What makes the boundary of uniform concentration zones ?

A plume of pollutant that spreads in a turbulent boundary layer does not mix homogeneously. Large regions remain with little variation of the concentration inside, separated by sharp boundaries where the concentration jumps: uniform concentration zones. In experiments on a turbulent boundary layer in a water tunnel we simultaneously measure the full three-dimensional velocity field using tomographic PIV and the concentration of fluorescent dye that comes from a point source using LIF.

Lagrangian coherent structures are attractive candidates for making the boundaries of these zones. They come in the shape of transient ridges that form barriers to turbulent mixing. However, their role appears to be tiny. The edges of uniform concentration zones correlate much stronger with regions of large shear.

Understanding the organization of scalar dispersion, and its relation with coherent events in the velocity field is of great relevance for understanding the spread of pollution inside the atmospheric turbulent boundary layer.