Relationship between drag reduction and flow structure in a Taylor–Couette flow with polymer solution

Abstract Polymer-induced drag reduction, commonly referred to as the Toms effect, has been extensively studied in wall-bounded turbulent flows such as pipe and channel flows. In contrast, fewer investigations have focused on rotating shear flows. In this study, we experimentally investigate the effect of polyethylene glycol (PEG), a water-soluble and non-toxic polymer, on turbulent Taylor–Couette flow, which develops between concentric cylinders where only the inner cylinder rotates. Torque measurements were conducted while varying the Reynolds number to quantify the degree of drag reduction due to polymer addition. The drag reduction ratio was evaluated by non-dimensionalizing the measured torque with respect to the baseline water case, revealing up to 30% drag reduction. This reduction is considered to be associated with the suppression of radial velocity components by the polymer molecules. In addition to quantitative torque-based analysis, we employed Kalliroscope particles for flow visualization. High-speed camera recordings were used to generate space–time diagrams, enabling the observation of axial vortex structures and their temporal evolution at selected Reynolds numbers. The present study aims to deepen the understanding of polymer-induced drag reduction in rotating turbulence by integrating quantitative measurements with qualitative visualization techniques. Graphical abstract