Features of small-scale vortical structures from a fully resolved experimental dataset of von Kármán mixing flow

Intense vortical structures or ‘worms’ at the small-scales have been studied in a wide range of turbulent flows, mostly using DNS.In the present study, we investigate dynamics/statistics of the worms in a fully resolved three-dimensional experimental dataset of a turbulent mixing flow measured at the center of a large von Kármán mixing tank at a Re_λ=179. To avoid arbitrariness in the detection method, an objective detection method proposed by Haller et al. is implemented. In total, 1003 snapshots (volumes) of the flow acquired at random times with the spatial resolution of 1η, where η is the Kolmogorov length scale, have been studied. As a result, about 12500 structures were detected having an average radius of 5.1η, which is in a good agreement with previous findings. Different features of the turbulent flow were studied and compared relative to the whole volume and locally within the structures. Comparison of the joint probability density functions (J-PDFs) of the enstrophy and dissipation shows that within the structures, high vorticity and low shear are dominant. Alignment of the vorticity-eigenvectors inside the structures shows that the vorticity vector is well aligned with the intermediate eigenvector and is normal to the compressive and extensional eigenvectors demonstrating that the structures are quasi one-dimensional.The intermediate eigenvalue was found to be positive on average.This shows that enstrophy production inside the structures results from vortex stretching.We then consider the entrainment/detrainment at the boundary of the structures. This shows that the structures are entraining ambient fluid on average and that this entrainment is a result of the competing effects of non-viscous and viscous phenomena consistent with Burgers’ vortices.