Antagonistic effects of drag-reducing flow control on extreme events in turbulent flow

The effect of active flow control, such as external body forces in wall-bounded turbulent flows has been extensively studied, particularly for its role in reducing skin-friction drag. However, the impact of these forces on extreme events observed in various flow variables has received less attention. Despite numerous studies aimed at understanding and describing extreme events, there is lack of coherence in its definition across various fields. In this study, we introduce the definition based on wall shear stress and examine the evolution of extreme events using direct numerical simulations with the inclusion of external body forces up to a friction Reynolds number of 180. The results indicate that recurrent extreme wall-shear-stress events or very strong bursting phenomena are observed in both uncontrolled and controlled flows, as evidenced by high flatness values. Interestingly, in several cases, the magnitude of extreme events in drag-reducing flows exceeded the mean wall shear stress of the uncontrolled flow, indicating antagonistic or amplifying effects of flow control. A detailed analysis will be given to elucidate this amplifying effect by phase space dynamics. In addition, the dependence of Reynolds number on extreme events in drag-reducing flows will be explored.