Properties of Fluids

We know about fluid and students every fluid(liquids, vapours, gases) has definite properties which help us to describe its physical condition. Such properties are called properties of fluid. It also  helps us to analysis fluid flow problem.

PROPERTIES OF FLUIDS
There are main properties of fluids following :-

1. Density or Mass Density:- It is the ratio of mass of a fluid to its volume. It is de noted by ρ(ro).

    Mass Density,.   p = Mass of fluid/volume of fluid
                                 =m/v
SI Unit of density or Mass Density is Kg/cubic meter.
  The density of water is 1000kg/cubic meter or 1 gm/cubic centimeter.

2. Specific Weight:- It is defined as the ratio of the weight of a fluid to its volume. It is denoted by 'w' and is also called weight density.
         w =.       Weight of fluid/volume of fluid 
               = mass of fluid ×g/volume of fluid
              =     Density of fluid× g.
              =.   ρg
Unit of specific weight is N/cubic meter.
Specific weight of water is 9.81KN/cubic meter.
   It depends upon gravitational acceleration. It changes from one place to another.

3. Specific Volume:- It is the ratio of the volume of a fluid to its mass.
   Specific Volume = Volume of fluid/Mass of fluid                   =.  1/ρ
                         ρ  = density of fluid
Unit of specific Volume is cubic meter/Kg.

4. Specific Gravity:- it is defined as the ratio of the specific weight of fluid to that of a standard fluid. For liquid , the standard fluid is water and for gases it is air. It is denoted by S.
       S = specific weight of fluid/specific weight of standard fluid(water in case of liquid and air in case of gas).

It is dimensionless quantity.
In case of liquid, Specific gravity of liquid is the ratio of specific weight of liquid to specific weight of water.
          S(for liquid) = w of liquid/ w of water.
                   Specific weight of liquid = S(for liquid)× Specific weight of water.
And as we know that specific weight of water is 9.81KN/cubic meter.
    So,     Specific weight of liquid = S × 1000 × 9.81N/cubic meter.
     So,   Density of liquid. = S × density of water.
          Density of liquid = S× 1000kg/cubic meter.

5. Viscosity:-  If the layers of a fluid slip or tend to slip on another layers in contact, then any two such layers exert a tangential force on each other.The direction of these forces are such that the relative motion between the layers are opposed.This property of the fluid to oppose relative motion between it's layers is called viscosity. The forces between the layers opposing the relative motion between them are called the forces of viscosity.Thus, viscosity may be thought to be an internal friction of the fluid in motion​. It is the property of a fluid by virtue of which it offers resistance to shear stress.
  Coal tar, colgate, honey and mud are highly viscous where as water and alcohol have very low viscosity.

        As you see that in above figures. As we move from the lowermost layer to upward the flow velocities of the liquid layers go on increasing. Hence we can say that every lower layer has lesser velocity than consecutive upper layers.In fact every upper layer tries to drag forward every consecutive lower layer, but a lower layer tries to pull consecutive upper layer backwards.
      If a liquid flows in a cylindrical pipe or tube  a velocity curve is obtained which is parabolic and the velocity of the mid. Layer is the maximum and decrease for other adjacent layers.
So, velocity is the internal characteristics of liquid and gases which are produced by the cohesive forces of the molecules. When the distance among the molecules increases and it is opposed by the cohesive forces and thus viscosity produced. In gases viscosity is generated due to molecules transfer from one to another place. That is why, viscosity of gases is less as compare to liquid.

Shear resistance, τ = F/A
  A is area of contact between the two layers.
       The shear stress is produced between the layers is found to be proportional to the rate of change of velocity in direction perpendicular to the layers with respect to distance y.
              
                  τ is directly proportional to du/dy
       
         τ = μdu/dy
u is the constant of proportionality and is known as dynamic viscosity or absolute viscosity.
   if the velocity variation of fluid layers is uniform, then
              τ = μdu/dy.
            μ = τ/du/dy.
Unit of viscosity is Pas(Pascalsecond) or  Nsec/square meter.

6. Kinematic viscosity:- it is the ratio of dynamic viscosity of fluid to its mass density( density). It is denoted by v(nu).
        ν = viscosity/density = μ/ρ

Units of kinematic viscosity is square meter/second.
    
            The viscosity of liquid is decrease with increase in temperature and viscosity of gases increase in increase in temperature.

7. Cohesion is a property of a liquid due to which the molecules of the liquid are attracted towards each other. Due to this property  liquid tends to gather into droplets when poured on smooth horizontal surface.

8. Adhesion is a property of a liquid which enables it to adhere to another body in contact. Due to this property liquids don't gather into droplets when poured on smooth horizontal surface.

9. Surface Tension :- It is a property of the surface of liquid that causes it to behave as an elastic sheet. It is defined as the force required to maintain unit length of the sheet in complete equilibrium and is equal to the product of the surface tension co efficient and the length of the free surface. It is denoted by sigma.

Unit of surface tension is N/m.

10.Capillarity:- It is defined as the phenomenon of the rise or fall of a liquid in a thin tube(capillary tube)relative to the adjacent general level of liquid. The rise of liquid surface is known as capillary rise and fall of liquid surface is known as capillary depression.
Factor affecting capillarity
1. Diameter of the tube
2. Specific weight of liquid
3. Surface tension of liquid.

11.Vapour Pressure:- the pressure at which liquid is converted into vapour due to evaporation is called vapour Pressure.

12. Compressibility:- compressibility of fluid is its ability of change its volume under pressure.when we apply pressure on liquid it compress and then we remove pressure it expands. The change in density in liquid is very small so for all practical cases, liquids are considered  as incompressible. The gas is considered as compressible due to change in density with pressure variation.