Effects of Cross-Sectional Shape on Drag Reduction of a Novel Three-Dimensional Riblet

To address the challenges brought by the Green Aviation, the riblets technique is introduced as an effective method that can reduce the surface drag by 6–10%, and may save significant number of fuels with installation on aircraft. This paper aims at further enhancing the riblet drag reduction performance by designing new 3-dimensional configuration riblets. Conventional riblets usually have no variations but lines straight-up in the streamwise direction. The newly proposed “Longitudinal Variation Riblets” (LV riblets) superimpose the carefully designed periodic sawtooth-like structures on the classic straight riblets. And two different cross section shapes of blade and triangle are studied. The quasi-Direct Numerical Simulation (DNS) method is employed to calculate riblets drag reduction performances in turbulent channel flow case with Reτ = 180. The results shows that the cross-sectional shape significantly affects the drag characteristics of LV riblets. The blade shape case shows a drag reduction of merely 0.63%, while triangular shape case has a significant drag reduction of 14.11%. Further analysis of turbulent statistics reveals that the coherent structures of triangular shape case are much less chaotic than that of blade shape case, which could reduce the intensity and possibility of sweeping and ejecting event and therefore reduce the high shear stress. Moreover, the surface area with high wall shear stress of blade shape case is quite larger than the triangular shape case, especially at the riblet bottom valley. This might imply that the lifted streamwise vortices forced by sawtooth structure of blade case could fall into the riblet valley, and future works will focus on adjusting riblet parameter to achieve better performance.