Loss of effectiveness of transverse wall oscillations for drag reduction in pipe flows with Reynolds number

This paper investigates the effect of Reynolds number on drag reduction in a turbulent pipe flow using transverse wall oscillation. Three Reynolds numbers are presented, Re_τ = 180, 360, and 720. The drag reduction values decline from 28.8% at Re_τ = 180 to 22.9% at Re_τ = 720. It is demonstrated that all three Reynolds numbers are effective in suppressing small and intermediate scales of motion in and below the buffer layer; however, large scales of motions are enhanced in the log layer and in the outer layer, an effect that exacerbates with the Reynolds number. A spectral decomposition of a turbulent contribution to bulk mean velocity from Fukagata-Iwamoto-Kasagi identity [1] shows that a comparable amount of drag reduction for all three Reynolds numbers comes from suppressing small and intermediate scales of motion. However, large scales of motion contribute negatively to drag reduction, which causes a loss of performance of transverse wall oscillations with Reynolds number.