PhD: Fluctuation induced forces in turbulence

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.

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