Jeremie Bec - CNRS, LAGRANGE - Keywords: turbulence, transport, dust

Jeremie Bec - CNRS, LAGRANGE - Keywords: turbulence, transport, dust

Contribution title: Dusty turbulence

Numerous environmental, industrial or astrophysical situations involve impurities such as dust, droplets, sediments, and other kinds of colloids that are transported by a turbulent fluid. When the suspended particles have finite sizes and masses, they detach from the flow by inertia and form uneven distributions where intricate interactions and collisions take place. The physical processes at play are rather well established, leading to quantitative predictions on the rates at which cloud droplets coalesce, dust accrete to form planets, or heavy sediments settle in a turbulent environment.

Still, basic and important questions remain largely open as to the backward influence of particles on the carrier flow structure and geometry. Some situations involve particle mass loadings so large that the fluid turbulent microscales are altered and, in turn, several macroscopic processes are drastically impacted. These include spray combustion in engines, aerosol saltation in dust storms, biomixing by microorganisms in the oceans, and formation of planetesimals by streaming instabilities in circumstellar disks. Currently such systems are unsatisfactorily handled by empirical approaches or specific treatments. A better modelling requires identifying and understanding the universal physical mechanisms at play in turbulence modulation by dispersed particles.

In this spirit, we study the influence of tiny, heavy, dust-like particles onto the small scales of a turbulent flow. We show that the velocity is unstable in regions with a high particle density contrast, leading to energy transfers shortcutting the classical turbulent cascade. A remarkable feature of this turbulent enhancement is the creation of small-scale eddies whose signature is a power-law range with exponent -2 in the kinetic energy spectrum. These vortices profoundly affect particle concentration. On the one-hand, their spatial distribution tends to weaken large-scale inhomogeneities, to reduce potential barriers to transport and enhance mixing. On the other hand, the dispersion in the flow and the interactions between these long-living structures trigger density fluctuations that are much more intense than in the absence of coupling between the two phases.