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 degrees and 135 degrees to the principal flow direction) of a turbulent boundary layer at moderate Reynolds number (Re-tau similar to 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-uu) and streamwise-wall-normal (R-uw) 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 strearnwise correlations in R-uu are dominated by slower strearnwise structures. Additional analysis of R-ww correlations suggests that the long strearnwise slow-moving regions contain discrete zones of strong upwash over extended strearnwise distances, as might occur within packets of angled hairpin vortices. At a wall-normal location outside of the log region (z/delta = 0.5), the correlations are shorter in the strearnwise direction and broader in the spanwise direction. Correlations in the inclined cross-stream plane data reveal good agreement with the streamwise-spanwise plane. R-uu in the 45 degrees plane is more elongated along the in-plane wall-normal direction than in the 135 degrees plane, which is consistent with the presence of hairpin packets with a low-speed region lifting away from the wall.