Influence of surface geometry on secondary flows in turbulent boundary layers*

Secondary flows have recently earned an increased consideration due to their observations in a wide range of rough-wall turbulent flows. While the mean flow universality was previously shown to be violated, the effect of surface geometry on the secondary flows has not been documented yet. This experimental study is designed to look at the effect of smooth ridges of different shapes and widths, with Stereo-PIV used to investigate the flow field and oil-film interferometry to assess the drag. Their maximum heights and spanwise spacings were fixed to match 0.1 and 1 times the boundary layers thickness δ, in order to maximize potential secondary flows.  Large-scale modifications in the mean and turbulent flow are observed in each case and are characterized by the circulation of the secondary motions, local correlation maps and dispersive stresses. These modifications are shown to highly depend on the geometrical characteristics of the surface heterogeneity, which in turn impact the wall drag owing to the mean flow heterogeneity.