Numerical analysis of turbulence characteristics in a flat-plate flow with riblets control

A comparative study about riblets-controlled turbulent boundary layers has been performed to investigate the turbulence characteristics associated with drag reduction in a compressive flat-plate flow (where the free-stream Mach number is 0.7) by means of direct numerical simulations (DNSs). With a setting of the triangular riblets (s⁺ ≈ 30.82, h⁺ ≈ 15.41) settled on the Re_τ ≈ 500 turbulent boundary layer, an effective global drag reduction was achieved. By comparing velocity and its fluctuation distribution, vorticity fluctuation and streaks structures between the smooth and riblets flat-plate cases, two roles of lifting and rectification in terms of riblets drag control are revealed that the micro-scale riblets can lift up logarithmic-law region of the boundary layer, which leads to a smaller wall friction velocity and thus a drag reduction. The streamwise vortices and its fluctuation structures are shifted upward, thus the interactions between them and the wall surface are weakened, which causes the suppressed intensity of Reynolds normal stresses, streamwise vorticity and turbulent kinetic energy production inside the riblets. Moreover, the streaks associated with streamwise velocity or 3D vortices are ruled from the distorted to long and straight structures as they pass through the riblets, indicating an ability of riblets to turn turbulence into a more ordered state.