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.