Abstract

Spanwise surface oscillation has been shown to be an effective drag reduction strategy for wall-bounded turbulent flows at low to modest Reynolds numbers (friction Reynolds numbers up to 1,000). Here, we present experimental measurements of substantial drag reduction at friction Reynolds numbers up to 12,800 using spanwise surface oscillations. The drag reduction was measured via a combination of drag balance and hot-wire anemometry. The drag reduction was found to occur via two distinct physical pathways. The first pathway, recognised from previous studies, involved actuating the surface at high frequencies, comparable to those of the small-scale eddies that dominate turbulence near the surface. The second pathway, however, was new and leveraged actuation at low frequencies comparable to those of the large-scale eddies farther from the surface. Importantly, this is accompanied with net-power savings due to the low actuation frequencies used. Insights into the underlying mechanism for the drag reduction will be discussed.