Direct numerical simulation of turbulent flow in an annular pipe with large-scale control using buoyancy force

In this study, we investigated the mechanism of drag reduction in turbulent flow in an annular pipe subjected to large-scale buoyancy-driven control through direct numerical simulations. The control was achieved by imposing alternating heating and cooling along the outer wall, which induces a buoyancy force that generates large-scale vortical motion. Two radius ratios (ξ = 0.3 and 0.5) were examined at a bulk Reynolds number of 5600. A maximum reduction in skin friction of 20.1% was obtained at ξ = 0.3 with a Richardson number Ri = 1.25 × 10−2 and azimuthal wavenumber Ω = 3. The results of an analysis of the flow fields revealed that the drag was reduced in the formation of two pairs of large-scale counter-rotating vortices and the establishment of stable thermal stratification. A three-component decomposition of the Reynolds shear stress and an annular Fukagata-Iwamoto-Kasagi (FIK) identity analysis indicated that the reduction primarily resulted from the suppression of the turbulent component near the outer wall. These findings demonstrate that buoyancy-induced large-scale control is an effective and environmentally sustainable strategy for reducing frictional drag in wall-bounded turbulent flows.