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Here, we present measurements of turbulent drag reduction in boundary layersat high friction Reynolds numbers in the range of $4500 \le Re_\tau \le 15000$.The efficacy of the approach, using streamwise travelling waves of spanwisewall oscillations, is studied for two actuation regimes: (i) inner-scaledactuation (ISA), as investigated in Part 1 of this study, which targets therelatively high-frequency structures of the near-wall cycle, and (ii)outer-scaled actuation (OSA), which was recently presented by Marusic et al.(Nat. Commun., vol. 12, 2021) for high-$Re_\tau$ flows, targeting thelower-frequency, outer-scale motions. Multiple experimental techniques wereused, including a floating-element balance to directly measure theskin-friction drag force, hot-wire anemometry to acquire long-time fluctuatingvelocity and wall-shear stress, and stereoscopic-PIV (particle imagevelocimetry) to measure the turbulence statistics of all three velocitycomponents across the boundary layer. Under the ISA pathway, drag reduction ofup to 25% was achieved, but mostly with net power saving losses due to thehigh-input power cost associated with the high-frequency actuation. Thelow-frequency OSA pathway, however, with its lower input power requirements,was found to consistently result in positive net power savings of 5 - 10%, formoderate drag reductions of 5 - 15%. The results suggest that OSA is anattractive pathway for energy-efficient drag reduction in high Reynolds numberapplications. Both ISA and OSA strategies are found to produce complexinter-scale interactions, leading to attenuation of the turbulent fluctuationsacross the boundary layer for a broad range of length and time scales.

Turbulent drag reduction by spanwise wall forcing. Part 2: High-Reynolds-number experiments