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The effect of spanwise heterogeneous surface geometry on turbulent boundary layer secondary flows and on skin friction is investigated experimentally. The surfaces consist of smooth streamwise-aligned ridges of different shapes and widths with spanwise wavelengths comparable to the boundary layer thickness (
). Cross-stream stereoscopic particle image velocimetry combined with oil-film interferometry is used to investigate the flow field and assess the drag. Results show that the spanwise distribution of the skin friction varies as a consequence of the mean flow heterogeneity and is highly dependent on surface geometry. The swirling strength maps revealed remarkable changes in the secondary flow structures for different ridge shapes. For wide ridges, topological changes occur showing the appearance of tertiary vortices coexisting with the large-scale secondary structures. An imbalance in favour of these tertiary structures occurs over a certain width which take over the secondary structures, causing a swap in the locations of the low- and high-momentum pathways. Furthermore, the results indicate that the spanwise spacing alone is insufficient to characterise the surface heterogeneity. A new parameter (
), which is based on the ratio of the perimeter over and below the mean surface height, is shown to adequately capture the changes in skin friction and streamwise circulation of the secondary motions. Triple decomposition allowed the quantification of the dispersive stresses for all these cases, which can contribute up to
of the total shear stress when strong secondary motions occur.
An experimental investigation of a turbulent boundary-layer flow over a heterogeneous surface is carried out to examine the mean flow and turbulence characteristics, and to document the variation of skin friction that might affect the applicability of traditional scaling and similarity laws. The heterogeneity is imposed along the spanwise direction and consists of streamwise-aligned smooth raised strips whose spanwise spacing
is comparable to the boundary-layer thickness (
). Single-point velocity measurements alongside direct skin-friction measurements are used to examine the validity of Townsend’s similarity hypothesis. The skin-friction coefficients reveal that the drag of the heterogeneous surface increased up to 35 % compared to a smooth wall, while velocity measurements reveal the existence of a log layer but with a zero-plane displacement and a roughness function that vary across the spanwise direction. Lack of collapse in the outer region of the mean velocity and variance profiles is attributed to the secondary flows induced by the heterogeneous surfaces. Additionally, the lack of similarity also extends to the spectra across all scales in the near-wall region with a gradual collapse at small wavelengths for increasing
. This suggests that the effect of surface heterogeneity is not necessarily felt at the smaller scales other than to reorganise their presence through turbulent transport.
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