Topographically-driven secondary flows and very-large-scale motions interactions at high Reynolds number boundary layers

Spanwise heterogeneous roughness-wall flows have now been fairly well explored due to the emergence of the secondary flows, known to result in highly three-dimensional flows in the cross-plane, causing considerable changes in the heat and momentum transfer properties. Most studies, however, are presented at moderate Reynolds numbers, hence limiting the range of scale separation. In this study, a turbulent boundary layer flow developing over a spanwise heterogeneous ridge-type topography is examined at high Reynolds numbers. The skin-friction coefficient and turbulent flow properties are measured using a floating-element drag balance together with two simultaneous single-point hot-wire anemometry at the two symmetry planes (ridge and valley), respectively. To investigate the impact of large-to-small scale separation and strength of the secondary motions on the flow behavior, three spanwise spacings are considered at increasing Reynolds numbers. The results revealed that the skin friction varies depending on the surface spanwise characteristic wavelength, owing to changes in the strength of the secondary flows while the turbulence properties showed changes in distribution of the spectral density across different wavelengths. Changes in the amplitude and frequency modulations due to differences in the inner-outer interactions will further be discussed.