Concept of Fluid System

CONCEPT OF SYSTEM

A system or fluid system is a certain mass of fluid with in a closed surface. When fluid flows through a system or when fluid is compressed in the system, it is deformed. So the size and shape of the system is changed with respect to time, i.e. the fluid system moves and also deforms.

1. Continuum concept of system

The molecules are closely spaced which create strong intermolecular cohesive forces in the liquid. Thus the liquid behaves as a continuous mass. Therefore it is not needed to find out motion of individual molecules but the overall motion of the fluid. The fluid as continuous and homogeneous medium is called continuum. From the continuum view point, the overall properties and behavior of fluids can be studied without considering its atomic and molecular structure.

2. Concept of control volume

A control volume is a fixed region in space which does not move or change shape as shown in figure 1.19. The fluid flow in and out of this fixed region. Its closed boundaries are called control surface. A control volume may either be infinitesimally small or as large as desired. It should have one inlet and one outlet. Therefore, it is a properly selected region in a space. There is no specific rule for selecting control volume, but proper choice will make the analysis easier.

An infinitesimal control volume is called a differential control volume. A differential control volume is assumed to have very small but non-zero dimensions. The shape of the differential control volume may be chosen arbitrarily: a parallelepiped for the rectangular coordinates and segment of a hollow cylinder for the cylindrical coordinates. Fluid may enter and leave the control volume across the control surface. The net rates of change of fluid mass, momentum and energy across the control surface are called mass flux, momentum flux and energy flux. There may in addition be rate of mass generation within a control volume this may or may not be accompanied by rates of momentum and energy generations.

For the purpose of understanding the changes that takes place in the fluid characteristics we may introduce a control volume. So that we may compare the flow characteristics of a fluid just before it enters the control volume and just after it leaves the control volume.