Experimental study on secondary flow in turbulent boundary layer over spanwise heterogeneous microgrooves

Convergent-divergent riblets (C-D riblets) are a type of grooved surface pattern with directionality and spanwise heterogeneity. In the cross-stream plane, we apply stereoscopic particle image velocimetry to study the characteristics of the secondary flow over C-D riblets. Three dierent heights of h+ = 8, 14 and 20 are applied in the turbulent boundary layers at Re = 723 to reveal the eect of riblet height on the flow field. In the cross-stream plane, increasing the riblet height intensifies the heterogeneity of turbulent properties, i.e., a wider downwelling region, a stronger spanwise flow, a narrower upwelling region and a
stronger deceleration eect. Compared with the smooth-wall case, the magnitude of spanwise velocity fluctuations is larger over the converging region. The dispersive momentum transfer is primarily contributed by the secondary-flow-induced stress compared with the roughness-induced stress, and it becomes more intense as the riblet height increases. Compared with the smooth-wall case, the near-wall streamwise turbulent events are slightly
wider over the diverging region and much narrower over the converging region. Overall, the higher C-D riblets generate a more intense secondary flow, and the mechanism of an increasing riblet height is attributed to the greater capability of deeper yawed microgrooves. In light of the results from our study, we propose a dierent way of categorizing the surface patterns with spanwise heterogeneity from the perspectives of surface geometry, roll mode and secondary flow generation mechanisms.