Lesson 3 - Principles of Fluid Mechanics 1 SAQ

* Newton’s law of viscosity defines the viscosity of a Newtonian fluid as what? (More than one answer may be appropriate):

a)    A constant

b)    A constant of proportionality

c)     Independent of shear rate

d)    Independent of shear stress

e)    A function of shear rate

f)      A function of shear stress

g)    The ratio of shear stress to shear rate

h)    The ratio of shear rate to shear stress

Response:

The correct answers are a), b), c), d), and g).

A reminder of Newton’s law of viscosity:

Where  is shear stress, µ is dynamic viscosity and du/dy is shear rate.

You can see that dynamic viscosity µ, is both a constant and from the origins of Newton’s law, a constant of proportionality between shear stress and shear rate.

It is also independent of (not a function of) shear stress and shear rate.

If you rearrange Newton’s law of viscosity with µ as the subject, you get:

So, viscosity is the ratio of shear stress to shear rate (not the other way around).