Spatial features of the wall-normal structures in a turbulent boundary layer

Coherent wall-normal structures in a turbulent boundary layer (TBL) have been investigated by scrutinizing a direct numerical simulation (DNS) dataset with Re-theta = 2500. It was found that groups of hairpin legs induce wall-normal-aligned structures and these large-scale regions are mostly induced by the combination of the attached and detached vortical structures. By using the feature extraction algorithm, we found that more than approximately 60% of the attached momentum regions extend beyond the log region, while most of the detached regions formed locally away from the wall have relatively short wall-normal extent. The two-point correlation of the streamwise velocity fluctuations showed that the wall-normal length scales vary linearly with the distance from the wall, and, finally, the two-point correlations between the signed swirling motions showed that interactions between counterrotating vortices are predominant throughout the boundary layer and especially frequent in the near-wall region.