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_τ\,~ 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 ( Rᵤᵤ ) and streamwise–wall-normal ( Ruw ) 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 Rᵤᵤ are dominated by slower streamwise structures. Additional analysis of Rww 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/δ \,=\, 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. Rᵤᵤ 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.