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

Planar particle image velocimetry measurements are 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, 2018) revealed that large density gradients were linked to tall wall-normal velocity structures extending across the entire boundary layer height. It was postulated that they result from 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 distance from the wall is linked to the phase of large-scale streamwise velocity fluctuations, and they convect with different velocities. Further, filtering of the density gradient signal reveals a strong scale separation in both wall-normal and streamwise velocities, allowing a multi-scale analysis of the velocity structures associated with large scalar gradients.