Velocity structures associated with large streamwise density gradients in an incompressible turbulent boundary layer (with air as the working fluid, ) are analysed experimentally using planar image velocimetry and aero-optic measurements. The resulting flow topologies for the velocity fluctuations associated with large negative and positive density gradients are in excellent agreement with results for coolings and heatings in time, respectively (Antonia & Fulachier, J. Fluid Mech., vol. 198, 1989, pp. 429-451). The current results are complimentary to those from Saxton-Fox et al. (AIAA J., vol. 57 (7), 2019, pp. 2828-2839), on the signature of the vertical velocity structures associated with large density gradients. In the present work, these structures are shown to exhibit a sign change, consistent with the scalar gradient, and are localised in the wall-normal direction with an average height of approximately , almost constant for increasing distance from the wall. The corresponding small-scale streamwise fluctuations also exhibit a consistent sign change, which is found to originate, on average, from upstream leaning structures. The emerging picture for the velocity field is then that of a multiscale phenomenon, where small-scale structures, responsible for large optical aberrations, are superimposed on the back of large-scale bulge-like structures that are known to populate the outer layers. The proposed conceptual model is consistent with early ideas of 'typical' eddies (Falco, Phys. Fluids, vol. 20 (10), 1977, pp. S124-S132), the hairpin vortex model and associated shear layers (Adrian et al., J. Fluid Mech., vol. 422, 2000, pp.1-54), as well as with notions of multiscale velocity organisation in shear layers (Klewicki & Hirschi, Phys. Fluids, vol. 16 (11), 2004, pp. 4163-4176; Saxton-Fox et al. 2019), and it provides new insight into the geometry of the small-scale velocity structures.

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).