Types of fluid

The fluid may be classified into the following five types :

1. Ideal fluid
2. Real fluid
3. Newtonian fluid
4. Non-Newtonian fluid
5. Ideal plastic fluid

Ideal fluid :

Ideal fluid is a fluid which is incompressible and is having no viscosity.

Ideal fluid is only an imaginary fluid.

Real fluid :

The real fluid is a fluid which possesses viscosity.

All fluid is in actual practice is real fluid.

Newtonian fluid :

A real fluid, in which the shear stress is directly proportional to the rate of strain is known as a Newtonian fluid.

Non-Newtonian fluid :

A real fluid, in which the shear stress is not proportional to the rate of strain is known as a Newtonian fluid.

Ideal plastic fluid :

A fluid, in which shear stress is more than the yield value and shear stress is proportional to the rate of shear strain is known as ideal plastic fluid.

What is specific gravity

Specific gravity is defined as the ratio of the weight density of a fluid to the weight density of a standard fluid.

For liquid standard fluid is taken as water and for gas is air.

Specific gravity is also called relative density.

Specific gravity is a dimensionless quantity.

It denoted by S.

S ( for liquid ) = Weight density of liquid / Weight density of water

S ( for gases ) = Weight density of gas / Weight density of air

What is specific volume

A 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 is called specific volume.

Specific volume = Volume of fluid / Mass of fluid 

Specific volume is also reciprocal of mass density. 

The unit of specific volume is m³/kg.

Specific volume is mostly applied to gases.

What is specific weight

The specific weight of a fluid is the ratio between the weight of a fluid to its volume.

Specific weight is also called weight density.

Weight per unit volume of a fluid is called weight density.

It denoted by w.

w = Weight of fluid / Volume of fluid 

What is density

Density is a degree of compactness of a substance.

Density is defined as the ratio of the mass of a fluid to its volume. thus mass per unit volume of a fluid is called density.

It denoted as ρ.

ρ = Mass of fluid / Volume of fluid

The unit of density :

SI Unit : kg / cubic meter = kg/m³

The density of liquids may be considered as constant while that of gases changes with the variation of pressure and temperature.

The density of water is 1 gm/cm³ or 1000 kg/m³.

Density and viscosity both of them have a certain similarity. 
Don't get confused with each other terms. You can go to the difference between viscosity and density and clear your confusion. 

What is viscosity

Viscosity definition :

Viscosity is the property of a fluid which determines its resistance to shearing stresses.

Cause of Viscosity: 

It is due to cohesion and molecular momentum exchange between fluid layers.

Newton’s Law of Viscosity: 

It states that the shear stress (τ) on a fluid element layer is directly proportional to the rate of shear strain.

T = µ (du/dy)

Where µ = Coefficient of dynamic viscosity 

Units of Viscosity :

S.I Units : Pa.s or N.s/m²

C.G.S Unit of viscosity is Poise = dyne-sec/cm²

One Poise = 0.1 Pa.s

1/100 Poise is called centipoises.

Dynamic viscosity of water at 20 ⁰C is approx= 1 cP

Kinematic Viscosity :

Kinematic viscosity is the ratio between the dynamic viscosity and density of the fluid.

It is denoted by ν.

ν = dynamic viscosity / density 

Units of Kinematic Viscosity :

S.I units: m²/s

C.G.S units: stoke = cm²/sec

One stoke = 10^-4 m²/s

Viscosity and density are two same terms but there is a difference between them. Let we check it out the difference between viscosity and density.

Characteristics of fluid

What is fluid :

A fluid is a substance which deforms continuously when it is subjected to external shearing forces.

Characteristics of Fluid :

• It has no proper shape of its own but conforms to the shape of the containing vessel.
• A fluid can be defined unambiguously as a material that deforms continuously and permanently under the application of shearing stress, no matter how small.
• The fluid has elastic properties only under direct compression.

1. Ideal Fluid

An ideal fluid is one which has

• No viscosity
• No surface tension
• Incompressible

2. Real Fluid

A Real fluid is one which has

• Viscosity
• Surface tension
• Compressible

Naturally available all fluids are real fluid.

Flow and Pressure

There must be a minor difference between flow and pressure. 

Water Flow :
Water Flow is a measurement of how much water is delivered at a particular outlet over a set period of time. For example, if you place a 10 litres bucket under the tap in a sink and it takes 10 seconds to fill the bucket you can see that the flow rate is 1 litre per second.

Water Pressure :
Water Pressure is a measurement of the force exerted by the water. 
We understand it by one example a cold water storage cistern in the attic may be used to supply water to a basin in a bathroom and a basin in a downstairs cloakroom. Assuming everything else is equal you will notice that the pressure at the downstairs tap is considerably more than that at the one upstairs. The increased pressure is due to the height of the cistern in relation to the tap.

Higher pressure will cause greater flow through any given pipe size, but as the flow increases, the pressure will decrease downstream due to friction loss because water velocities increase as well.

For any flow to happen, there is a requirement of pressure gradient and not the pressure. Higher the pressure gradient, keeping all other things (fluid, pipe diameter, length) constant, higher is the mass flow rate. But higher pressure does not reveal anything about the flow.

• The pressure is defined as the force acting perpendicular to the surface of an object per unit area over which the force is distributed.
• In the case of gases, this force is because of the collision of the gas particles with the surface. So, the pressure exerted by the gases in a given environmental condition is more of a statistical average than average value.
• In terms of liquids, the pressure exerted is the weight of the liquid over a surface, acting on that surface.

• The definition of flow is subjective depending on the time and length scales considered. Since we defined flow as the bulk movement of fluid particles.

If we need more water, so increase the pipe size so we don't lose more pressure to friction loss. 

PSI:
Pounds per square inch, the standard measurement of pressure in the United States.

Water Velocity:
The accepted standard for water velocity in piping systems is 5 feet per second or less. As flow increases in any given pipe size, the velocity of that water also increases. As velocity and/or flow increases in any given pipe size, the PSI loss also increases. The means of decreasing pressure loss for a given flow is to increase pipe size. (diameter)

Friction Loss:
The PSI loss which results from friction against the interior walls of pipes, directional fittings, valves or any other obstruction to the irrigation water. Once again, as flow increases so do friction loss. Friction loss is synonymous with PSI loss.

GPM, GPH, GPD:
Gallons Per Minute, the standard measure of flow; Gallons Per Hour, often used for low-volume flow such as drip irrigation; Gallons Per Day, a measure of overall water use on a daily basis.

Feet of Head:
Another term for water pressure. The pressure is directly affected by elevation change, and every 2.31 vertical feet of change upwards will decrease pressure by 1 psi in a holding tank. That is why such enormous pressures exist in the deep ocean; enough to crush a submarine as depth increases. Another way to look at it: each 1 foot elevation change equals .433 "feet of head".

Total Dynamic Head:
TDH is a measure of overall head (pressure) loss in a water system. When an irrigation designer or engineer calculates all of the friction (pressure) losses and outlet pressure required for an irrigation system, they will express the number as TDH. If an irrigation system has a maximum TDH of 250, that means that just over 108 PSI will be required to power the system.

Conversion Factors:

Pressure
To Convert From:	Into:	Multiply By:
PSI	Feet of Water	2.307
PSI	Pounds/Sq.Foot	144
PSI	Atmospheres	.06805
PSI	Bars (metric)	.06895
PSI	Inches Water @ 39.2 F	27.681
PSI	Millimeters Mercury @ 0 C	51.715
Feet of Head	PSI	.433501
Bars (metric)	PSI	14.5038
Bars	Feet of Head	33.4883
Bars	Pounds/Square Foot	2089
Bars	Atmospheres	.98692
Bars	Centimeters Mercury @ 0 C	75.0062
Bars	Inches Mercury @ 32 F	29.53

Flow
To Convert From:	Into:	Multiply by:
GPM	Gallons/Hour	60
GPM	Cu. Feet/Second	.002228
GPM	Cu. Feet/hour	8.0208
GPM	Cu. Meters/Second (metric)	.000063
GPM	Cu. Meters/Hour	.2268
GPM	Liters/Second (metric)	.06308
GPM	Liters/Minute	3.7853
GPM	Acre-Feet/Day	.0044192
GPM	Millions Gallons/Day	694.444
Millions Gallons/Day	Acre-Feet/Day	3.0689
Millions Gallons/Day	Acre-Inches/Day	36.8266
Millions Gallons/Day	Gallons/Hour	41,666.667
Millions Gallons/Day	GPM	.00144
Liters/Minute (metric)	GPM	.26418
Liters/Minute	Gallons/Second	.004403
Liters/Minute	Cu. Feet/Second	.000588
Liters/Minute	Cu. Feet/Minute	.0353