  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.
