Drag reduction in azimuthally oscillating turbulent pipe flow

One promising method of significant drag reduction for turbulent flows is the implementation of spanwise wall oscillations at specially tuned frequencies and amplitudes. This mechanism typically uses high-frequency spanwise oscillations at time-scales that can be coupled to the small eddies near the surface. In the past this research has been limited to low friction Reynolds number (Re_τ) findings (defined here as Re_τ < 2,000); however, recent studies have shown that tuning frequencies to timescales that couple with the larger eddies in the flow may be a more effective way to reduce drag in high Re_τ applications, where these larger-scale motions begin to account for an increasing share of drag. We report results from an experimental study in an azimuthally oscillating pipe to observe drag reduction at Re_τ values up to ~6000. The experimental set up allows for a large range of amplitude of oscillation, frequency of oscillation, and Re_τ in order to elucidate a new relationship between these factors and the resulting drag reduction in both high and low Re_τ flows.