Scalar absorption by particles advected in a turbulent flow

TL;DR

This study uses numerical simulations to analyze how turbulence influences scalar absorption by particles, revealing increased uptake and fluctuation behaviors relevant for microorganisms in aquatic environments.

Contribution

It extends classical shear flow models to turbulent flows, quantifies turbulence effects on scalar uptake, and examines fluctuations and correlations in a Lagrangian framework.

Findings

Turbulence increases scalar uptake compared to stationary flow.

Large fluctuations in local uptake occur at high Peclet numbers.

Uptake fluctuations follow velocity fluctuations with a Kolmogorov delay.

Abstract

We investigate the effects of turbulent fluctuations on the Lagrangian statistics of absorption of a scalar field by tracer particles, as a model for nutrient uptake by suspended non-motile microorganisms. By means of extensive direct numerical simulations of an Eulerian-Lagrangian model we quantify, in terms of the Sherwood number, the increase of the scalar uptake induced by turbulence and its dependence on the Peclet and Reynolds numbers. Numerical results are compared with classical predictions for a stationary shear flow extended here to take into account the presence of a restoring scalar flux. We find that mean field predictions agree with numerical simulations at low Peclet numbers but are unable to describe the large fluctuations of local scalar uptake observed for large Peclet numbers. We also study the role of velocity fluctuations in the local uptake by looking at the temporal correlation between local shear and uptake rate and we find that the latter follows fluid velocity fluctuations with a delay given by Kolmogorov time scale. The relevance of our results for aquatic microorganisms is also discussed.