Efficiency loss in viscoelastic turbulent flows due to polymer degradation

Introducing small amounts of long-chain polymers to a viscous liquid has been shown to dramatically reduce skin-friction drag in turbulent flows. One of the main challenges in polymer-induced drag reduction is the mechanical degradation of polymers due to strong elongational strain and shear stresses generated by turbulence. During this process, polymer chains are likely to split, which significantly diminishes drag reduction (DR) effects. For this study, we perform direct numerical simulations of viscoelastic flows using the FENE-P model to investigate the effects of polymer degradation. The simulations are performed at a friction Reynolds number of 100 while other parameters such as Weissenberg number (Wi) and polymer length/concentration are adjusted based on reported experimental observations to model the degradation process. As expected, the degradation contributes to a significant loss of DR effects, where DR% decreases by a factor of at least 3.8 after the first degradation pass. It is also found that the more flexible the polymers (high Wi number), the more loss of DR%. However, there seems to be an apparent saturation reached at sufficiently high Wi numbers. During the second degradation pass, the loss of DR effects becomes less dramatic due to the resulting very low-Wi regime. We will also discuss the effects of the initial polymer length and concentration on the loss of DR effects.