Question

Which of these laws is obeyed by a Newtonian fluid?

(a) Shear stress law

(b) Newtonian law

(c) Incompressible fluid law

(d) Prandtl law

I got this question during an interview.

This intriguing question originated from Couette Flow topic in section Numerical Techniques for Nonlinear Supersonic Flow, Elements of Hypersonic Flow, Couette and Poiseuille Flow of Aerodynamics

Answer 1

The correct answer is (a) Shear stress law

To explain I would say: Newtonian fluids are characterized by those fluids that obey the shear stress law. Aeronautics domain makes use of newtonian fluids only (air, other gases). Those fluids that don’t obey shear stress law are known as non – newtonian fluids (blood).

The shear stress law is given by:

τ = μ(\(\frac {∂u}{∂y}\))

This formula helps in obtaining the shear stress at any point in the fluid flow.