How a scalar puff that is written in turbulence disperses: theory and experiment

When a blob of passive scalar is released in turbulence, it will spread due to the combined action of turbulence and molecular diffusion. It is a still unresolved question whether molecular diffusion helps or suppresses the spreading of the blob. We write a scalar puff in a strongly turbulent flow of air using molecular tagging with two crossed UV laser beams. The puff is made by fusing N$_2$ and O$_2$ molecules to NO, which is then used as a tracer. The dispersion of the puff is followed using laser--induced fluorescence. When the blob is small (size $\approx 10 \: \eta$), the evolution of its Gaussian parameters $\Gamma$ satisfies a simple linearized equation driven by the gradients of the turbulent velocity field [1]. It was computed using the velocity field of a direct numerical simulation (Re$_{\lambda} = 400$). At short times we find striking agreement between experiment and numerical simulation. A question is whether the strongly anomalous statistics of the gradients will endow the fluctuations of $\Gamma$ with special properties. A preliminary conclusion is that this is not the case, with the fluctuations being close to log--normal. [1] H. Tennekes and J.~L. Lumley, {\em A First Course in Turbulence}.