Drag reduction via a spanwise body force and its connections to turbulent dynamics

An external body force has been successfully used to achieve turbulent drag reduction. In this study, we investigate the effects of a spanwise body force on skin-friction drag reduction in a turbulent channel flow and its connection to turbulent dynamics. A form of a traveling wave is considered for the spanwise body force, which consists of four control parameters that are the magnitude of force, penetration depth, period of oscillation, and wavelength. Direct numerical simulations (DNS) were performed to investigate the effect of these parameters on drag reduction. The current DNS results show that the skin friction is reduced by as much as 20%. An optimal condition of the spanwise body force for drag reduction is further discussed. Interestingly, it is observed that in low-drag intervals, where the wall shear stress is much less than its mean value, the spanwise body force appears to significantly affect turbulent dynamics to make the wall shear stress not as chaotic as in high-drag events. The power analysis of the body force for actual energy savings will be performed to clarify the roles of the body force for the drag reduction process.