Measurements of Accelerations of Large Neutrally-buoyant Particles in Intense Turbulence

TL;DR

This study measures the acceleration of neutrally buoyant particles of various sizes in intense turbulence, revealing how acceleration scales with particle size and providing models to predict these behaviors.

Contribution

It introduces experimental measurements of large particle accelerations in turbulence and compares them with theoretical models, highlighting the transition from tracer-like to inertial-range behavior.

Findings

Acceleration variance decreases with particle size as d^{-2/3}

Model based on pressure differences over particle diameter matches transition behavior

Scaled acceleration PDFs show little dependence on particle size

Abstract

We measure acceleration statistics of neutrally buoyant spherical particles with diameter 0.4 < d/eta <27 in intense turbulence (400< R_lambda <815). High speed cameras image polystyrene tracer particles in a flow between counter-rotating disks. The measurements of acceleration variance, <a^2>, clearly resolve the transition from the tracer like behavior of small particles to the much smaller accelerations of large particles. For d>5 eta, <a^2> decreases with diameter as d^{-2/3} in agreement with inertial range scaling arguments. A model relating <a^2> to the pressure structure functions matches the transition from small to large particle behavior if the particles respond to pressure differences over (1.7 +- 0.3) d. A model relating <a^2> to the fluid acceleration averaged over the particle diameter predicts the transition with no free parameters, but does not show clean inertial range scaling in the size range studied. Consistent with earlier work, we find that the scaled acceleration probability density function shows very little dependence on particle size.