摘要：Experience shows that the Newtonian fluid model can be adequately employed for flow conditions of gases and small molecule liquids such as water. However, the behavior of a vast number of fluids cannot be considered under Newtonian fluid model. There are some non-Newtonian fluids with shear-independent viscosity, which nonetheless exhibit normal stress differences or other non-Newtonian behaviors. Many polymer solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as ketchup, custard, toothpaste, starch suspensions, paint, blood, and shampoo. In a Newtonian fluid, the relation between the shear stress and the shear rate is linear, passing through the origin, with the constant of proportionality being the coefficient of viscosity. In a non-Newtonian fluid, the relation between the shear stress and the shear rate is different and can even be time dependent. Interestingly, the same fluid can behave as Newtonian or non-Newtonian under different flow conditions.
