Learning by Research

Thesis Title: Inertial Particle Dynamics in the Turbulent/Non-Turbulent Interface

The vast majority of natural fluid flows, whether they occur in the atmosphere, the ocean, or rivers, are turbulent. Turbulence is characterized by eddies, vortices, and rapid fluctuations in fluid velocity. Due to their random nature, these flows are very complex to study and model. Moreover, many turbulent flows in the environment involve a second phase of matter, such as atmospheric clouds advecting water droplets. This dispersed phase does not exactly follow the fluid motions. On the other hand, the inclusions have their own trajectories, due to their inertia caused by their size or density. They are known as inertial particles and interact in a complex manner with turbulence.

Over the past few decades, most studies on particle/turbulence interactions have focused on the simplified case of homogeneous and isotropic turbulence. However, many natural flows exhibit non-homogeneous turbulence with a turbulent/non-turbulent interface. A turbulent interface refers to a region of the fluid separating two flows with different turbulent states. These interfaces are ubiquitous in nature but have been scarcely studied when loaded with inertial particles. This thesis aims to understand the dynamics of inertial particles when they are advected by a turbulent interface.

During wind tunnel experiments, we created a turbulent interface and introduced water droplets only on the highly turbulent side of the interface. The dynamics of these inertial particles are then modified by the presence of the interface compared to the case of homogeneous turbulence. We particularly analyzed the impact of the interface on the particle settling velocity and their ability to aggregate into dense groups, known as clusters. We found that droplets at the interface form large and dense clusters and experience a significant increase in their settling velocity.
The results obtained can be applied to processes occurring at the boundaries of atmospheric clouds. Indeed, in the case of cumulus clouds, the interior of the cloud, laden with water droplets, is more turbulent than the surrounding dry air. These results are important because understanding the mechanisms of air entrainment at cloud boundaries, even at small scales, has repercussions on cloud macroscopic properties and development. In the long term, these results could allow for improved precision in meteorological and climate models regarding these sub-grid processes.

Key Words: Turbulence, experimental study, inertial particles, turbulent/non-turbulent interface

Doctoral School: ED I-MEP² - Ingénierie - Matériaux, Mécanique, Environnement, Énergétique, Procédés, Production
Research laboratory: Laboratoire des écoulements géophysiques et industriels (LEGI - CNRS, Grenoble INP-UGA, UGA)
Thesis supervision: Martín OBLIGADO, Alberto ALISEDA and Nathanaël MACHICOANE

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