8. Specific heats : It is the amount of energy required for a unit mass of a fluid for unit rise in temperature. Since the pressure, temperature and density of a gas are interrelated, the amount of heat required to raise the temperature from T₁ to T₂ depends on whether the gas is allowed to expand during the process so that the energy supplied is used in doing the work instead of raising the temperature. For a given gas, two specific heats are defined corresponding to the two extreme conditions of constant volume and constant pressure.

1. (a)  Specific heat at constant volume (c_v)

(b)  Specific heat at constant pressure (c_p)

The following relation holds good for the specific heat at constant volume and constant pressure. For air ; c_p = 1.005 KJ/kg.K c_v = 0.718 KJ/kg. K

(1.1.10)

9. Speed of sound (c): An important consequence of compressibility of the fluid is that the disturbances introduced at some point in the fluid propagate at finite velocity. The velocity at which these disturbances propagate is known as “acoustic velocity/speed of sound”. Mathematically, it is represented as below;

(1.1.11)

In an isothermal process,

(1.1.12)

In isentropic process,

(1.1.13)

10. Vapour pressure (p_v): It is defined as the pressure exerted by its vapour in phase equilibrium with its liquid at a given temperature. For a pure substance, it is same as the saturation pressure. In a fluid motion, if the pressure at some location is lower than the vapour pressure, bubbles start forming. This phenomenon is called as cavitation because they form cavities in the liquid.

11. Surface Tension (σ): When a liquid and gas or two immiscible liquids are in contact, an unbalanced force is developed at the interface stretched over the entire fluid mass. The intensity of molecular attraction per unit length along any line in the surface is called as surface tension. For example, in a spherical liquid droplet of radius (r), the pressure difference (Δp) between the inside and outside surface of the droplet is given by,

(1.1.14)

In SI system the unit and dimension of pressure can be written as, N/m and MT^-2, respectively.