Description du sujet de thèse

Particle aggregation in turbulent flows is a central question in various applications in environmental sciences, going from cloud formation by collision of ice crystals and/or water droplets to pollutant dispersion at the surface of the ocean like the sargassum patch in the sargassum sea, or in industrial flows like in water treatment, fiber flocculation for paper formation, etc. Several models have been proposed to estimate the particle collision rate for small particles. The most well known is the Saffman model [1] for small inertialess particles that do not disturb the flow. Since this pioneering work several models have been proposed to consider particle inertia [2] but much less is
known on the role of the hydrodynamic interactions between particles. What is the exact role of such interactions in setting the particle collision rate and aggregation rate ? For instance, an object larger than the Kolmogorov length scale, the dissipative length scale of the turbulence, immersed in a turbulent flow will affect the velocity field and modify locally the turbulent fluctuations. When two objects are close enough to each other, the spectrum of the fluctuations of the velocity field in the space between the objects should be different than elsewhere. This can be at the origin of an interaction force between the two objects.

The goal of the FLIRT project is to characterize and model Fluctuations Induced Force (FIF) acting on bodies immersed in turbulent flows with the aim of quantifying the importance of this force on the aggregation process and go beyond the current models of aggregation which neglect hydrodynamic interactions between particles. To achieve this goal 3 objectives have been identified : characterizing this force in turbulence which has been predicted numerically [3] and relate the evolution of the FIF to the characteristics of the turbulent flows and the turbulent spectra, characterizing the influence of the shape of the object as particle involved in particle aggregation are not straight plates, and investigating the dynamical aspects of FIF in relation with the aggregation of particles in turbulence.

The objective of the PhD is to relate the evolution of these forces with the 3D-turbulent spectrum and to investigate both static and dynamic regimes in connection with particle aggregation in non-stationary flows. To achieve this, state-of-the-art measurement techniques such as Particle Tracking Algorithms and high speed stereo-PIV will be used to characterize the flow both between and around the objects. The forces experienced by the plates will be measured using force sensors and several experimental setups will be used to characterize the influence of large-scale flow structures on the FIF. The recruited PhD student will be responsible for conducting the experiments, acquiring and analyzing the experimental data and contributing to the modeling of the phenomenon.

[1] P.G. Saffman and J.S. Turner. On the collision of drops in turbulent clouds. J. Fluid Mech., 1(1), 1956.
[2] Alain Pumir and Michael Wilkinson. Collisional aggregation due to turbulence. Annu. Rev. Condens. Matter Phys., 7(1), 2016.
[3] V. Spandan et al., Fluctuation-induced force in homogeneous isotropic turbulence, Sci. Adv., 14(6), 2020.

Contexte de travail

The FLIRT project is a collaboration between the LOMA in Bordeaux and IRPHE in Marseille funded by the French funding agency (ANR). This project has started in January 2026. At LOMA, they will be in charge of the investigation of FIF in 2D turbulence. At IRPHE, preliminary experiments in 3D turbulence have been performed in a cubic tank in spring 2025. Our measurements show that such FIF indeed exists in 3D turbulence and our results are inline with previous numerical simulation.

Contraintes et risques

The candidate should hold a master degree in Physics, Mechanics or an equivalent qualification. A background in fluid mechanics is required, and prior experience with experimental studies would be an advantage. The PhD will start in fall 2026.