Coherent structures and drag-reduction mechanism in wall turbulence

Abstract

The discovery of coherent structures in turbulent shear flows is one of the most important advances in turbulence research of the last century. These large-scale structures play important role in the physics of wall turbulence, and suggest a new direction for turbulence control. High skin friction in wall-bounded turbulent flows is closely associated with the coherent structures in the near-wall region. The control strategy based on the near-wall physics successfully achieves drag reduction, yet becomes less effective as the Reynolds number increases. It was discovered recently that large-scale coherent motions exist in the outer layer of the high-Reynolds number wall turbulence. These motions have important influence on turbulence in the near-wall region and the skin friction, and bring new challenges to the control of turbulent flow at high Reynolds number. In the present paper, we briefly review the research history on coherent structures in wall turbulence, and mainly focus on dis- cussing the near-wall coherent structures and their control mechanism, the recent research developments on the large-scale motions in the outer region of high-Reynolds number wall turbulence, and the key issues concerning the drag-reduction control of the high-Reynolds number turbulent flows.