Pair separation in high Reynolds number turbulence

The separation of two nearby particles in a turbulent flow is fundamental in our everyday lives. Turbulent mixing is important everywhere from mundane applications like stirring milk into a cup of tea to technological processes such as the mixing of chemicals in reactors, combustion engines, or jet turbines. Environmental problems such as the spread of pollutants or bioagents in the atmosphere and oceans are fundamentally turbulent mixing processes. Even biological organisms use it to survive in marine ecosystems. Despite intense scientific inquiry, however, no convincing agreement has been found with the Richardson and Batchelor two-particle dispersion predictions over a wide range of timescales. Here we report measurements in a laboratory water flow at very high turbulence intensities (Taylor microscale Reynolds numbers of R_lambda = 690 and 815) that show excellent agreement with a refinement of Batchelor's prediction. We find that even for large initial spatial separations Batchelor scaling is fulfilled. We observe no Richardson scaling regime. Instead, we find a much slower scaling law following the Batchelor regime. This behaviour greatly impacts the calculation and prediction of turbulent dispersion