Underlying mechanisms of drag reduction and enhancement in turbulent flow via external body forces

The effects of externally-applied traveling-wave body forces on turbulent flows are investigated using direct numerical simulations at a friction Reynolds number of 134. In this study, we aim at elucidating the underlying mechanisms behind drag reduction (DR) and drag enhancement (DE) caused by the external body force. It is found that the low forcing frequency results in DE, while the drag continues to decrease with increasing frequency. At sufficiently high forcing frequencies, DR is ultimately achieved. To elucidate DR/DE mechanisms, the wall shear stress of DR, DE, and no-control cases are compared. Using the frequency analysis, it is observed that as expected, the DR case exhibits the forcing and intrinsic bursting frequencies, while the no-control case presents only the bursting frequency. Interestingly, the DE case shows three frequencies: the bursting frequency, a frequency close to the forcing frequency, and a very high frequency. It is assumed that this very high frequency is responsible for drag enhancement. In addition, we investigate the wall shear stress characteristics. Counter-intrusively, the DR case shows a significantly large increase during bursting compared to no-control and DE cases. However, the bursting magnitude of the DE case is very similar to the no-control case. Reynolds number dependence of DR/DE mechanisms will be discussed.