Direct numerical simulation of turbulent flow above zigzag riblets

We investigate the drag-reducing performance of a zigzag riblet inspired by the “Miura fold” and its influence on the streamwise vortices above a wall-bounded turbulent flow through direct numerical simulations of channel flows. The employed channel wall geometries are a flat plate, straight riblet, and zigzag riblet. The drag-reducing performance of various zigzag riblets with adjusted configuration parameters is superior or at least comparable to that of the straight riblet. We assume an analogy between the drag reduction mechanisms of spatially periodic forcing and the zigzag riblet, because both methods induce similar sinusoidal velocity profiles. To investigate the characteristics of the zigzag riblets, we apply a conditional sampling technique to the near-wall streamwise vortices that is also applied to the wall-bounded flow under spatially periodic forcing. The flow toward the wall of the rotating motion of a vortex becomes relatively small, as the main flow levitates at the tips of the zigzag riblet. Thus, the ejection increases and the sweep decreases. In the phase where the direction of the induced spanwise velocity coincides with the vortical motion, the latter is weakened because the vortex tilts in the spanwise direction at the phase; then, vortex stretching is suppressed. The zigzag riblet can reduce the drag via a mechanism analogous to active control, such as periodic oscillation. However, the effect of the upward flow is dominant above the zigzag riblet, and the analogous mechanisms are less remarkable. The considered zigzag riblet has potential application as a kind of active feedback control.