MODIFYING TURBULENT STRUCTURE WITH DRAG-REDUCING POLYMER ADDITIVES IN TURBULENT CHANNEL FLOWS

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(tau)2 almost-equal-to -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.