Turbulence attenuation by large neutrally buoyant particles

TL;DR

This study experimentally investigates how large neutrally buoyant particles influence turbulence in a von Kármán flow, revealing that increasing particle volume fraction attenuates turbulence and suggesting a surface-based interaction mechanism.

Contribution

It provides new insights into turbulence attenuation by large particles and identifies a potential transition regime related to particle spacing and boundary layer thickness.

Findings

Turbulence attenuation scales as /3 with particle volume fraction.

Small-scale turbulence properties remain unaffected by particles.

Evidence of a transition between regimes based on particle spacing and boundary layer thickness.

Abstract

Turbulence modulation by inertial-range-size, neutrally-buoyant particles is investigated experimentally in a von K\'arm\'an flow. Increasing the particle volume fraction $\Phi_\mathrm{v}$, maintaining constant impellers Reynolds number attenuates the fluid turbulence. The inertial-range energy transfer rate decreases as $\propto\Phi_\mathrm{v}^{2/3}$, suggesting that only particles located on a surface affect the flow. Small-scale turbulent properties, such as structure functions or acceleration distribution, are unchanged. Finally, measurements hint at the existence of a transition between two different regimes occurring when the average distance between large particles is of the order of the thickness of their boundary layers.