DNS of turbulent drag reduction in a pressure-driven rod-roughened channel flow by microfiber additives

The turbulent drag reduction caused by microfiber additives in a pressure-driven rod-roughened channel flow at a nominal shear Reynolds number 395 is presented. Both channel walls are roughened by spanwise rods of square cross section with a height of k+=13.6 which translates to 1.7% of channel width. A pitch-to-height ratio of 8 is chosen. The effect of suspended fibers is modeled by an algebraic closure model, which allows the direct simulation of such complicated physics by significantly reducing the computational costs. Three simulations have been conducted; the Newtonian flow and the drag-reduced flows with two different fiber concentrations. As a result, 12.71% and 16.78% increase in the mean bulk velocities throughout the channel could be achieved for the two concentrations considered here. Also, the mean streamwise and wall-normal velocity profiles as well as Reynolds stresses at various locations between two consecutive roughness elements are presented and discussed. In the drag-reduced flow, the streamwise Reynolds normal stress is increased whereas the spanwise and wall-normal normal stresses are decreased. Also, the magnitude of the Reynolds shear stress is decreased in the fibrous flow.