2016 Volume 11 Issue 3 Pages JFST0015
This paper proposes a new riblet configuration. A traditional sinusoidal riblet was modified with the aim of reducing pressure drag. The height of the side wall in the newly configured riblet is lowered toward the node of the sinusoidal curve to reduce the large pressure drag, while the riblet height is maintained at the anti-node position as it has been reported to be the most effective for straight or traditional sinusoidal riblets. In this paper, effective configuration parameters and the drag-reducing performance of the modified sinusoidal riblet are investigated through parametrically conducting direct numerical simulation (DNS). First, the optimal combination of the wavelength and amplitude of the spanwise sinusoidal curve configuration is determined with a fixed riblet spacing s and height h, which are located at around the design point of straight riblets. The effective combination obtained is similar to that reported in previous studies on traditional sinusoidal riblets. Second, the optimal vertical amplitude a of the modified sinusoidal riblet is determined with the effective settings mentioned above. Unfortunately, with a/h < 0.2, the drag-reducing performance is similar to that of traditional sinusoidal riblets. Furthermore, the performance deteriorates with a/h ≥ 0.2. However, the drag-reducing performance of the modified sinusoidal riblet is improved when s+ deviates from the design point compared to that of comparable straight or traditional sinusoidal riblets. This indicates that flow-condition robustness is improved for the modified sinusoidal riblet. Finally, the mechanism of robustness is investigated. The contributions of the bottom and side walls to drag are calculated, and the difference of each contribution between the traditional and modified sinusoidal riblets is shown to discuss the influence caused by the lowered side wall. In addition, a quadrant analysis is conducted to examine the change in momentum transfer above the modified sinusoidal riblet.
