Experimental Observation of Batchelor Dispersion of Passive Tracers

We report the first detailed experimental observation of the Batchelor regime [G. K. Batchelor, J. Fluid. Mech. 5, 113 (1959)], in which a passive scalar is dispersed by a large scale strain, at high Péclet numbers. The observation is performed in a controlled two-dimensional flow, forced at large scale, in conditions where a direct enstrophy cascade develops [J. Paret, M.-C. Jullien, and P. Tabeling, Phys. Rev. Lett. 83, 3418 (1999)]. The expected $k^{-1\mathrm{}}$ spectrum is observed, along with exponential tails for the distributions of the concentration and concentration increments and logarithmlike behavior for the structure functions. These observations, confirmed by using simulated particles, provide a support to the theory.