Abstract
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.
Highlights
Riblets, which are developed to mimic drag-reducing shark skin, have recently attracted renewed attention as realizable flow control devices
We evaluate the drag-reducing performance of a Miura-fold zigzag riblet and investigate its drag reduction mechanism by conducting direct numerical simulation (DNS) for different geometries
This configuration was explored because the expandable and tractable structure of the Miura fold is advantageous for adjusting the spanwise spacings of zigzag riblets by an actuator in response to flow conditions determined by a sensor
Summary
Riblets, which are developed to mimic drag-reducing shark skin, have recently attracted renewed attention as realizable flow control devices. Okabayashi[16] and Peet et al.[13,14] have separately reported that the friction drag reduction rate of sinusoidal riblets with a tuned geometry is 2% superior to that of a straight riblet Those researchers have found that the optimal wavelength (λ) and amplitude (a) are approximately 1000 and 5–30 wall units, respectively. Jung et al.[26] first reported that the friction drag in a turbulent boundary layer is reduced by oscillating the wall in the spanwise direction Those researchers performed direct numerical simulation (DNS) of the channel flow, finding that the maximum friction drag reduction of 40% was obtained for an oscillation period of 100 wall units. We evaluate the drag-reducing performance of a Miura-fold zigzag riblet and investigate its drag reduction mechanism by conducting DNSs for different geometries. We investigate modification of the near-wall streamwise vortices above straight or zigzag riblets, by applying the conditional sampling technique.[31,34]
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