Direct numerical simulation for investigation on yaw angle effects on riblets

Abstract

The yaw angle (φ) effect on riblets are investigated by parametrically conducted direct numerical simulation (DNS). Three configurations are adopted: standard straight riblet, sinusoidal riblet, and modified sinusoidal riblet. The height of the side wall in the modified sinusoidal riblet is lowered toward the node of the sinusoidal curve to reduce the pressure drag, whereas the riblet height is maintained at the anti-node as it has been reported to be the most effective for straight or traditional sinusoidal riblets. This study is the first investigation on yaw angle effect on both traditional and modified sinusoidal riblets. The increase and decrease in drag caused by riblets are calculated by comparing the drag of the upper and lower walls in a channel. To reproduce inclined flow, pressure gradients P_g cosφ and P_g sinφ are applied in the x and z directions, respectively, where P_g is the pressure gradient applied in the x direction in the zero-yaw-angle case as the driving force. Under moderate misalignment of φ ≤ 10°, straight riblet is more robust than the other two configurations against the change of yaw angle. Nevertheless, the drag-reducing performance of both the traditional and modified sinusoidal riblets is still maintained. It should be noted that the total drag reduction rates of the modified sinusoidal riblet are better than those of the traditional sinusoidal riblet. Under larger misalignment of φ = 20°, the total drag reduction rates of the three configurations are similarly degraded. To discuss the reason for the change of drag-reducing performance, the contributions of the pressure drag and friction drag to the total drag reduction rates, which cannot be measured separately, are investigated by DNS.