Real and Ideal fluid introduction, characteristics and types

What is Fluid mechanics?

Fluid mechanics is a branch of engineering science which deals with the behavior of fluids under the conditions of rest and motion. Hydraulics is a part of fluid mechanics where fluid considered is only water. Actually, the word hydraulics is derived from the Greek word 'Hudour which means water. Therefore, hydraulics is one of the most ancient branches of civil engineering and its practical knowledge is very essential. When the fluid is at rest, it is called fluid static whereas fluid in motion while pressure forces are not considered is called fluid kinematics. But when the pressure forces are considered for the fluid in motion, it is called fluid dynamics.

What is Fluid?

The fluid is a substance that deforms continuously when subjected to shear force. This continuous deformation of a substance is called fluidity or fluid flow. The fluid is generally in the form of either only on a liquid or a gas form available. The common examples of fluid are water, mercury, oils, air, gases, and vapors, etc.

Characteristics of a fluid

The following are the characteristics of a fluid given below.

1. It has no definite shape of its own. It takes generally always the form of the container during which it's contained.
2. It offers no resistance to shear deformation. A shearing force on fluid changes its shape and when a shearing force acts on a fluid, it flows but when fluid is at rest no shear force acts on it.

Types of fluids

There are two main types of fluids given below.

1. Ideal fluids
2. Real fluids

1. Ideal fluids (Perfect fluid)

The fluids which are without viscosity, surface tension, shearing force, and compressibility are called ideal fluids or perfect fluids.

Ideal fluids are all imaginary fluids and do not exist in nature with such properties. However, a few fluids such as air and water have exceedingly low viscosity, surface tension, and great resistance to compression are treated as ideal fluids for all practical purposes as their assumptions are helpful in simplifying the mathematical analysis.

2. Real fluids or practical fluids or actual fluids

The fluids which have viscosity, surface tension, shearing force, and compressibility are called real fluids. All the fluids, in actual practice, are real fluids and they offer a certain amount of resistance during fluid motion. Real fluids are further sub-divided as given below.

i.Newtonian fluids

The real fluids, in which the shear stress is directly proportional to the rate of shear strain or velocity gradient are known as Newtonian fluids as shown in Fig. 1.1. Examples of Newtonian fluids are water, air, kerosene, thin lubricating oils, and glycerine, etc. These fluids follow Newton's law of viscosity.

ii.Non-Newtonian fluids

The real fluids in which the shear stress is not proportional to the rate of shear strain or velocity gradient are known as Non-Newtonian fluids. These fluids don't obey Newton's law of viscosity.

iii. Ideal plastic or Bingham fluids

The fluids during which shear stress is over the yield stress and also the shear stress is proportional to the rate of shear strain or velocity gradient are mainly known as ideal plastic fluids. The examples of ideal plastic fluids are water suspension of clay and fly ash.

iv.Thixotropic fluids

These are Non-Newtonian fluids that have a non-linear relationship between the shear stress and also the rate of angular deformation beyond certain yield stress. The examples of thixotropic fluids are crude oils, bentonitic drilling fluid and printer's ink, etc.

Main Differences between liquids and gases

The points of differences between liquids and gases are as explained the following below. 

Liquids

1. A given mass of liquid has a definite volume but no size or shape. It generally takes the shape of a container in which it is contained and changes its shape easily to acquire the shape of its container.
2. A free surface is formed if the volume of the container is always bigger than that of the liquid.
3. Liquids are mainly considered as incompressible for all practical purposes.
4. The pressure and temperature changes have practically no effect on the volume of a liquid
5. The liquids have an always high specific mass.
6. Water, oils, kerosene, petrol, and mercury are liquids.

Gases

1. A given mass of gas always some fluctuation it has no fixed volume. It expands continuously to adjust completely to fill the container during which it's placed.
2. The gas is mainly no free surface is formed. 
3. Gases are readily compressible.
4. A gas expands infinitely in the absence of pressure and contracts easily under liquid pressure.
5. The gases generally have a really very low specific mass.
6. Air, ammonia, and carbon dioxide are gases.

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