The reducing and enhancing effects of turbulence on the settling velocities of single particles is discussed in terms of theory and the available experimental data. The coherent eddy structures of turbulence are shown to have a strong delaying effect on bubbles, which have a tendency to be trapped inside steady vortices. The vortex-trapping mechanism is less efficient for heavy particles because they tend to escape due to the centrifugal effect. Furthermore, heavy particles in arrays of steady vortices tend to be fast tracked along certain paths (Maxey and Corrsin 1986) and thus experience an increase in settling velocity. Experiments with grid turbulence indicate that settling velocity is decreased by relatively weak turbulence but increased by strong turbulence. The measured changes in settling velocity range from a 40% decrease to an increase by a factor of 4.

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