Modifying turbulent structure with drag-reducing polymer additives in turbulent channel flows

Abstract

New power spectra computed from LDA measurements of the fluctuating u - and v -velocity signals in a turbulent channel flow with and without drag-reducing polymer (polyethylene oxide) injection are presented. LDA data rates were sufficiently high to reconstruct the simultaneous time-dependent u - and v -velocity signals along with the time-dependent Reynolds stress signal. Time-averaged statistics of the turbulent flow are presented in conjunction with the power spectral measurements which show a dramatic reduction in both the v -velocity fluctuations and Reynolds stress fluctuations throughout the channel over all frequencies. There is also a redistribution of energy in the u -velocity fluctuations from high frequencies to low frequencies throughout the channel. Different injection conditions were examined: different polymer concentrations were injected at different flow rates such that the total amount of polymer in the channel remained constant. For certain polymer concentrations, ‘large’ negative Reynolds stress, -〈 uv 〉/ u τ 2 ≈ − 0.2, was measured in the near-wall region. In addition, there is a marked difference in the u -velocity spectra and the Reynolds stress spectra close to the wall for the different injection conditions.