Local anisotropy of laboratory two-dimensional turbulence affects pair dispersion

Experimental investigation of particle pair separation is conducted in two types of laboratory two-dimensional turbulence under a broad range of experimental conditions. In the range of scales corresponding to the inverse energy cascade inertial interval, the particle pair separation exhibits diffusive behaviour. The analysis of the pair velocity correlations suggests the existence of coherent bundles or clusters of non-diverging fluid particles. Such bundles are also detected using a recently developed topological tool based on the concept of braids. The bundles are observed as meandering streams whose width is determined by the turbulence forcing scale. In such locally anisotropic turbulence, the particle pair dispersion depends on the initial particle separation and on the width of the bundles.