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Accueil du site > Évènements > Séminaires > Séminaires IRPHE > Archives IRPHE > 2023

Vendredi 27 Janvier 2023 / IRPHE

publié le

Webinaire IRPHE

Particle transport along the non-wavy free surface above turbulent water

Orateur : Filippo Coletti / Department of Mechanical and Process Engineering, ETH Zurich

Abstract : Every year, millions of tons of plastics enter the ocean. This poses a serious threat to the marine and freshwater ecosystem, and to human health. Limiting such an issue has become one of the defining challenges of our times. Devising effective strategies to mitigate this type of pollution, from product bans to clean-up initiatives, requires the quantitative understanding of how these substances travel and spread. Because a large fraction of all plastics are less dense than water, the fate of plastic pollution depends on the dynamics of free-surface turbulence. Past studies in this area have mostly focused on the influence exerted by the surface on the flow underneath, while the characterization of the transport along the surface itself remains incomplete. I will summarize our recent experiments on tracer particles floating in turbulent water, using laboratory-scale and field-scale facilities : grid turbulence in a large open-channel flow, homogeneous turbulence in a zero-mean flow tank, and an outdoor stream. We focus on regimes in which surface waves are too small to affect the transport. Single-point and two-point statistics in both Eulerian and Lagrangian frames are explored. Although the tracers move in two dimensions, their motion is consistent with the hallmarks of Kolmogorov’s theory for three-dimensional turbulence, as revealed by the velocity fluctuations, structure functions, energy cascade, and Lagrangian dispersion. Due to the compressibility of the free surface, the tracers cluster strongly and do so over spatial and temporal scales comparable to the integral scales of the turbulence. Capillarity-driven attraction breaks the equilibrium between cluster formation and breakup, thus the aggregates steadily grow in size. Particle size also impacts the transport : larger particles filter the small-scale velocity fluctuations, which results in a more time-correlated motion and, in turn, faster dispersion. Taken together, these findings show how, along with aspects specific to the problem, small floating objects on a turbulent non-wavy free surface display strikingly similar behaviours to particles in three-dimensional turbulence. This allow us to tap into decades of fundamental research to gain a predictive understanding of the urgent issue of plastic pollution in waters.

Date et lieu : le vendredi 27 janvier à 11h00 / en ligne