DNS Study of Drag Reduction Effect on Ultra-Fine Rough Surfaces

The ultra-fine rough surfaces, even if it is randomly-distributed in space, sometimes delay transition and reduce drag, when their size is within the viscous sublayer (Tani (1988), Oguri et al. (1996), Tameike et al. (2021) and Hamada et al. (2022)). We name that surface structure to reduce drag as Distributed Micro Roughness (DMR). We demonstrated direct numerical simulation of Tollmien–Schlichting (T-S) transitional flow over the sand-grind roughness surface, that was implemented in the real shape we used in the wind tunnel experiment. Firstly, we confirmed the drag reduction effect over the sand-grid rough surface, by observation of the turbulent kinetic energy (TKE) distribution and friction drag with calculation of the integral value of the averaged Reynolds shear stress. Secondary, we analyzed the mechanism of the statistic change, by applying the statistical decomposition of temporal phase-averaging of the TS wave and other three-dimensional components. The results show that the TS vortex tends to break down into turbulence over the sand-grind rough surface, although TKE and friction drag were suppressed compared to the smooth surface. In the presentation, we will discuss the mechanism more in detail.