Lecture 1 : Basic Concepts


Properties of Fluid

Any characteristic of a system is called property . It may either be intensive (mass independent) or extensive (that depends on size of system). The state of a system is described by its properties. The number of properties required to fix the state of the system is given by state postulates. Most common properties of the fluid are:

1. Pressure (p): It is the normal force exerted by a fluid per unit area. More details will be available in the subsequent section (Lecture 02). In SI system the unit and dimension of pressure can be written as, N/m2 and ML-1T-2, respectively.

2. Density: The density of a substance is the quantity of matter contained in unit volume of the substance. It is expressed in three different ways; mass density , specific weight (ρg) and relative density/specific gravity . The units and dimensions are given as,

For mass density; Dimension: ML-3 Unit: kg/m3

For specific weight; Dimension: ML-2T-2 Unit: N/m3

The standard values for density of water and air are given as 1000kg/m3 and 1.2 kg/m3, respectively. Many a times the reciprocal of mass density is called as specific volume .

3. Temperature (T): It is the measure of hotness and coldness of a system. In thermodynamic sense, it is the measure of internal energy of a system. Many a times, the temperature is expressed in centigrade scale (°C) where the freezing and boiling point of water is taken as 0°C and 100°C, respectively. In SI system, the temperature is expressed in terms of absolute value in Kelvin scale (K =°C+ 273).

4. Viscosity (u): When two solid bodies in contact, move relative to each other, a friction force develops at the contact surface in the direction opposite to motion. The situation is similar when a fluid moves relative to a solid or when two fluids move relative to each other. The property that represents the internal resistance of a fluid to motion (i.e. fluidity ) is called as viscosity . The fluids for which the rate of deformation is proportional to the shear stress are called Newtonian fluids and the linear relationship for a one-dimensional system is shown in Fig. 1.1.2. The shear stress (τ) is then expressed as,


where, is the shear strain rate and μ is the dynamic (or absolute) viscosity of the fluid.

The dynamic viscosity has the dimension ML-1T-1 and the unit of kg/m.s (or, N.s/m2 or Pa.s). A common unit of dynamic viscosity is poise which is equivalent to 0.1 Pa.s. Many a times, the ratio of dynamic viscosity to density appears frequently and this ratio is given by the name kinematic viscosity . It has got the dimension of L2T-1 and unit of stoke (1 stoke = 0.0001 m2/s). Typical values of kinematic viscosity of air and water at atmospheric temperature are 1.46 x 10-5 m2/s and 1.14 x 10-6 m2/s, respectively.

Fig. 1.1.2: Variation of shear stress with rate of deformation.