupled Level Set-Volume of Fluid (CLSVOF) method. The comparisons indicate that DUGKS is a promising scheme for direct numerical simulations of immiscible two-phase flows

Subgrid Liquid Flux and interface modelling for LES of Atomization

[EN] Traditional Discrete Particle Methods (DPM) such as the Euler-Lagrange approaches for modelling atomization, even if widely used in technical literature, are not suitable in the near injector region. Indeed, the first step of atomization process is to separate the continuous liquid phase in a set of individual liquid parcels, the so-called primary break-up. Describing two-phase flow by DPM is to define a carrier phase and a discrete phase, hence they cannot be used for primary breakup. On the other hand, full scale simulations (direct simulation of the dynamic DNS, and interface capturing method ICM) are powerful numerical tools to study atomization, however, computational costs limit their application to academic cases for understanding and complementing partial experimental data. In an industrial environment, models that are computationally cheap and still accurate enough are required to meet new challenges of fuel consumption and pollutant reduction. Application of DNS-ICM methods without fairly enough resolution to solve all length scales are currently used for industrial purpose. Nevertheless, effects of unresolved scales are generally cast aside. The Euler-Lagrange Spray Atomization model family (namely, ELSA, also call, Σ − �� or Ω − ��) developed by Vallet and Borghi pioneering work [1], and [2], at the contrary aims to model those unresolved terms. This approach is actually complementary to DNS-ICM method since the importance of the unresolved term depends directly on mesh resolution. For full interface resolution the unclosed terms are negligible, except in the far-field spray when the unresolved terms become dominant. Depending on the complexity of the flow and the available computational resources, a Large Eddy Simulation (LES) formalism could be employed as modelling approach. This work focus on the two main terms that drive these different modelling approaches namely the subgrid turbulent liquid flux and the resolved interface. Thanks to the open source library OpenFoam® this work is an attempt to review and to release an adapted modelling strategy depending on the available mesh resolution. For validation, these solvers are tested against realistic experimental data to see the overall effect of each model proposal.This work was partly supported European Union’s Horizon 2020 research and innovation program under the Sklodowska-Curie grant agreement No. 675676. Simulations were carried out at TGCC (The Curie supercomputer, owned by GENCI and operated into the TGCC by CEA), and at CRIHAN (Centre de Ressources de Haute Normandie).Anez, J.; Ahmed, A.; Puggelli, S.; Reveillon, J.; Brändle De Motta, JC.; Demoulin, F. (2017). Subgrid Liquid Flux and interface modelling for LES of Atomization. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 385-393. https://doi.org/10.4995/ILASS2017.2017.4694OCS38539

Numerical simulation of particle laden turbulent flows with finite-sized particles in dense regime

Un grand nombre d'écoulements naturels et industriels mettent en jeu des particules (sédimentation,lit fluidisé, sprays...). Les écoulements chargés en particules sont bien décrits numériquement sous l'hypothèse des particules plus petites que toutes les échelles de l'écoulement. Cette thèse consiste à simuler numériquement une turbulence homogène et isotrope soutenue chargée en particules dont la taille est supérieure à l'échelle de Kolmogorov. Pour se faire une méthode de simulation a été développée au sein du code Thétis puis validée. L'originalité de cette méthode consiste en l'utilisation d'une approche de pénalisation associée à la viscosité dans la zone solide. Les particules sont transportées de façon lagrangienne. Les principaux résultats concernent trois simulations faisant varier le rapport de densité entre le fluide et le solide. Chaque simulation simule le mouvement de 512particules avec un diamètre 22 fois plus grand que l'échelle spatiale de Kolmogorov remplissant ainsi3% du volume total. La dispersion des particules est étudiée et montre des comportements comparables à ceux observés pour des particules ponctuelles. Un intérêt particulier est porté sur le régime collisionnel. On observe que la corrélation des vitesses avec le fluide environnant réduit le nombre de chocs frontaux par rapport au cas théorique de particules d'un gaz dense. L'effet de la prise en compte du fluide visqueux entre les particules (couche de lubrification) lors de la collision a été étudiée. L'écoulement moyen à l'échelle des particules est aussi analysé, mettant en évidence l'existence d'une couche de dissipation autour des particules.Many applications and natural environment flows make use of particles (sedimentation, fluidized bed,sprays...). Particle laden flows are described correctly by numerical methods when the particles are smaller than all other spatial scales of the flow. This thesis involves the numerical simulation of a particle laden sustained homogeneous isotropic turbulence whose particle's size is larger than the Kolmogovov spatial scale. A numerical method has been developed and validated in the numerical code Thetis. The novelty of this method is the viscosity penalization approach. The particles are tracked by a Lagrangian way. The main results obtained are related to three simulations where the density ratio between the solid and the fluid varies. Each simulation reproduces the movement of 512particles whose diameter is 22 times the Kolmogorov spatial scale (3% volumetric solid fraction).The dispersion of particles is studied and has similar behavior than those observed with point particles simulations. The collision regime is also investigated. It is shown that he number of frontal collision is lower than its estimate for kinetic theory of gazes because there is a correlation between the particles velocity and the surrounding fluid. The modification of the collision regime when the lubrication film between particles at collision is taken into account is studied. Finally, the averaged flow around particles is analyzed and shows that there is a dissipation layer around particles