How to find patterns written in turbulent air

In molecular tagging velocimetry, patterns of tagged molecules are written in a flow using lasers. They can be lines, crosses or grids. When time proceeds, these patterns are deformed and displaced by the flow. The patterns are followed in time, and from their deformation and displacement, the velocity field can be deduced. In strongly turbulent flow, written lines may be severely deformed or even ruptured, and then, it becomes a challenge to find them in images. We discuss the technique of active contours to trace deformed lines and crosses in turbulence. These patterns can be used to measure the statistical properties of the flow. We illustrate our technique using a variant of molecular tagging in a turbulent flow of air in which NO molecules are formed from N₂ and O₂ using UV laser beams. A short while later, these patterns are visualized by inducing fluorescence of the NO molecules.