Investigation of large-scale coherence in a turbulent boundary layer using two-point correlations

Stereoscopic particle image velocimetry (PIV) measurements are made in streamwise–spanwise and inclined cross-stream planes (inclined at 45^ and 135^ to the principal flow direction) of a turbulent boundary layer at moderate Reynolds number (Re_\, 1100). Two-point spatial velocity correlations computed using the PIV data reveal results that are consistent with an earlier study in which packets of hairpin vortices were identified by a feature-detection algorithm in the log region, but not in the outer wake region. Both streamwise–streamwise (R ₔₔ) and streamwise–wall-normal (Rₔₖ) correlations are significant for streamwise displacements of more than 1500 wall units. Zero crossing data for the streamwise fluctuating component u reveal that streamwise strips between zero crossings of 1500 wall units or longer occur more frequently for negative u than positive u, suggesting that long streamwise correlations in R ₔₔ are dominated by slower streamwise structures. Additional analysis of Rₖₖ correlations suggests that the long streamwise slow-moving regions 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/ \, =\, 0. 5), the correlations are shorter in the streamwise direction and broader in the spanwise direction. Correlations in the inclined cross-stream plane data reveal good agreement with the streamwise–spanwise plane. R ₔₔ in the 45^ plane is more elongated along the in-plane wall-normal direction than in the 135^ plane, which is consistent with the presence of hairpin packets with a low-speed region lifting away from the wall.