Contrary to Newtonian trends: Early flow transition and drag enhancement at low to intermediate Reynolds number flows of structured fluids

Abstract

The flow of structured fluids, such as polymeric and micellar solutions, involves a strong two-way coupling between flow kinematics and internal microstructural dynamics, including polymer stretching, micellar scission, and reformation. These interactions yield complex nonlinear rheological responses, including viscoelasticity, shear-thinning, and thixotropy. In this study, we perform high-fidelity numerical simulations of micellar solution flow past a circular cylinder using the Modified Bautista-Manero (MBM) model, which couples a viscoelastic constitutive equation with a kinetic equation for fluidity to capture reversible micellar breakage and reformation. Model parameters are derived from quantitative fitting of experimental data for the EHAC-NaSal system. Our results reveal substantially richer flow dynamics than in Newtonian fluids under the same conditions. Transitions to unsteady flow occur at significantly lower Reynolds numbers, indicating greater instability driven by microstructural effects. At intermediate Reynolds numbers, micellar flows exhibit quasi-periodic behaviour, contrasting with classical periodic vortex shedding in Newtonian cases. Unlike the monotonic drag reduction in Newtonian fluids, micellar solutions show an anomalous drag increase. Wake characteristics reverse with regime: larger recirculation zones at low Reynolds numbers but more compact wakes in unsteady flows. Lift coefficients and Strouhal numbers are consistently increased. Vorticity fields display pronounced spatial localisation within thin near-wake shear layers. A dynamic mode decomposition analysis reveals coexisting unstable time-decaying and self-sustained modes in micellar flows, whereas only self-sustained modes appear in Newtonian cases.