Direct numerical simulations of turbulent flow over the converging and diverging riblets

Different from conventional streamwise-aligned riblets, converging and diverging (C-D) riblets are arranged obliquely along the main-flow direction, intending to induce a large-scale secondary flow that modulates or controls the existing large-scale vortical structures in wall turbulence. In this study, we perform direct numerical simulations of turbulent flow over the C-D riblets at a low Reynolds number and investigate the impacts of the C-D riblets on aerodynamic forces, turbulence statistics, and coherent structures. The results show that the C-D riblets can reduce the skin-friction drag, but at the expense of a significantly increased pressure drag in the diverging region, which leads to an increase in the total drag. We present detailed mean and statistical flow-fields to assess the flow characteristics in the presence of the C-D riblets, including modifications on the distributions of velocities, pressure, Reynolds stresses, and turbulence kinetic energy. Moreover, the re-arrangement of turbulence structures over the C-D riblets is examined in terms of instantaneous flow-fields and two-point correlations. The findings provide more insight into the physics of flow over C-D riblets and may contribute to the further development of this flow control strategy.