Unsteady flow of Carreau fluids around an impulsively moving circular cylinder is numerically investigated in this study. Both shear-thinning and shear-thickening fluids are used with the range of the power-index number 0.4 ≤ n ≤ 2. Unsteady laminar flow with the influence of shear-dependent viscosity and impulsive motions of the cylinder are analyzed with various Carreau numbers 10 ≤ Cu ≤ 40 and Reynolds numbers 10 ≤ Re ≤ 100. Both starting and stopping flows are numerically simulated. Systematic validation is conducted for current numerical approaches using literature data before the investigation of the unsteady flow. The effects of major Carreau fluid flow parameters, namely, n, Cu, and Re, on the unsteady vortical flow are studied with a focus on the formation of vortices, zero-shear point on the cylinder, and transitory hydrodynamic loads. In general, increased shear-thickening (n > 1 with higher Cu and lower Re) leads to higher viscous effect, delaying the formation of additional vortices and promoting the drag force due to the dominant viscous drag. More interesting flow aspects are noticed with shear-thinning fluids. Increased shear-thinning (n < 1 with higher Cu and higher Re) results in complicated vortical flow including secondary, tertiary, quaternary vortices, and even more. Highly oscillated drag force is obtained in such vortical flow due to the oscillated pressure drag, which dominates the total drag in the current unsteady flow. It is expected that the results reported in this study could be used for a better understanding of unsteady non-Newtonian fluid flow and for the validation of numerical simulation of unsteady non-Newtonian fluid flow.
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