Cássio Machiaveli Oishi
August 8, 2025
From Yielding to Elastic Effects: Transition and Turbulence in Complex Fluids
This lecture presents recent numerical investigations of viscoplastic and viscoelastic materials in benchmark turbulent flows. Rather than treating non-Newtonian fluids as mere perturbations of Newtonian dynamics, the analysis emphasizes fundamental modifications in disturbance growth, the formation and persistence of coherent structures, and the redistribution of kinetic energy arising from the intricate coupling of stress, flow geometry, and material properties. In viscoplastic turbulence, the coexistence of yielded and unyielded regions alters classical transition thresholds and generates long-lived heterogeneous states that challenge conventional turbulence modeling frameworks. In viscoelastic turbulence, particularly within the elasto-inertial regime, elastic stresses introduce instability mechanisms capable of sustaining turbulence even at low Reynolds numbers. Direct numerical simulations reveal a complex energy redistribution among the mean flow, turbulent fluctuations, and elastic stresses. This nonlinear exchange gives rise to sheet-like polymer extension structures, whose persistence and spatial organization are closely associated with the onset and evolution of flow instabilities. For both classes of materials, accurate characterization of high-shear rheology is essential for calibrating constitutive models used in predictive simulations. These findings underscore the need for tailored diagnostic methodologies and modeling strategies when addressing complex fluids such as pastes, slurries, and polymer solutions, while providing a consistent set of benchmark data to guide future research.
