Sorry Sir, I know for shear thinning fluids you have rheological equations linking share stress to shear rate. For example the power law model : T = k j^n. But when you have a shear thinning fluid flowing in a complex duct, you need a viscosity to calculate the Reynolds number. And to have the viscosity (apparent = T/j = kj^(n-1)) you need to calculat j in the duct ! It was the meening of my question. How to calculate the shear rate j ?