Preferential concentration of inertial sub-kolmogorov particles. The roles of mass loading of particles, Stokes and Reynolds numbers

Turbulent flows laden with inertial particles present multiple open questions and are a subject of great interest in current research. Due to their higher density compared to the carrier fluid, inertial particles tend to form high concentration regions, i.e. clusters, and low concentration regions, i.e. voids, due to the interaction with the turbulence. In this work, we present an experimental investigation of the clustering phenomenon of heavy sub-Kolmogorov particles in homogeneous isotropic turbulent flows. Three control parameters have been varied over significant ranges: $Re_{\lambda} \in [170 - 450]$, $St\in [0.1 - 5]$ and volume fraction $\phi_v\in [2\times 10^{-6} - 2\times 10^{-5}]$. The scaling of clustering characteristics, such as the distribution of Vorono\"i areas and the dimensions of cluster and void regions, with the three parameters are discussed. In particular, for the polydispersed size distributions considered here, clustering is found to be enhanced strongly (quasi-linearly) by $Re_{\lambda}$ and noticeably (with a square-root dependency) with $\phi_v$, while the cluster and void sizes, scaled with the Kolmogorov lengthscale $\eta$, are driven primarily by $Re_{\lambda}$. Cluster length $\sqrt{\langle A_c \rangle}$ scales up to $\approx 100 {\eta}$, measured at the highest $Re_{\lambda}$, while void length $\sqrt{\langle A_v \rangle}$ scaled also with $\eta$ is typically two times larger ($\approx 200 {\eta}$). The lack of sensitivity of the above characteristics to the Stokes number lends support to the "sweep-stick" particle accumulation scenario. The non-negligible influence of the volume fraction, however, is not considered by that model and can be connected with collective effects

Microstructure Clustering in Multiphase Materials: Effect of Initial Configuration

In disperse multiphase systems, the dispersion/distribution of particles (microstructure) is regarded as a key factor, both affected by processing and in its turn determining performance. Among many miscrostructural features, clustering, that is, the tendency of dispersed particles to agglomerate forming clusters of differnt size, is considered of primary significance. Our purpose in this study is both, to understand the evolution of clustering generated by Monte-Carlo procedure with the change of the initial configuration of the particles...

Microstructure Clustering in Multiphase Materials: The role of dimensionless temperature and surface fraction

The significant development and intensified use of composite materials (reinforced plastics, extruded materials and mechanically blended thermoplastics) over the last 30 years has provided the impetus for intense research on their processability as well as on the durability and properties of the final products. In disperse multiphase systems, the dispersion/distribution of particles (microstructure) is regarded as a key factor, both affected by processing and in its turn determining performance

Microstructure Clustering in Multiphase Materials: Particle size effect

The significant development and intensified use of composite materials (reinforced plastics, extruded materials and mechanically blended thermoplastics) over the last 30 years has provided the impetus for intense research on their processability as well as on the durability and properties of the final products. In disperse multiphase systems, the dispersion/distribution of particles (microstructure) is regarded as a key factor, both affected by processing and in its turn determining performance..

Computational Analysis of Transport Across Flake-Filled Composites of Realistic Microstructure

In this paper we present the results of a computational study of diffusion across disordered flake composites in which the flakes are misaligned with respect to the direction of bulk diffusion. We evaluate the effect of flake orientation as well as the influence of boundary conditions and unit-cell types on the predicted barrier properties. Flake orientation impacts very significantly on the barrier properties in flake-filled composites, and usually the key objective in their fabrication is to orient them as close as possible to being perpendicular to the direction of macroscopic diffusion. Our computations are carried out in two-dimensional, doubly-periodic unit cells, each containing up to 3000 individual flake cross-sections. We consider high aspect ratio (alpha) systems with alpha=1000, from the dilute (alpha phi=0.01) and into the very concentrated (alpha phi=40) regime. The effective diffusivity of the corresponding unit cells is computed from the imposed concentration difference and the computed mass flux, using Fick's Law. We show that use of cyclic boundary conditions and doubly-periodic unit cells results in effective diffusivities which are in agreement with theory and invariant of the shape of the unit cell. We also show that the use of adiabatic boundary conditions produces erroneous results at high flake concentrations. Finally we compare our results to the predictions of existing literature models and find that the latter deviate significantly from computation at high flake concentrations

Concentration préférentielle de particules inertielles : la structure et la dynamique de clusters

Cette thèse étudie les phénomènes de concentration préférentielle et de sédimentation de particules inertielles transportées dans un écoulement turbulent. Pour cela, des expériences ont été menées en soufflerie dans une turbulence engendrée en aval d’une grille active et ensemencée avec des gouttelettes d'eau. La concentration préférentielle se manifeste par la ségrégation spatiale des particules qui bien qu’initialement ensemencée de façon homogène, tendent à se regrouper en amas, laissant en déplétion d’autres zones de l’écoulement. Un effort particulier a été consacré à séparer les mécanismes liés à l’inertie des particules, à la turbulence et aux effets collectifs impactant la formation des amas et modifiant la vitesse de sédimentation des particules. Quatre principaux paramètres non-dimensionnels ont été variés afin d’établir le rôle spécifique de chacun d’entre eux sur les processus de concentration préférentielle et de sédimentation : le nombre de Rouse Ro , représentant le rapport de la vitesse de sédimentation des particules à la vitesse fluctuante de l’écoulement; le nombre de Stokes St, quantifiant l'inertie des particules comme le rapport entre le temps de réponse des particules et le temps dissipatif de l’écoulement; le nombre de Reynolds RE lambda représentant le degré de turbulence et enfin la fraction volumique de la phase dispersée phi v.Deux techniques expérimentales (suivi Lagrangien des particules et interférométrie à phase Doppler) ont été utilisées pour l'acquisition des données et pour le diagnostic de la concentration préférentielle et de la sédimentation des gouttelettes dispersées. Le suivi Lagrangien de particules a été réalisé par visualisation à haute vitesse cadence des gouttelettes dispersées dans une nappe de laser. Cela donne accès aux statistiques simultanées de la distribution spatiale des particules et de leur vitesse. La niveau de clustering a été quantifié à l’aide de tessélation de Voronoï. Nous établissons des lois d’échelles quantitatives caractérisant la dépendance du degré de clustering et de la géométrie des amas en fonction des paramètres de l’étude (St, Re lambda et phi v. Ces lois d’échelles indiquent une forte influence de Re lambda et de phi v, mais un faible effet de St. Ce résultat est cohérent avec un rôle dominant du mécanisme « sweep-stick » comme origine de la concentration préférentielle, tel que proposé par Vassilicos. En outre, l'analyse conditionnelle des vitesses de sédimentation des particules en fonction de leur appartenance ou non à des amas montre que les zones à fortes concentration tendent à sédimenter plus rapidement que les zones peu concentrées, suggérant un possible rôle des effets collectifs dans l’augmentation de la vitesse de chute. Les mesures par interférométrie de phase Doppler ont ensuite permis d’analyser plus en détail les statistiques de vitesse et de concentration de particules conditionnées à la taille des particules. Ces mesures montrent une augmentation de la vitesse de sédimentation pour les particules de petits diamètres, en accord avec des études précédentes. En revanche, la sédimentation est ralentie pour les particules de plus grand diamètre. Ceci indique une subtile intrication de plusieurs mécanismes possibles affectant la sédimentation turbulente de particules.This PhD thesis investigates the phenomena of preferential concentration and settling of sub-Kolmogorov inertial particles transported in a turbulent flow. To this end, experiments have been carried out in active-grid-generated turbulence in a wind-tunnel, seeded with water droplets. Preferential concentration manifests itself as the emergence of spatial segregation of the particles, which where initially homogeneously seeded in the carrier flow, leading to clusters and voids. A particular effort has been put in disentangling the roles of particles inertia, of turbulence and of collective effects on the emergence of clustering and the modification of settling velocity and in investigating the interplay between clustering and settling. Four main non-dimensional parameters have been varied to establish the role of each in the clustering process and on the settling of the particles: the Rouse number Ro, representing the ratio of the settling velocity of the particles to the fluctuating velocity of the fluid ; the Stokes number St , quantifying particle inertia as the ratio of the particle response time to the flow dissipative time scale ; the Reynolds number Re lambda representing the degree of turbulence and the volume fraction phi v representing the concentration of the particles in the two-phase flow.Two experimental techniques (Lagrangian Particle Tracking and Phase Doppler Interferometry) are used to acquire data and diagnose the clustering and settling properties of the dispersed droplets.2D-Lagrangian Particle Tracking has been performed using high-speed visualization of the dispersed droplets in a laser sheet. This gives access to simultaneous statistics of particles spatial distribution and velocity. Clustering has been quantified using Voronoï tessellation and quantitative scalings on the dependency of clustering intensity and clusters dimensions on St, Re lambda and phi v are found. They show a strong influence of Re lambda and volume fraction phi v but a weak effect of St. This finding is consistent with a leading role of the “sweep-stick” mechanism in the clustering process, as proposed by Vassilicos. Furthermore, conditional analysis of the velocities of particles within clusters and voids has been performed showing that clusters tend to settle faster than voids, pointing to the role of collective effects in the enhancement of settling.Phase Doppler Interferometry has then been used to further analyse velocity statistics, and particle concentration field conditioned on particle diameter. Enhancement of the settling velocity for small diameters is observed, in agreement with previous studies. On the contrary, for larger particles settling velocity is found to be hindered. This indicates a subtle intrication of several possible mechanisms affecting the settling, including preferential sweeping, loitering and collective effects

Clustering of inertial sub-Kolmogorov particles : structure of clusters and their dynamics

This PhD thesis investigates the phenomena of preferential concentration and settling of sub-Kolmogorov inertial particles transported in a turbulent flow. To this end, experiments have been carried out in active-grid-generated turbulence in a wind-tunnel, seeded with water droplets. Preferential concentration manifests itself as the emergence of spatial segregation of the particles, which where initially homogeneously seeded in the carrier flow, leading to clusters and voids. A particular effort has been put in disentangling the roles of particles inertia, of turbulence and of collective effects on the emergence of clustering and the modification of settling velocity and in investigating the interplay between clustering and settling. Four main non-dimensional parameters have been varied to establish the role of each in the clustering process and on the settling of the particles: the Rouse number Ro, representing the ratio of the settling velocity of the particles to the fluctuating velocity of the fluid ; the Stokes number St , quantifying particle inertia as the ratio of the particle response time to the flow dissipative time scale ; the Reynolds number Re lambda representing the degree of turbulence and the volume fraction phi v representing the concentration of the particles in the two-phase flow.Two experimental techniques (Lagrangian Particle Tracking and Phase Doppler Interferometry) are used to acquire data and diagnose the clustering and settling properties of the dispersed droplets.2D-Lagrangian Particle Tracking has been performed using high-speed visualization of the dispersed droplets in a laser sheet. This gives access to simultaneous statistics of particles spatial distribution and velocity. Clustering has been quantified using Voronoï tessellation and quantitative scalings on the dependency of clustering intensity and clusters dimensions on St, Re lambda and phi v are found. They show a strong influence of Re lambda and volume fraction phi v but a weak effect of St. This finding is consistent with a leading role of the “sweep-stick” mechanism in the clustering process, as proposed by Vassilicos. Furthermore, conditional analysis of the velocities of particles within clusters and voids has been performed showing that clusters tend to settle faster than voids, pointing to the role of collective effects in the enhancement of settling.Phase Doppler Interferometry has then been used to further analyse velocity statistics, and particle concentration field conditioned on particle diameter. Enhancement of the settling velocity for small diameters is observed, in agreement with previous studies. On the contrary, for larger particles settling velocity is found to be hindered. This indicates a subtle intrication of several possible mechanisms affecting the settling, including preferential sweeping, loitering and collective effects.Cette thèse étudie les phénomènes de concentration préférentielle et de sédimentation de particules inertielles transportées dans un écoulement turbulent. Pour cela, des expériences ont été menées en soufflerie dans une turbulence engendrée en aval d’une grille active et ensemencée avec des gouttelettes d'eau. La concentration préférentielle se manifeste par la ségrégation spatiale des particules qui bien qu’initialement ensemencée de façon homogène, tendent à se regrouper en amas, laissant en déplétion d’autres zones de l’écoulement. Un effort particulier a été consacré à séparer les mécanismes liés à l’inertie des particules, à la turbulence et aux effets collectifs impactant la formation des amas et modifiant la vitesse de sédimentation des particules. Quatre principaux paramètres non-dimensionnels ont été variés afin d’établir le rôle spécifique de chacun d’entre eux sur les processus de concentration préférentielle et de sédimentation : le nombre de Rouse Ro , représentant le rapport de la vitesse de sédimentation des particules à la vitesse fluctuante de l’écoulement; le nombre de Stokes St, quantifiant l'inertie des particules comme le rapport entre le temps de réponse des particules et le temps dissipatif de l’écoulement; le nombre de Reynolds RE lambda représentant le degré de turbulence et enfin la fraction volumique de la phase dispersée phi v.Deux techniques expérimentales (suivi Lagrangien des particules et interférométrie à phase Doppler) ont été utilisées pour l'acquisition des données et pour le diagnostic de la concentration préférentielle et de la sédimentation des gouttelettes dispersées. Le suivi Lagrangien de particules a été réalisé par visualisation à haute vitesse cadence des gouttelettes dispersées dans une nappe de laser. Cela donne accès aux statistiques simultanées de la distribution spatiale des particules et de leur vitesse. La niveau de clustering a été quantifié à l’aide de tessélation de Voronoï. Nous établissons des lois d’échelles quantitatives caractérisant la dépendance du degré de clustering et de la géométrie des amas en fonction des paramètres de l’étude (St, Re lambda et phi v. Ces lois d’échelles indiquent une forte influence de Re lambda et de phi v, mais un faible effet de St. Ce résultat est cohérent avec un rôle dominant du mécanisme « sweep-stick » comme origine de la concentration préférentielle, tel que proposé par Vassilicos. En outre, l'analyse conditionnelle des vitesses de sédimentation des particules en fonction de leur appartenance ou non à des amas montre que les zones à fortes concentration tendent à sédimenter plus rapidement que les zones peu concentrées, suggérant un possible rôle des effets collectifs dans l’augmentation de la vitesse de chute. Les mesures par interférométrie de phase Doppler ont ensuite permis d’analyser plus en détail les statistiques de vitesse et de concentration de particules conditionnées à la taille des particules. Ces mesures montrent une augmentation de la vitesse de sédimentation pour les particules de petits diamètres, en accord avec des études précédentes. En revanche, la sédimentation est ralentie pour les particules de plus grand diamètre. Ceci indique une subtile intrication de plusieurs mécanismes possibles affectant la sédimentation turbulente de particules

Microstructure Clustering in Multiphase Materials: The role of dimensionless temperature and surface fraction

The significant development and intensified use of composite materials (reinforced plastics, extruded materials and mechanically blended thermoplastics) over the last 30 years has provided the impetus for intense research on their processability as well as on the durability and properties of the final products. In disperse multiphase systems, the dispersion/distribution of particles (microstructure) is regarded as a key factor, both affected by processing and in its turn determining performance

Microstructure Clustering in Multiphase Materials: Effect of Initial Configuration

In disperse multiphase systems, the dispersion/distribution of particles (microstructure) is regarded as a key factor, both affected by processing and in its turn determining performance. Among many miscrostructural features, clustering, that is, the tendency of dispersed particles to agglomerate forming clusters of differnt size, is considered of primary significance. Our purpose in this study is both, to understand the evolution of clustering generated by Monte-Carlo procedure with the change of the initial configuration of the particles...

Enhancement versus hindering: impact of the turbulence characteristics on the settling behavior of heavy sub-Kolmogorov particles in a turbulent flow

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