Analysis of Vortex Packets and Reynolds Stress in a Turbulent Boundary Layer

Sets of stereo PIV measurements were obtained in streamwise-spanwise planes of a turbulent boundary layer with Reτ = 1060. Two-point spatial correlations obtained from fields in the logarithmic region revealed that both streamwise-streamwise (Ruu) and streamwise-wall-normal (Ruw) correlations were significant for streamwise displacements of more than 1500 wall units. Zero crossing data for the streamwise fluctuating component u revealed that streamwise strips between zero crossings of 1500 wall units or longer occurred more frequently for negative u than positive u. This result suggested that the long streamwise correlations in Ruu are dominated by slower streamwise structures. Additional analysis of Rww correlations suggested that the long slow-moving streamwise structures contain discrete zones of strong upwash over extended streamwise distances as might occur within packets of angled hairpin vortices. At a wall-normal location outside of the log region (z/δ = 0.5), the correlations were shorter in the streamwise direction and broader in the spanwise direction. All of the correlation results are consistent with earlier studies (Ganapathisubramani et al, 2003) in which a feature detection algorithm identified packets of hairpins in the log region but not in the outer region.