Pair Dispersion in Turbulence

Turbulent mixing of liquids and gasses is ubiquitous in nature. It is the basis of all industrial fluid mixing processes, and it determines the spread of pollutants or bioagents in the atmosphere and oceans. Biological organisms even use it to survive in marine ecosystems. A fundamental component of turbulent mixing is the separation of two nearby fluid elements, i.e., pair dispersion. Despite almost eighty years of intense scientific inquiry, no clear understanding of this fundamental aspect of turbulence has emerged. One critical unresolved question is the extent to which the initial separation of the fluid particles influences their subsequent motion. Surprisingly, our measurements in a laboratory water flow at very high turbulence levels (Taylor microscale Reynolds numbers up to R_lambda = 815) suggest that the initial separation remains important for all but the most violent flows on Earth. This observation has important consequences for such varied problems as pollution control, combustion modelling, hazardous chemical control, and even the understanding of how animals locate food, predators, and mates.