Controlling turbulent wall flows via composite active excitations

A sustained drag reduction rate upper R greater than 28 percent sign R > 28 % $R\gt 28\,\%$ (with net power saving greater than 26 percent sign > 26 % $\gt 26\,\%$ ) is obtained for turbulent channels at a series of Reynolds numbers (i.e. italic Re Subscript tau Baseline equals 180 Re τ = 180 $\textit{Re}_\tau =180$ , 395 395 $395$ , 550 550 $550$ ) via a composite drag control (CDC). This strategy combines opposition control (OC) and large-scale spanwise opposed wall-jet forcing (SOJF), enabling superior suppression of drag-producing near-wall turbulent structures. Parameter searching over 114 simulations is conducted here, and the maximum drag reduction obtained from CDC is much higher than that produced by either SOJF (e.g. 14 % at italic Re Subscript tau Baseline equals 550 Re τ = 550 $\textit{Re}_\tau =550$ ) or OC (e.g. 20 % at italic Re Subscript tau Baseline equals 550 Re τ = 550 $\textit{Re}_\tau =550$ ). Flow analysis shows that CDC is more effective in suppressing random turbulence than OC, and it further weakens the secondary flows between colliding motions of SOJF. Interestingly, the pre-multiplied spectrum of the wall shear stress fluctuation ( tau prime Subscript w