Voronoi analysis of vortex clustering in homogeneous isotropic turbulence

Clustering of fine-scale dissipative vortices is quantified using a new technique based on the Voronoi diagram. We consider datasets of numerical simulations of decaying and forced homogeneous turbulence up to R e λ ≈ 400. Velocity gradient events are more intense and intermittent if fine-scale vortices have a higher number density. However, clustering is observed only if the vortices are thresholded according to their intensity. Therefore, a fine-scale vortex cluster is the accumulation among stronger vortices rather than the accumulation of all of the vortices in the flow. Turbulence statistics in intense vortex clusters share characteristics with those of the outside when they are renormalized by their local magnitude. This suggests that the clusters are the product of passive amplification of the swirling intensity by underlying larger-scale motions. Comparing the decaying and forced flows reveals that the large-scale forcing affects the vortex distribution although other turbulence statistics are unaffected. The rotation axes of the vortices in the cluster are not preferentially aligned for either flow, which implies that large-scale shear layers are not a vital feature of the clusters.