In this study, direct numerical simulations are conducted to exhibit the viscous drag reduction effect of equilateral triangle riblets at Mach 0.85 and investigate the optimal groove width for maximum drag reduction. Additionally, the Mach number effect on their drag reduction performance is elucidated by comparing the drag reduction effect of riblets at Mach 0.85 with that at Mach 0.2. The computational results indicate that largest reduction in drag, approximately 11%, is achieved at Mach 0.85 at Formula: see text, and 24, where Formula: see text represents the riblet groove width Formula: see text in viscous wall units. Furthermore, the results indicate that the drag reduction is greater in the case of Mach 0.85 than in the case of Mach 0.2 for the same Formula: see text because the ejection and sweep intensities on the riblet surface decrease more significantly than on the smooth surface. Additionally, the robustness of the drag change ratio against the change in Formula: see text is improved for Mach 0.85 as compared to Mach 0.2.
Kaneko et al. (Wed,) studied this question.