- Properties
- Viscosity
- Fluid Flow in a pipe
- Static, Dynamic & Total Pressure
- Head Loss
- Fluid Hammer
- Reynolds Number
- Laminar & Turbulent Flow
- Critical Velocity

Matter is primarily found in three forms: solids, liquids and gases. Gases and liquids together are called fluids. The molecules of a solid are usually mutually closer than those of a fluid. The attractive forces between the molecules of a solid are so large that a solid tends to retain its shape. This is not the case for a fluid, where the attractive forces between the molecules are smaller. There are plastic solids, which flow under the proper circumstances, and even metals may flow under high pressures. On the other hand there are certain viscous fluids liquids that do not flow readily and is easy to confuse them with plastic solids. The distinction is that any fluid, no matter how viscous will yield in time to the slightest stress. But a solid, no matter how plastic, requires a certain magnitude of stress to be exerted before it will flow. Also when the shape of a solid is altered by external forces, the tangential stresses between adjacent particles tend to restore the body to its original configuration. With a fluid, these tangential stresses depend on the velocity of deformation and vanish as the velocity approaches zero. When motion ceases, the tangential stresses disappear and the fluid does not regain its original shape.

**Properties of
fluids:**

- Density, Specific weight, specific
volume and specific gravity: The
*density*of a fluid is its mass per unit volume, while the*specific weight*is the weight per unit volume. They are related as g = r g. The*specific volume*n is the volume occupied by a unit mass of fluid. Specific volume is the reciprocal of density. n = 1/*r*.*Specific gravity**s*of a^{0}F as this temperature. Specific gravity of a - Compressible and incompressible fluids: Fluids may be either of a constant or variable density. There is nothing like an incompressible fluid, but the term is applied to those fluids whose density changes negligibly with pressure. Liquids are ordinarily considered incompressible fluids, but sound travels through them demonstrating that they are elastic.
- Compressibility: This is defined as
the change in volume due to change in pressure. It is inversely
proportional to its volume modulus of elasticity, also known
*as bulk modulus*. The bulk modulus is analogous to the modulus of elasticity of solids; however for fluids it is defined on a volume basis. - Vapour pressure of liquids: As all liquids tends to evaporate or vaporize, which they do by projecting molecules into the space above their surfaces. The pressure exerted by the molecules increases till some of the molecules start re-entering the liquid. This pressure is called the vapour pressure.

- Kinematic viscosity
- Dynamic viscosity

where P is the pressure, r is the density and Z is the head of the fluid.

C is any arbitrary constant.

where the subscripts 1 and 2 denote any two positions.

This equation is called Euler's equation and it implies that at different
points in the pipe the energy of the fluid is constant. This equation does
not take the frictional losses in the pipe into account.

Where P

P = static pressure

r = density of the fluid

v = velocity of flow.

The term (1/2) r v^{2} has the dimensions of pressure and is called
the dynamic pressure of the fluid.

p_{h} = f (r ,
u_{0}, c_{p} ).

The fluid hammer causes pressure fluctuations in the fluid in the pipe because of which the pipe expands and contracts. This is a critical problem in case of power plants, where the flow of water must be varied rapidly in proportion to the load changes on the turbine. Incidentally, the pressure wave is always set up as a result of abrupt decrease in velocity.

For circular pipes flowing full, Reynolds number

where V = mean velocity of fluid

D = diameter of the pipe

n = kinematic viscosity f the fluid

r = mass density of the fluid

m = absolute viscosity of the fluid.

For non circular cross sections, the ratio of the cross sectional area to
the wetted perimeter is used as the Reynolds number.

In turbulent flow the particles of the fluid move in a haphazard fashion in all directions. It is impossible to trace the motion of an individual particle. There is more uniform distribution of velocity. The Reynolds number determined for a turbulent flow is greater than 2000.