Fluids in Motion:

Whether it is the flow of water in rivers or in pipes, or the flow of blood in the blood vessels, these all motions of fluids are governed by the principles of fluid dynamics. In reality, the motion of fluids is a rather complex phenomenon. To keep our discussion of fluid motion simpler, we make certain simplifying assumptions as given below:

(a) The fluid is incompressible. This condition is ordinarily satisfied by liquids but not by gases. Therefore, if a gas is not subjected to large change in pressure, it could also be considered incompressible.

(b) The fluid is non-viscous. That is, fluid motion does not suffer any friction. This condition is similar to ignoring rolling or sliding friction in mechanics. (The motion of viscous fluid has been elaborates further on viscosity.)

(c) The flow of fluid is steady. This means which the fluid velocity does not change with time at a given position. Note that this does not mean that the velocity of fluid is same at all positions in the body of the fluid.

(d) The fluid motion is irrotational. This implies that if a paddle wheel is placed in the flowing fluid, it will not rotate.

Above simplifying assumptions enable us to analyse the behaviour of a fluid in motion without using complex mathematical techniques. Further, before you study the dynamics of a incompressible, non-viscous, steady and irrotational fluid, it is advisable to understand the concepts of streamline motion and turbulent motion.