Fluids may be classified as follows:
• Ideal fluid
• Real fluid
• Newtonian fluid
• Non-Newtonian fluid.
An ideal fluid is one, which is incompressible and has no viscosity. Such a fluid is only imaginary, as all existing fluids possess some viscosity.
Simply speaking, a fluid which possesses viscosity is known as a real fluid. All fluids, in actual practice are real fluids.
During the course of our discussion on viscosity, we have seen that shear stress is proportional to the velocity gradient i.e. ra dv/dy. A real fluid in which the shear stress is proportional to the velocity gradient is known as a Newtonian fluid.
A real fluid in which the shear stress is not proportional to the velocity gradient is known as a non-Newtonian fluid.