Impact of turbulence on the stratified flow around small particles

TL;DR

This paper investigates how background turbulence influences the flow around small particles in stratified fluids, revealing that turbulence can negate stratification effects and restore classical flow patterns.

Contribution

It provides an analytical framework showing how turbulence modifies stratified flow, including an exact expression for fluctuations and conditions where turbulence dominates stratification effects.

Findings

Stronger turbulence cancels stratification effects.

Turbulence can restore unstratified Stokes flow.

Derived an exact expression for flow fluctuations.

Abstract

We study the turbulent flow of the density-stratified fluid around a small translating (either passively or self-propelled) particle. It was found recently [A. M. Ardekani and R. Stocker, Phys. Rev. Lett. vol. 105, 084502 (2010)] that without turbulence, the familiar Stokes flow is dramatically altered by the stratification. Stratification-induced inhomogeneity "turns on" the buoyancy introducing a new "cutoff" or "screening" length scale for the flow, yielding closed streamlines and a faster (exponential-like) decay of velocity. This result, however, did not account for the potential role of the background turbulence, intrinsically present in many aquatic environments. Turbulence mixes the density opposing the effect. Here we derive and solve the advection-diffusion equation that describes the interplay of turbulent mixing, diffusion of the stratifying agent and buoyancy. We derive an exact expression for fluctuations due to weak background turbulence and show that stronger turbulence can completely change the flow around the particle, canceling the effect of stratification and restoring the unstratified Stokes flow.