Time resolved visualization of structures of velocity gradients measured with near Kolmogorov-scale resolution in turbulent free-shear flows

A three-dimensional visualization of velocity gradients at the finest scales in a turbulent free-shear flow is obtained using a time-resolved scanning PIV system with near Kolmogorov-scale resolution. A minimal observation volume capable of capturing single velocity gradient structures is used. The structures are identified by iso-surfaces revealing four basic shapes in the makeup of the velocity gradients: sheets, tubes, square ribbons and spherical blobs. Their size is also obtained. The 3D local acceleration structure is visualized showing strong anti-alignment with the convective acceleration. The statistics of the velocity gradient, and higher order moments (skewness and kurtosis) are calculated and compared favorably with published experimental work and with homogeneous isotropic theory.