Fluid - Fluid is a substance which offers no resistance to shear deformation and will continue to from when subjected to shear stresses . It doesn't have a definite shape.If change its shape due to shear force and is said to flow .
Fluid can be classified into liquid and gases .liquid have comparatively high mass density compressibility  as compared to those of gases. Liquids have one free surface while gases do any free surfaces.

Gas -  It Possessed no definite volume and is compressible .

Vapour -  It is a gas whose temperature and pressure are such that it is very near the liquid state .

Ideal fluid - An ideal fluid is one which has no viscosity and tension and is incompressible  

{ ideal fluid has no surface tension }
{ Ideal fluid are imaginary and do not exist in nature }

Real fluid -  A real fluid is one which has viscosity ,surface tension and compressibility in addition to the density .

liquid & there properties 
A liquid can be easily distinguished from a solid or a gas .
A solid has a definite shape .
A liquid takes the shape of vessel into which it is poured .
A gas completely fills the vessel which contains it .

The properties of water are of much importance because the subject of hydraulics is mainly concerned with it. Some important properties of water which will be considered are.
1. Density        2. Vapour pressure     3. Specific gravity         4. Viscosity          
5. Capillarity    6. Cohesion                7. Surface tension      

Types of fluid - 
There are types of fluid .
1. Ideal fluid .
2. Real fluid .
3. Newtonian fluid .
4. Non-Newtonian fluid .
5. Ideal plastic fluid .

Different types of head (or energies ) of a liquid in Motion -
There are three types of energies or heads of flowing liquids ;
1. Potential head or potential energy .
2. Velocity head or kinetic energy .
3. Pressure head or pressure energy .

Total head,    H= z+ v²/ 2g + p/w     m of liquid

Total energy ,  E= z+v²/2g +p/w  Nm/kg  of liquid .

Fluid Machines 
Hydraulic Coupling - is a device which is employed for transmission of power form one shaft to another though a liquid medium .It has no mechanical connection or face to face contact . The magnitudes of input and output torques are equal .

Uses of hydraulic coupling - Althrough a hydraulic or fluid coupling has a low valve of transmission efficiency when compared to mechanical coupling yet it is widely used in the following fields .

Automobiles , marine , ropeway .

Fluid pump -
A pump is contrivance which provides energy to a fluid assists to increase the pressure energy or both of the fluid by converting the mechanical energy .

classification of pump 
on the basic of transfer of mechanical energy the pumps can be broadly classified as follow.
1. Rotodynamic pumps .
(i) Radial flow pumps (ii) Axial flow pumps
(iii) mixed flow pumps.
2. Positive displacement pumps.


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