Start The onset of Dean instability in laminar secondary flow in 180° curved rectangular cross section channel was studied experimentally and numerically in Newtonian and non-Newtonian fluids. The development of the instability was observed; we showed that the Dean vortices develop first in the near wall zone on the concave wall, where the viscosity is weak and the shear rate is high, and than they penetrate the cross-section center characterized by a high viscosity for the pseudoplastic fluids and a solid (unsheared) zone for the yield fluids. Based on the complete formation of the Dean vortices, the critical value of the Dean number decreases with the increase of the power law index and with the decrease of the Bingham number. Contrarily to what is reported in the literature where the instability threshold was usually obtained visually, in this study a new criterion based on the radial gradient of the axial velocity to detect the instability threshold was defined. Comparison was made between numerical and experimental results concerning the instability threshold, obtained with the new criterion. Also, a comparison between the instability threshold using this new criterion and using the visualization technique is presented. We show that the value of the Dean number using the new criterion is comparable for the two studies, numerical and experimental. These values are weaker than those obtained with the visualization technique for the same conditions.your abstract here...
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