FLUID PROPERTIES IN FLUID MECHANICS AND ARCHIMEDES PRINCIPLE,BUOYANCY


PROPERTIES OF FLUID IN FLUID MECHANICS


There are various properties which are aught to be understood when we are studying fluid mechanics. These properties of fluid are:-

  1. Mass density 
  2. Weight density 
  3. Specific gravity 
  4. Viscosity 
  5. Vapour pressure 
  6. Surface tension 
  7. Capillarity 

One by one each topic is elaborated.

1. Mass density:-

It is defined as mass contained in unit volume of substance. In "v" volume of substance "m" kg of mass is present, then density can be given as:-

DENSITY= MASS/VOLUME

unit is kg/m^3 or gram/liter or gm/cc


Points to remember:-

As the pressure increases the mass density increases (for compressible fluids) With increase in temperature mass density decreases and it is unaffected by the location.

2. WEIGHT DENSITY:-

It is also called specific weight and defined as weight per unit volume. Let "w" be the weight in NEWTON of "V" m^3 of fluid, then weight density may be given as



                             mg          m
Wt density=      -----------  =(----)g =density*g
                             V             V



unit=N/m^3

density of water = 1000 kg/m^3
wt density of water = 9.81*100 N/m^3
density of air = 1.92 kg/m^3
Wt density of air = 11.71 N/m^3


3. SPECIFIC GRAVITY:-

It is nothing but the mass density of fluid to the mass density of water.

                              DENSITY OF FLUID
Specific gravity=   ------------------------
                               Density of water


It is dimensionless quantity.
If specific gravity ratio is greater then 1, it means it is heavier than water.

4. VISCOSITY:-

In the last blog we have discussed newtons law of viscosity.

It is the resistance is the path of fluid flow. When a layer of fluid obstruct the flow of subsequent layer of fluid, it restricts its flow. This is called viscosity.




These are of two types:-

Absolute viscosity
Kinematic viscosity



5. VAPOUR PRESSURE :-



  • It is the pressure exerted by its own vapour in the case of liquid .
  • With increase in temperature vapour pressure increases. 
  • mercury has low vapour pressure and high density, hence used in barometer.
6. CAPILLARITY:-
It is the rise or fall of liquid in small diameter tube. There is cohesion between liquid molecule and adhesion between liquid and glass molecule.    



fig:-capillarity


BUOYANCY AND ITS FACT



Before I start it is very important to know principle of Archimedes:-


According to Archimedes,when a body is immersed in fluid completely of partially, it is forced upward by the amount of fluid displaced by object.

 







What is buoyancy?
It is the resultant force on body or object which is partially or wholly immersed in static fluid.

As we move down to fluid column, the pressure increases. This is due to weight of overlying fluid. If a object is kept at bottom of fluid column it will experience greater force as compared to the fluid object kept at upper surface.


The layer of fluid of above layer creates pressure due to its weight on the lower level of fluid, this creates difference in pressure and then a new term comes in light called BUOYANCY.
The difference in pressure generates net force leading to net upward force on the object.


You would be thinking what would be the magnitude of force?

The magnitude of net force will be equal to the difference in pressure of top and bottom level of fluid.
 The major factor that plays vital role in fluids is DENSITY. If the density of object is greater than that of fluid then the object tends to sink. If the density of object is approximately similar to that of the fluid then it tends to float (As in case of boat).

buoyancy force = weight of object in empty space - weight of object when immersed in fluid.


CENTER OF BUOYANCY:-



It is point of application of resultant buoyancy force. It is also the center of gravity of volume of fluid displaced.

  
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