Numerical and experimental investigations on drag-reducing effects of riblets

The numerical simulation and force measurement experiment are conducted in this work. The direct numerical simulation method with high-order schemes is performed to resolve the incompressible turbulent flow over riblets. According to the turbulent statistics, behaviors of the large-scale streamwise vortices above riblets are analyzed. In drag-reducing cases, the population density of streamwise vortices near the wall decreases, and the ratio of contributions of the large-scale streamwise vortices to the total mean shear is also lowered. In addition, streamwise vortices are situated near riblet tips, and spanwise motions of the vortices are weakened. Consequently, they are anchored at the riblet surface. In the experimental investigation, the drag characteristics of a transport aircraft mounted with riblets are studied in a low-speed wind tunnel. The angle of attack (AoA) ranges between -2 degrees and 20 degrees, and the test speed is up to 70 m/s. A maximum of nearly 40% decline in drag coefficient is achieved at 10 degrees AoA. Because the riblet surface makes the fluid more irrotational and the vortices are anchored at the wall, the flow separation is weakened at moderate AoAs, which indicates that the pressure drag is also reduced in the circumstance.