Simulation of fully resolved finite-size particles in a turbulent flow

In this study we are interested on the statistics of finite sized particles. A sustained homogeneous isotropic turbulence with particles is numerically simulated in order to obtain those statistics

Wake instabilities behind an axisymmetric bluff body at low Reynolds numbers

This paper aims at understanding the dynamical process that leads to the onset of chaos in the flow past a blunt based axisymmetric bluff body. On the ba- sis of direct numerical simulations, conducted for Reynolds numbers ranging from 100 to 800, we show that the flow undergoes multiple transitions, successively giv- ing rise to the SS, RSPa, RSPb and RSPc wake states. In particular, the RSPc state, revealed in this work via long-term computations, is characterized by intermittent vortex stretching denoting the onset of chaos and the potential occurence of a third instability that superimposes to the first and second instability associated with state RSPa and RSPb respectively. Interestingly, the reflectional symmetry plane that characterizes the RSP states is still retained. Hence, chaos is triggered before the symmetry breaking and the occurence of the RSB state observed at higher Reynolds numbers

Toward direct numerical simulation of high speed droplet impact

International audienceWhen a liquid drop impacts a solid surface or a liquid film, several outcomes are possible. In particular, splashing phenomena can exhibit complex behaviors, like the formation of very thin crowns and emission of small droplets. Underlying mechanisms are hard to elucidate, partly due to the difficulty for current experimental devices to access the very small length scales involved. Here, we use direct numerical simulation to explore low and high velocity drop impact phenomena. We show that classical incompressible two-phase methods can be sufficient to address low energy impacts and take into account wetting phenomena. However, dedicated robust and conservative methods are needed to simulate splashing phenomenon at higher velocities. In our test cases, we show that an impact on a thick liquid film exhibits thick crown formation and delayed splashing. On a dry wall, on the other hand, splashing phenomenon can be difficult to reproduce even with high velocity impacts. We show however how a higher value of the surrounding air density may trigger splashing. The presence of a very thin liquid film on the wall strongly modifies impact outcome, forcing the ejection of a thin crown and subsequent secondary droplet emission

Direct numerical simulation of particles dispersion in supersonic shear layers

Particles dispersion plays an important role in many industrial applications such as combustion, pollution control and also in experimental measurements like Laser Doppler Velocimetry. In this last case, particles are supposed to have the same behavior as fluid particles in order to give relevance to the experimental measure. However it has been shown (Jacquin et al. 1991) that noticeable errors can appear in the rms velocity measurement of supersonic jet or shear layer, even if care has been taken concerning particle seeding of the flow. The aim of this paper is to use direct numerical simulation of particle-gas flow to investigate this phenomenon

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

Étude de stabilité et simulation numérique de l'écoulement interne des moteurs à propergol solide simplifiés

Cette thèse vise à modéliser les instabilités hydrodynamiques générant des détachements tourbillonnaires pariétaux (ou VSP) responsables des Oscillations De Pression dans les moteurs à propergol solide longs et segmentés par interaction avec l acoustique du moteur. Ces instabilités sont modélisées en tant que modes de stabilité linéaire globaux de l écoulement d un conduit à parois débitantes. En supposant que les structures pariétales émergent d une perturbation de l écoulement de base, des modes discrets et indépendants du maillage utilisé sont calculés. Dans ce but, une discrétisation par collocation spectrale multi-domaine est implémentée dans un solveur parallèle afin de s affranchir de la croissance polynomiale des fonctions propres et de la présence de couches limites. Les valeurs propres ainsi calculées dépendent explicitement des frontières du domaine, à savoir la position de la perturbation et celle de la sortie, et sont ensuite validées par simulation numérique directe. On montre alors qu elles permettent bien de décrire la réponse à une perturbation initiale de l écoulement modifié par une rupture de débit pariétale. Ensuite, la simulation d une réponse forcée par l acoustique se fait sous forme de structures tourbillonnaires dont les fréquences discrètes sont en accord avec celles des modes de stabilité. Ces structures sont réfléchies en ondes de pression de même fréquences remontant l écoulement. Finalement, la simulation numérique et la théorie de la stabilité permettent de montrer que le VSP, dont la réponse est linéaire vis-à-vis d un forçage compressible comme l acoustique, est le phénomène moteur des Oscillations De Pression.The current work deals with the modeling of the hydrodynamic instabilities that play a major role in the triggering of the Pressure Oscillations occurring in large segmented solid rocket motors. These instabilities are responsible for the emergence of Parietal Vortex Shedding (PVS) and they interact with the boosters acoustics. They are first modeled as eigenmodes of the internal steady flowfield of a cylindrical duct with sidewall injection within the global linear stability theory framework. Assuming that the related parietal structures emerge from a baseflow disturbance, discrete meshindependant eigenmodes are computed. In this purpose, a multi-domain spectral collocation technique is implemented in a parallel solver to tackle numerical issues such as the eigenfunctions polynomial axial amplification and the existence of boundary layers. The resulting eigenvalues explicitly depend on the location of the boundaries, namely those of the baseflow disturbance and the duct exit, and are then validated by performing Direct Numerical Simulations. First, they successfully describe flow response to an initial disturbance with sidewall velocity injection break. Then, the simulated forced response to acoustics consists in vortical structures wihich discrete frequencies that are in good agreement with those of the eigenmodes. These structures are reflected into upstream pressure waves with identical frequencies. Finally, the PVS, which response to a compressible forcing such as the acoustic one is linear, is understood as the driving phenomenon of the Pressure Oscillations thanks to both numerical simulation and stability theory.TOULOUSE-ISAE (315552318) / SudocSudocFranceF

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

Simulation numérique des écoulements de suspension à l'aide d'une méthode de pénalisation des équations de Navier-Stokes

Une méthode de simulation des écoulements particulaires avec des particules de taille finie est proposée. Il s'agit d'une méthode à un fluide pour laquelle la partie solide est représentée par une viscosité très élevée. Le calcul se réalise sur maillage cartésien décalé. Les particules sont repérées de façon lagrangienne. Cette méthode de domaines fictifs à comme particularité que le déplacement de particules se fait implicitement par la résolution de Navier-Stokes. Malgré le rapport de viscosités, la méthode du lagrangien augmenté converge tout en gardant la contrainte d'incompressibilité

Particle resolved direct numerical simulation of a liquid–solid fluidized bed: Comparison with experimental data

Particle-resolved direct numerical simulations of a 3-D liquid–solid fluidized bed experimentally investigated by Aguilar-Corona (2008) have been performed at different fluidization velocities (corresponding to a range of bed solid volume fraction between 0.1 and 0.4), using Implicit Tensorial Penalty Method. Particle Reynolds number and Stokes number are O(100) and O(10), respectively. In this paper, we compare the statistical quantities computed from numerical results with the experimental data obtained with 3-D trajectography and High Frequency PIV. Fluidization law predicted by the numerical simulations is in very good agreement with the experimental curve and the main features of trajectories and Lagrangian velocity signal of the particles are well reproduced by the simulations. The evolution of particle and flow velocity variances as a function of bed solid volume fraction is also well captured by the simulations. In particular, the numerical simulations predict the right level of anisotropy of the dispersed phase fluctuations and its independence of bed solid volume fraction. They also confirm the high value of the ratio between the fluid and the particle phase fluctuating kinetic energy. A quick analysis suggests that the fluid velocity fluctuations are mainly driven by fluid–particle wake interactions (pseudo-turbulence) whereas the particle velocity fluctuations derive essentially from the large scale flow motion (recirculation). Lagrangian autocorrelation function of particle fluctuating velocity exhibits large-scale oscillations, which are not observed in the corresponding experimental curves, a difference probably due to a statistical averaging effect. Evolution as a function of the bed solid volume fraction and the collision frequency based upon transverse component of particle kinetic energy correctly matches the experimental trend and is well fitted by a theoretical expression derived from Kinetic Theory of Granular Flows