Synergy Mechanism in Combined Polymer and Microbubble Drag Reduction: Decreased Polymer Diffusion

Drag reduction (DR) experiments with combined micro-bubble and polymer solution injection were conducted in a water tunnel at the ARL/Penn State. Previous research has shown that gas injected upstream of polymer produced higher levels of DR than expected based on DR levels observed with the independent injection of micro-bubbles or polymer alone. This synergy between the two injection processes was speculated to have resulted from a decrease in the diffusion rate of injected polymer away from the surface by the effects of the micro-bubbles. The slowed polymer diffusion should extend the length of the zone where large polymer DR occurs. In the current work, a confocal-style laser induced fluorescence based probe was developed and used to measure the wall concentration of injected polymer solutions and injected water with and without upstream micro-bubble injection. The local wall concentrations were increased with gas injection by more than an order of magnitude in many cases. These results show that synergistic drag reduction occurs with combined injection as a result of increased polymer wall concentrations.