Fluid Properties - Theory

FLUID PROPERTIES:

1.           Density or Mass density: Density or mass density of a fluid is defined as the ratio of the mass of a fluid to its volume. Thus mass per unit volume of a is called density.

Mass density fluid / Mass of  Density of  fluid

The unit of density in S.I. unit is kg/m3. The value of density for water is 1000kg/m

2.Specific weight or weight density: Specific weight or weight density of a fluid is the ratio between the weight of a fluid to its volume. The weight per unit volume of a fluid is called weight density.

Weight density   = Weight of  fluid / Volume of  fluid

w = Mass of  fluid  x g / Volume of  fluid

The unit of specific weight in S.I. units is N/m3. The value of specific weight or weight density of water is

9810N/m3.

3.)Specific Volume: Specific volume of a fluid is defined as the volume of a fluid occupied by a unit mass or volume per unit mass of a fluid.

Specific volume = Volume of  a fluid / Mass of  fluid

Thus specific volume is the reciprocal of mass density. It is expressed as m3/kg. It is commonly applied to gases.

4.)Specific Gravity: Specific gravity is defined as the ratio of the weight density of a fluid to the weight density of a standard fluid.

Specific gravity = Weight density of liquid / Weight density of water

VISCOSITY

Viscosity is defined as the property of a fluid which offers resistance

to the movement of one layer of fluid over adjacent layer of the fluid. When two layers of a fluid, a distance 'dy' apart, move one over the other at different velocities, say u and u+du as shown in figure. The viscosity together with relative velocity causes a shear stress acting between the fluid layers.

COMPRESSIBILITY:

Compressibility is the reciprocal of the bulk modulus of elasticity, K which is defined as the ratio of compressive stress to volumetric strain.

Consider a cylinder fitted with a piston as shown

in figure. Let  V= Volume of a gas enclosed in the

cylinder

P= Pressure of gas when volume is V

Let the pressure is increased to p+dp, the volume of gas decreases

from V to V-dV. Then increase in pressure =dp kgf/m2

Decrease in volume= dV

Volumetric Strain = d / V

- ve sign means the volume decreases with increase of pressure.

Bulk modulus K = Increase pressure / Volumetric Strain

= dp / dV/V

Compressibility is given by = 1/K

Relationship between K and pressure (p) for a Gas:

The relationship between bulk modulus of elasticity (K) and pressure for a gas for two different processes of comparison are as:

(i)          For Isothermal Process: The relationship between pressure (p) and density (?) of a gas as

 p = Constant

V  = Constant

Differentiating this equation, we get  (p and V are variables)

PdV +Vdp = 0

or   

pdV= - Vdp      

or

p/dV         = Vdp        

Substituting this value K =p                                      

(ii) For adiabatic process.  For adiabatic process                                   

p    Constant  or         pVk = Constant  

SURFACE TENSION:

Surface tension is defined as the tensile force acting on the surface of a liquid in contact with a gas or on the surface between two two immiscible liquids such that the contact surface behaves like a membrane under tension

CAPILLARITY

Capillarity is defined as a phenomenon of rise or fall of a liquid surface in a small tube relative to the adjacent general level of liquid when the tube is held vertically in the liquid. The rise of liquid surface is known as capillary rise while the fall of the liquid surface is known as capillary depression. It is expressed in terms of cm or mm of liquid. Its value depends upon the specific weight of the liquid, diameter of the tube and surface tension of the liquid.