Large-scale control in turbulent channels over surface riblets

As a passive drag control method, surface riblets have been studied for a long time. However, its drag reduction rate is no more than 10%, affected by many riblets issues such as shape, size and arrangement. Recently, a large-scale control scheme has been developed via spanwise opposed wall-jet forcing, generating a pair of counterrotating streamwise swirls that combine near wall streaks together and hence reduce drag. Thus, it is interesting to check whether a pair of large-scale swirls over surface riblets would lead to drag reduction higher than each of the individual control method. Here, we report direct numerical simulations of turbulent channel at Re_τ=180, with equicrural-triangle riblets (height h⁺=10, width s⁺=20) and large-scale swirls (controlling-height y_c⁺=30, spanwise size λ⁺=1200). It is found that the new method leads to at least 16% drag reduction (could be higher by different placements of riblets in the spanwise direction), in contrast to the mere surface riblets control (which is around 3%). As a conceptual validation, our results support the perspective of drag control for higher Reynolds number by generating large-scale swirls over various surface modifications.