Akira Oyama

The Mach number effect on the riblets’ drag reduction performance in turbulent transitional flow regimes is investigated by direct numerical simulations. We focus on the transitional flow occurred by the Tollmien-Schlichting instability. For freestream Mach numbers of 0.2,0.6and 0.85, it is found that the riblets reduce the frictional drag in the turbulent flow region independently of the Mach number, while they tend to increase it in the transitional flow regions. Interestingly, the rate of the drag reduction in the turbulent region decreased with increasing the Mach number. This is because the non-dimensional groove width in the turbulent region changes as the Mach number changes. In other words, the relation between the groove width of the riblets and the size of the longitudinal vortices in the turbulent flow changes as the Mach number changes. The turbulent kinetic energy spectrum in the turbulent region supports these results. The difference in the spectrum between the smooth and riblet surfaces became smaller as the Mach number increased, indicating that the flow structure changes as the Mach number changes. From these results, it is recommended that for high-speed vehicles such as transonic aircraft, riblets be designed in compressible flow rather than incompressible flow.

In this study, direct numerical simulations are conducted to reveal the optimal groove width of the riblet at the mainstream Mach number ! = . , the typical cruise speed of a transonic aircraft. Additionally, the study aims to clarify the effect of Mach number on the drag change ratio by comparing it with the incompressible flow condition, ! = . . The results at ! = . show that the lowest drag change ratio, around − %, is observed at " =, and . As for the Mach number effect, the drag change ratio is smaller in the case of ! = . than in the case of ! = . for the same non-dimension groove width ". This is because the drag reduction amount with increasing Mach number is larger on the riblet surface than on the smooth surface since the ejection and sweep intensities on the riblet surface decrease more with increasing Mach number than on the smooth surface. It is also found that the robustness of the drag reduction effect against the change in " is improved for ! = . compared to ! = . .