Clustering of inertial sub-Kolmogorov particles : Structure of clusters and their dynamics
Supervisors
M. Alain Cartellier, directeur de rechercher au CNRS
M. Mickael Bourgoin, directeur de rechercher au CNRS (ENS Lyon, précédemment LEGI)
Abstract
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 were 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 representing the degree of turbulence and the volume fraction Phi 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 Re and Phi are found. They show a strong influence of Re, and volume fraction Phi 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.