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

We present measurements of turbulent drag reduction (DR) in boundary layers at high friction Reynolds numbers in the range of 4500 <= Re-iota <= 15 000. The efficacy of the approach, using streamwise travelling waves of spanwise wall oscillations, is studied for two actuation regimes: (i) inner-scaled actuation (ISA), as investigated in Part 1 of this study, which targets the relatively 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-iota flows, targeting the lower-frequency, outer-scale motions. Multiple experimental techniques were used, including a floating-element balance to directly measure the skin-friction drag force, hot-wire anemometry to acquire long-time fluctuating velocity and wall-shear stress, and stereoscopic particle image velocimetry to measure the turbulence statistics of all three velocity components across the boundary layer. Under the ISA pathway, DR of up to 25% was achieved, but mostly with net power saving (NPS) losses due to the high-input power cost associated with the high-frequency actuation. The low-frequency OSA pathway, however, with its lower input power requirements, was found to consistently result in positive NPS of 5-10% for moderate DRs of 5-15%. The results suggest that OSA is an attractive pathway for energy-efficient DR in high-Reynolds-number applications.

Automated summary

This paper presents measurements of turbulent drag reduction in boundary layers at high friction Reynolds numbers. The study finds that using streamwise travelling waves of spanwise wall oscillations can effectively reduce turbulence drag, especially with lower frequency actuation that saves energy.