Underlying Drag Reduction Mechanisms of Slip Surfaces: Temporal Approach

Turbulent flow control is important in fundamentals and applications due to the potential benefits, particularly regarding drag reduction for energy savings. In this study, we will investigate the slip control strategy to better understand its underlying mechanisms of drag reduction in turbulent channel flows. Direct numerical simulations are performed at low Reynolds numbers with the inclusion of the slip surface. The temporal analysis is exploited to elucidate the underlying drag-reduction mechanisms. Two temporal phases are classified, depending on wall shear stress. Based upon the lifetime events of intermittent phases, periods of high and low wall shear stress are represented by super active and hibernating phases, respectively. The slip surface plays a distinct role in drag-reduction mechanisms with regard to these two phases. As the slip length increases, the slip flow causes the hibernating phases to occur more frequently with a decrease in the duration between each successive interval, which is intuitive for drag reduction. For the super active phases, however, both the duration of super active intervals and the time between these intervals increase. As a result, the total fraction of time spent in the super active phase increases, which is counter-intuitive for drag reduction. A detailed analysis will be given to elucidate this counter-intuitive behavior.