Organisation of velocity structures associated with large passive scalar gradients in a turbulent boundary layer

Planar particle image velocimetry (PIV) measurements were performed in a thermal boundary layer with simultaneous measurements of the streamwise density gradient, averaged across the boundary layer height. Previous work on the topic (Saxton-Fox et al., 2019) revealed that large density gradients were associated with tall wall-normal velocity structures extending across the entire boundary layer height. The authors suggested that these structures were a result of a wall-normal average of smaller scales, which reside at different locations from the wall, and contribute to the same density gradient change. Results from the present work support this model and show that, if a second condition on the vertical location of the identified features is imposed, one can extract wall-normal velocity structures that are localized in the vertical direction. Their average wall-normal extent is found to be around 0.1δ, with no significant variation across the boundary layer height. The corresponding conditionally averaged streamwise velocity fluctuations also show a change in sign from the wall up to edge of the boundary layer, consistent with the underlying density gradient condition. This sign change is found to originate on average from upstream leaning structures of opposite signs, while the inferred flow topology is shown to agree with earlier results on the topic (Antonia and Fulachier, 1989).