Combined effects of polymers and active blowing/suction on drag reduction

In the present study, we investigate the combined effect of polymer additives and active blowing and suction at the wall on drag reduction using direct numerical simulation. Three different turbulent channel flows having the same bulk Reynolds number but different wall shear velocities are generated to see the role of the strength of near-wall streamwise vortices in polymer drag reduction. From this study, we show that the phenomenon of polymer drag reduction is closely related to the strength of near-wall streamwise vortices as well as the elasticity of polymer solution. On the other hand, the combined drag reduction obtained by both the polymer additives and the blowing and suction is larger than the individual drag reduction either by polymers or by blowing and suction, but is smaller than sum of the two separate drag reductions, indicating that the polymer solution is less efficient in producing drag reduction in the absence of strong near-wall streamwise vortices.