Effects of polymer addition on transition and length scales of flow structures in transitional channel flow

The effect of polymer addition on transition to turbulence in a two-dimensional water-flow channel was experimen-tally investigated by flow visualization using reflective flakes. The flow entering the channel test section maintains a high disturbance level by expanding laterally after reaching a high Reynolds number upstream the test section. In order to obtain the intermittency factor (turbulence fraction), the visualized images were classified into non-turbulent and turbulent regions, and the streamwise scale of the streaks appearing in the non-turbulent region was estimated from the autocorrelation coefficient computed by shifting the images in the streamwise direction. The visualization results show that similar to the pure water case, intermittent flow with a patch-like distribution of turbulent and non-turbulent areas clustered by streamwise streaks is observed. The Reynolds number at which the intermittency increases shifts toward higher Reynolds numbers with increasing polymer concentration, indicating a delay of transition. The streaks appearing in the non-turbulent region elongate with increasing polymer con-centration. At high concentrations, straight elongated streaks penetrate through the turbulent regions, suggesting that the polymer addition affects the stability of the streaks. These changes of the streak behavior indicate that the polymer affects not only the transition Reynolds number but also the flow structure during the transition process.