A fluid is classified as being Newtonian if it conforms to NEWTON’s friction law, i.e. if viscosity remains constant with agitation or varying shear rate and its shear rate being proportional to the velocity gradient vertical to the direction of flow. That means, that the viscosity is only affected by temperature changes.
It is, on one-dimensional flow:, physically defined as:
Of Newtonian fluid characteristics are e.g. the following:
If it is not known whether a fluid is of Newtonian flow characteristics or not, it should be laboratory-tested.