Circulation signature of vortical structures in turbulent boundary layers

The strength of vortical structures in a turbulent boundary layer is of interest in determining the generation and development of hairpin vortices. The dual-plane Particle Image Velocimetry (PIV) data at z ⁺ = 110 (z/δ = 0.09) and z/δ = 0.53 (z ⁺ = 575) in a turbulent boundary layer at Re _τ = 1160 obtained by Ganapathisubramani et al. [7] were used to characterize the strength of the vortical structures by their circulation. The 3Dswirl was used to identify the vortex cores. The average number of swirl cores per field identified at z ⁺ = 110 was approximately twice the average number at z/δ = 0.53. The mean radius of the cores was found to decrease with increasing wall-normal distance. The main eigenvector of the velocity gradient tensor was used to determine the orientation of each vortex core. Circulation of the vortical structures was then calculated using the vorticity vector projected onto the main eigenvector direction. At z/δ = 0.53, the mean circulation calculated using the eigenvector was almost the same as that using the full vorticity vector, but for z ⁺ = 110 the mean circulation calculated using the eigenvector was 12% less than the mean circulation calculated using the vorticity vector.