Study of spanwise vortices in turbulent boundary layer flow based on tomographic PIV

Tomographic particle image velocimetry (Tomo-PIV) is a novel laser technique that can be applied to measure a three-dimensional three-component (3D3C) velocity field. In the current work, Tomo-PIV was utilized to measure a plate turbulent boundary layer (TBL) at Re_τ=1768, and 150 velocity fields with each size of 80 mm×16 mm×45 mm were obtained. The swirl strength λ_ci was used to identify the local vortex, while its projection on the spanwise direction λ_ci ^z was used to identify the spanwise vortex. The spatial coherent structures of spanwise vortices and their population trends along the wall-normal direction were studied through swirling strength λ_ci ^z. The statistic results suggest that the strength of the spanwise vortices reduces with the increase of the wall-normal distance y⁺. The streamwise velocity of retrograde spanwise vortices is higher than that of the prograde vortices and there is a strong dependence between the streamwise velocity and the wall-normal velocity. High or low momentum regions of small scale in the streamwise and wall-normal plane are highly correlated with the spatial arrangement of spanwise vortices. Some 3D plots of instantaneous flow structures in TBL are presented and discussed at the end of the paper.