Time-varying secondary flows in turbulent boundary layers over surfaces with spanwise heterogeneity

Secondary flows form in turbulent flow over surfaces with spanwise heterogeneity. In this study, we use surfaces comprised of spanwise alternating smooth and rough (P-36 grit sandpaper) strips to investigate the behaviour of these secondary flows for various strip widths. PIV measurements are performed on the wall-parallel plane above surfaces with various strip widths $S$ ($0.3 \leq S/\overline{\delta} \leq 3.6$), where $\overline{\delta}$ is the spanwise-averaged boundary layer thickness. We find that, when $S/\overline{\delta} \approx 1$, these secondary flows not only strengthen but also exhibit a pronounced unsteadiness. This unsteadiness is consistent with a flapping from side-to-side of large-scale streaks with a streamwise wavelength of 3-4$\overline{\delta}$.