Effective composite control for drag reduction at high Reynolds numbers

We explore further the composite drag control (CDC) [Yao, Chen & Hussain, Phys. Rev. Fluids 2, 062601(R) (2017)] that combines opposition control (OC) and spanwise opposed wall-jet forcing (SOJF) methods together via direct numerical simulation of incompressible turbulent channel flow. For three Re_τ's (180, 395 & 550), we achieve drag reductions above 30% -- much higher than obtained by either individual method (e.g. 14% for SOJF and 22% for OC at Re_τ=550). The net power saving is >30% as less power is required. Flow analysis shows that CDC is more effective in suppressing the random turbulence than OC, and the coherent motion is also suppressed more compared to SOJF due to the reduction of near-wall vortical structures by OC. The distribution of wall shear stress shows structures periodic in the streamwise direction, and corresponding spectrum show a sharp peak at k_x^+_~7*10-4 . Vorticity field analysis shows that the turbulent transport of spanwise vorticity in the normal direction also displays periodic structures. Moreover, the triple decomposition (mean, coherent and random) supports our previous results that coherent vorticity transport and random transport of wall normal vorticity in spanwise direction reduce drag, while random normal transport of spanwise vorticity increases drag. In summary, our results strongly promote prospects for employing CDC for effective skin friction drag reduction at high Reynolds numbers.