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

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