Intermittency suppression in turbulence via forced light particles

We investigate how turbulence is reshaped by the presence of externally forced light particles, using high-resolution direct numerical simulations with four-way coupling. The particles are subject to an oscillatory force that in turn locally affects the fluid flow through momentum exchange at the position of the particles. Since the light particles preferentially concentrate in high vorticity regions, this leads to an intricate preferential turbulence modulation. We show that through this modulation, the forced light particles strongly reduce the intermittency of the flow, shedding new light on the delicate relationship between vortex filaments and turbulence intermittency.

💡 Research Summary

The authors investigate how turbulence can be actively controlled by embedding light particles that are externally forced with an oscillatory acceleration. Using high‑resolution direct‑numerical simulations of homogeneous isotropic turbulence (HIT) at Taylor‑scale Reynolds numbers up to Reλ≈168, they implement a full four‑way coupling: (i) particles are advected by the fluid, (ii) they exchange momentum with the fluid (two‑way coupling), (iii) they collide as hard spheres (excluded‑volume effect), and (iv) they receive an imposed force fE = ae sin(ωt) ex. Particles are modeled as point‑mass bubbles (density ratio β≈3) with Stokes number St≈1, which is the regime where light particles preferentially accumulate in vortex filaments (high‑enstrophy regions).

The study first characterises preferential concentration by measuring the ratio of enstrophy sampled by particles to the Eulerian enstrophy. In a pure one‑way coupling case this ratio peaks at St≈1, confirming strong filament trapping. Adding collisions (four‑way coupling) reduces the ratio because particles cannot occupy the same volume, while the external forcing further weakens trapping by periodically “kicking” particles out of the filaments. Nevertheless, the sampling remains highly non‑uniform, indicating that the Lagrangian feedback will be spatially biased.

To quantify turbulence intermittency the authors examine Eulerian longitudinal velocity increments δℓu and compute flatness F(p)(ℓ)=S(p)(ℓ)/