The effect of viscoelasticity on the development of spiral turbulent intermittent structures and the variation in critical Reynolds number was examined in annular pipe flow. Flow visualization was performed while varying the Reynolds number and Deborah number, and turbulent regions were extracted based on deviations from the mean image brightness. The intermittency rate γ was used as a quantitative index of transition, and the transition characteristics of γ at each concentration and its corresponding relaxation time λ were analyzed. At λ = 3.00 × 10-2，5.33 × 10-2 (s), γ increased rapidly with Reynolds number, indicating the early onset of turbulence due to elastic instabilities. In contrast, at λ = 0.161 (s) and λ = 0.204 (s), γ remained low over a wider range of Reynolds number, and transition was delayed. These results confirmed that the critical Reynolds number tended to increase with Deborah number, while the intermittency rate exhibited nonlinear modulation depending on concentration. It was suggested that at low concentrations, elastic instability arising from the elastic sheet of viscoelastic fluids contributes to the growth of turbulence, whereas at high concentrations, the formation of turbulent structures was suppressed by elastic effects. The present findings indicated that viscoelasticity altered the transitional dynamics through elastic behavior dependent on λ.
MORI et al. (Wed,) studied this question.