Location and scales of drag reduction in turbulent pipe flow with wall oscillations at low Reynolds number

Turbulent drag reduction by spanwise wall oscillations has been understood through reducing skin friction and manipulation of Reynolds stress and stress producing events. This work aims to contextualize spanwise wall oscillations around the optimal frequency in terms of Fourier spectra in streamwise and spanwise directions along with the wall normal distributions of coefficients spectra. Analysis of where drag reduction occurs and potential physical mechanisms is provided by determining scale contributions of the Reynolds stress to the bulk mean velocity via the scale dependent Fukagata-Iwamoto-Kasagi (FIK) identity and vorticity contribution to the gradient of turbulent Reynolds' stresses.