Scales of influence on the settling velocities of synthetic, industrial and natural particles in grid turbulence

TL;DR

This study investigates how turbulence affects the settling velocities of various particles, revealing size-dependent influences and supporting the 'fast tracking' theory through detailed flow and trajectory analysis.

Contribution

It provides experimental evidence linking particle size to flow scale interactions and demonstrates the influence of turbulence on settling velocities across different particle types.

Findings

Small particles' velocities scale with Kolmogorov scales.

Mid-sized particles' velocities scale with integral scales.

Large particles are unaffected by turbulence.

Abstract

The settling velocities of natural, synthetic, and industrial particles were measured in a grid turbulence facility using optical measurement techniques. Particle Image Velocimetry and 2D Particle Tracking were used to measure the instantaneous velocities of the flow and the particles' trajectories simultaneously. We find that for particles examined in this study (Rep = 0.4 - 123), settling velocity is either enhanced or unchanged relative to stagnant flow for the range of investigated turbulence conditions. The smallest particles scaled best with a Kolmogorov-based Stokes number indicating the dissipative scales influence their dynamics. In contrast, the mid-sized particles scaled better with a Stokes number based on the integral time scale. The largest particles were largely unaffected by the flow conditions. Using Proper Orthogonal Decomposition (POD), the flow pattern scales are compared to particle trajectory curvature to complement results obtained through dimensional analysis using Stokes numbers. The smallest particles are found to have trajectories with curvatures of similar scale as the small flow scales (higher POD modes) whilst mid-sized particle trajectories had curvatures that were similar to the larger flow patterns (lower POD modes). The curvature trajectories of the largest particles did not correspond to any particular flow pattern scale suggesting that their trajectories were more random. These results provide experimental evidence of the 'fast tracking' theory of settling velocity enhancement in turbulence and demonstrate that particles align themselves with flow scales in proportion to their size.