Schlieren Imaging of Mixing and Turbulence in Dilute Polymer Solutions

Dilute synthetic polymer solutions have the potential to reduce turbulent drag in pipelines and around marine structures. Biopolymeric extracts such as plant mucilage have recently been proposed as alternative material sources for drag reducing polymers due to their cost-effectiveness and lower environmental hazards. The very low concentrations of polymer employed in drag reduction studies makes direct imaging of the new flow structures an outstanding challenge. Here, we employ Schlieren imaging to enable direct flow visualization of the mixing dynamics and recirculating regions that develop in complex flows of dilute aqueous polymer solutions. With this technique, we are able to detect density differences of order 10^-3% for dilute aqueous solutions of synthetic polymers and biopolymers. This promising capability is used to understand the vortical structures associated with turbulent drag reduction of polymer solutions. In our experiments, the turbulent mixing of dilute polymer jets is visualized for different values of Reynolds number, polymer concentration, and molecular weight.  We compare these visualization results with previous theoretical studies of mixing layer development and experimental reports of the turbulent drag reducing properties of different polymer solutions.