Statistical Properties of Turbulence Under a Smart Lagrangian Forcing

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 particle. Since the light particles preferentially concentrate in high vorticity regions, this leads to an intricate preferential turbulence modulation. By systematically varying forcing amplitude, frequency, and particle volume fraction, we show that this "smart" Lagrangian forcing suppresses small-scale turbulence intermittency - a central feature of turbulent flows linked to extreme events and anomalous scaling laws. Our results demonstrate a resonant modulation mechanism: intermittency reduction is most effective when the forcing frequency aligns with the Kolmogorov timescale. Particle collisions, modeled through hard-sphere interactions enforcing volume exclusion, play a key role by amplifying feedback when particles fill coherent structures. Experiments with different Stokes numbers highlight that this effect is non-trivial, manifesting only when preferential concentration occurs (St ≈ 1), and not in the tracer or heavy-particle limits.