Drag reduction in turbulent wall-bounded flows of dilute polymer solutions

The transition from the laminar to the turbulent regime is responsi- ble for a significant increase in friction drag in wall-bounded flows. Among the many possibilities to mitigate the phenomenon, the addi- tion of a tiny amount of long-chain polymers to a Newtonian solvent is known to reduce friction. Despite more than 70 years since the first experimental evidence, the polymer drag reduction phenomenon is still not fully understood and the mechanism behind it is still debated. While the experiments do not allow the unveiling of the elusive inter- action between polymer and turbulence dynamics at the basis of the phenomenon, numerical simulations never attempted to replicate ac- tual experimental conditions. Thus far, simulations have shown only qualitative accordance with the experimental investigations, either for computational or modelling limitations. The issue of comparison be- tween direct numerical simulation and experiments is the main aspect of the present thesis, as a comparison is instrumental to get physical insights into polymer drag reduction. An alternative, hybrid Eulerian- Lagrangian approach is proposed for direct numerical simulations of dilute polymer solutions with the potential of a one-to-one matching between simulations and experiments. The Lagrangian description of the polymers, modelled as FENE dumbbells, overcomes the limi- tations of the constitutive Eulerian models used so far, it allows for the matching of all the polymer physical parameters and it is able to characterise solutions of polydisperse polymers, which is the typical situation encountered in the experiments. Thanks to the latest outstanding advances in high-performance computing, simulations of realistic polymer solutions in terms of both polymers and solvent parameters are presented to investigate the in- teraction between polymers and turbulence, highlighting the role of fully extended polymers as responsible for the drag reduction effect.