Microbubbles and microparticles are not faithful tracers of turbulent acceleration

TL;DR

This study investigates how microbubbles and microparticles behave as tracers in turbulent flows, revealing that their acceleration statistics deviate from traditional predictions due to drift effects, which questions their reliability as tracers.

Contribution

The paper introduces a theoretical framework explaining the drift-induced deviations in microbubble acceleration statistics in turbulence, supported by experiments and simulations.

Findings

Microbubbles deviate from tracer acceleration statistics at lower Reynolds numbers.

Drift effects enhance acceleration variance and reduce correlation times.

Microbubbles are not faithful tracers of turbulence acceleration.

Abstract

We report on the Lagrangian statistics of acceleration of small (sub-Kolmogorov) bubbles and tracer particles with Stokes number St << 1 in turbulent flow. At decreasing Reynolds number, the bubble accelerations show deviations from that of tracer particles, i.e. they deviate from the Heisenberg-Yaglom prediction and show a quicker decorrelation despite their small size and minute St. Using direct numerical simulations, we show that these effects arise due the drift of these particles through the turbulent flow. We theoretically predict this gravity-driven effect for developed isotropic turbulence, with the ratio of Stokes to Froude number or equivalently the particle drift-velocity governing the enhancement of acceleration variance and the reductions in correlation time and intermittency. Our predictions are in good agreement with experimental and numerical results. The present findings are relevant to a range of scenarios encompassing tiny bubbles and droplets that drift through the turbulent oceans and the atmosphere. They also question the common usage of micro-bubbles and micro-droplets as tracers in turbulence research.