What is Fluid? Properties & Types of Fluids: Basic Concept

What is Fluid? Let us discuss about the properties & types of fluids to understand the basic concept. The most common type of matter that can be classified as a solid, liquid, or gas is one that can be classified as a solid, liquid, or gas. Depending on the properties of the fluids, these substances are categorized according to their properties.

There is also the possibility that liquids and gases can be defined as fluids in general terms. The definition of fluid is something that can flow or that has a tendency to flow under certain circumstances.

What is Fluid? Introduction

As we all know, fluids refer to substances that are prone to flow and are considered to be fluids because of their fluid nature. As outlined in general definitions, fluids can loosely be defined as substances that are able to spread and change shape relative to their surrounding conditions without experiencing any internal resistance while doing so as long as they exhibit a certain amount of resistance within themselves.

  • An important part of the mechanics of fluids is the study of how fluids respond when they are subjected to external forces, in order to gain a better understanding of their behavior when applied to external forces.
  • The solid will flex when placed under the influence of a shearing force as soon as its internal shear resistance equals the external pressure that is being applied to it.
  • It has been observed that the fluid does not return to its original position after removing the external force. This property of fluid can be interpreted as a characteristic that enables it to be identified as a substance that is continuously deformed when put under the influence of an external shearing force. As a result of this constant deformation of fluid, it is referred to as flow.
  • There is no formation in a solid that cannot be cured by elastic limits or if the external force is removed, the solid will regain its original shape when stretched by elastic limits.
  • There is a difference between liquids and solids when it comes to the above aspect. An external force that is applied to a fluid externally in the form of a small shearing force will cause it to start bending and it will continue to do so as long as the external force remains.

Fluid Basics: Classification of Matters

Before learning to properties of fluids or types, we need to learn the basics of fluid as well as classification of matters. The molecules and atoms in a fluid make up the fluid itself. In gaseous phases, as well as liquid phases, the distance between molecules of the fluid gas varies according to the phase of the fluid gas.

  • In gaseous phases, the distance between molecules is the largest, and in liquid phases, the shortest. A continuum flow cannot be treated as a flow when the distance between molecules or the mean free path is approaching the characteristic size of the flow device when the flow medium’s size approaches that of the flow device.
  • There is a regular lattice of molecules in solids, and they oscillate around an equilibrium point where molecules form a regular lattice.
  • There is a strong attraction between the molecules, which in this phase of the matter cannot be overcome by the kinetic energy of the molecules. Upon receiving sufficient energy, such as by heating, the molecules will melt and, consequently, turn into a liquid. By adding heat to a mixture of molecules, they gain momentum and begin to move around in an irregular way.
  • On the other hand, there is not a great deal of difference between the density of liquids and solids, which means the molecular distances in these two different states. There is a drastic drop in volume density when a liquid vaporizes and turns into a gas phase due to the molecules moving freely between the collisions of intermolecular molecules once the liquid vaporizes and turns into a gas phase.

What are the Properties of Fluid?

There are many properties of fluids that are of great importance and some of the most important ones are as follows:

The density of the fluid

As the name implies, density refers to how much mass a fluid occupies in a given volume. Basically, this means that we do not necessarily have the same density throughout the volume of a substance as we do in the case of a liquid. There is no possibility, in general, for the density of a fluid to be equal across the entire fluid as well as within it.

The Viscosity of the fluid

A fluid’s viscosity is the property that determines how resistant a fluid will be to shear stress in the event that it is moved as a unit. Generally speaking, in-viscid fluids are fluids that do not exhibit any resistance to shear stress under any condition. It is because of this property that fluid offers resistance to the flow between two layers of fluid adjacent to one another.

The temperature of a fluid

An important fluid property is a temperature. It is a measure of how hot or cold a fluid is or how much heat is in a fluid. It can be used to measure the color of the fluid. An instrument called a thermometer is generally used in order to measure the temperature of a substance & also we can measure the temperature conversions between various units.

Temperatures are usually measured using one of the following scales:-

  • Celsius which is denoted by °C
  • Fahrenheit which is denoted by °F
  • Kelvin which is denoted by K

In terms of scientific purposes, Kelvin scales are predominantly used from the International System of Units based on convections. As the Kelvin scale is not dependent on the properties of the substance it is used for scientific purposes since these properties do not affect its use.

The pressure of a fluid

In most cases, Fluid Pressure is the stress that is exerted at some point within the fluid when compression occurs. There is an approximate way to approximate the pressure in open conditions as the presence of a fluid or gas in a static or stationary condition since motion creates minimal changes in the pressure. An additional term for hydrostatic pressure is that which occurs when a fluid is not in motion and is at a stationary stage at any given moment.

The Pressure in the fluid can occur under the following two situations:-

  • Open Condition: It means in the open atmosphere in the form of gases, swimming pools, or in the oceans.
  • Closed Condition: It means any condensed air tightening area for example; gas flow lines or water flow lines.

The Specific volume of a fluid

Generally speaking, the Specific Volume of a material can be defined by how large its volume is in relation to its mass. It is determined by multiplying the density of the material by the reciprocal of its density. There is an inverse relationship between specific volume and density.

The fact that matter has this intrinsic property is something that cannot be changed. It is important to note that the intrinsic property is independent of the sample size. Additionally, it is a property of intensive nature, which means that it is independent of both the amount of material present and the location in which it was sampled.

A cubic meter per kilogram is the standard unit of specific volume.

The Specific weight of a fluid

The mass density of the fluid is the amount of weight per unit volume in a fluid. Density has a direct relationship with specific weight, and it is denoted by the term Y.


  • Y denotes the weight of the fluid
  • ρ denotes the density of the fluid
  • g denotes the acceleration force due to gravity.

Gravitational force in a fluid

Specific gravity is defined as the ratio of the particular weight of a fluid to the particular weight of a standard fluid, where the standard fluid is defined as an inorganic solution. The term relative density is also used to describe this phenomenon. There is a letter ‘S’ associated with it, which denotes its status but no unit is assigned to it. 

Vapor pressure of a fluid

There is a certain pressure that a vapor exerts over a liquid that determines its vapor pressure. A liquid’s vapor pressure varies with the temperature at which it is vaporized. The vapor pressure increases with the temperature as a result of an increase in temperature. Liquids can be measured in various ways in order to determine their vapor pressure, which can vary.

Cavitation of a fluid

It is often called “cavitations” which is the process through which vapor bubbles or vapor-filled cavities are formed in the flow of a liquid when the pressure of the liquid is above the vapor pressure and these vapor bubbles collapse when the pressure of the liquid rises above the vapor pressure.

Essential Characteristics of the Fluid

Here are a few characteristics of fluids:-

  • Incompressibility is the property of a fluid that does not change density when its pressure changes due to its inability to change density with it.
  • When it comes to fluids, deformation is not a concern, as they are able to take on the shape of the container without opposing it, so they are able to adapt to its shape.
  • A fluid’s pressure is defined as the normal force exerted by the fluid against the area of its contact per unit area of contact. From a mathematical point of view, it can be defined as the ratio between force and area.
  • There are several types of fluid pressure and they are all related to the normal forces exerted by a fluid per unit surface area of contact. The force to area ratio can be considered mathematically as a ratio of force to area.
  • There are different factors that affect the atmospheric pressure, but one of them is the weight of a vertical column of air that has the same cross-sectional area as a horizontal column of air starting at one point and extending up to the upper atmosphere.
  • At sea level, we experience the highest levels of atmospheric pressure because the Earth’s surface has the highest atmospheric pressure, and as we move upwards from the surface of the earth’s atmosphere the atmospheric pressure continues to decrease.
  • The intermolecular forces in molecules can be divided into two types. One of the forces involved in the cohesive force is the force of attraction between molecules that have the same chemical properties. Another different force that exists is the adhesive force, which means that molecules of two different substances are attracted to each other by the force of attraction.

Various Types of Fluids

According to how the viscosity changes with various fluids based on their properties, they can be divided into five different types based on how viscosity changes with each of the fluids:-

types of fluids
Types of fluids

Ideal Fluid type

Fluids are ideal when they are incompressible, limitless, have zero viscosity, and have no internal resistance to flow when they are incompressible. Furthermore, there is no rotation of the particles in an ideal fluid about their center of mass and as such, they are not irrigational particles.

In an ideal situation, fluid particles can be rotated in a circular pathway, but a fluid particle itself can flow in a non-circular pattern. In order to approximate the behavior of real fluids, we will often model them as ideal fluids, which are thought to exhibit all of these properties to some extent. The result that is obtained when applying ideal fluid properties to non-ideal fluid properties needs to be applied with extreme caution when one is doing so.

Real Fluid type

When a fluid exhibits some degree of viscosity in addition to its liquid composition, it is defined as real fluid. It is actually true that all fluids are actually real fluids if their presence is present in their surroundings or if they occur naturally in their surroundings. Petrol, and air, for example, are some examples of products in this category.

Newtonian Fluid type

Depending on the local strain rate, the rate of change of the deformation of a Newtonian fluid over time can be described as a linear relationship between the viscous stress arising from its flow at every point and the local strain rate of that fluid. Depending on how fast the fluid’s velocity vector changes, the stress of the fluid will increase or decrease. 

It is only when there is a constant viscosity tensor in the description of the flow state and velocity of the fluid that the viscous stress tensor and the strain rate are described in Newtonian terms and a constant viscosity tensor does not depend on the stress state or speed of the flow. This tensor can be reduced to two real coefficients, based on the fact that the fluid is also isotropic, which can be used as a measure of the resistance to continuous compression and expansion respectively, of the fluid.

Non-Newtonian Fluid type

An example of a non-Newtonian fluid is one whose viscosity is not governed by Newton’s rule of viscosity, i.e. one whose viscosity remains constant regardless of the applied stress. There is a possibility of viscosity changing in a fluid that is not Newtonian if it is subjected to a force, either becoming more liquid or solid.

  • A non-Newtonian fluid is one that becomes runnier when shaken, for example, the taste of ketchup becomes runnier when shaken. As well as many common substances that we encounter every day such as custard, toothpaste, starch suspensions, corn starch, paint, etc, there are many that are non-Newtonian fluids.
  • Non-Newtonian fluids generally have a viscosity that is dependent on something other than shear rate or history of shear rate. Despite their shear-independent viscosities, a number of non-Newtonian fluids can still exhibit normal stress differences or other non-Newtonian behaviors even though they have shear-independent viscosities.
  • As discussed earlier, Newtonian fluids are linear, with the relationship between the shear stress and the shear rate passing through the origin with the coefficient of viscosity acting as a constant of proportionality.
  • Generally speaking, the relationship between the shear stress and the shear rate of a non-Newtonian fluid differs from that of a Newtonian fluid. There is even a possibility that the viscosity of a fluid can change with time. Hence, it is impossible to define a constant coefficient of viscosity that can be applied to all fluids.

Ideal Plastic Fluid

Known as an ideal plastic fluid, a fluid that exhibits a greater rate of a shear strain than its yield value at higher shear stress than that of the yield value is described as an ideal plastic fluid.

Incompressible fluid type

The density of fluid remains constant regardless of the change in pressure in an isothermal environment, i.e, the coefficient of compressibility remains zero for fluids under isothermal conditions.

Diabetic heating or cooling of an incompressible fluid will thus result in the expansion and contraction of the fluid. A more common problem is that fluids may or may not stratify in reaction to isothermal processes, but the movement of a parcel from one pressure level to another or vice versa will not affect its density if the parcel is moving from one pressure level to another.

Compressible Fluid type

There is a branch of fluid mechanics called compressible fluids, which is responsible for the study of flows with significant changes in density that take place in them. Even though all flows are compressible, when the Mach number is smaller than 0.3, most of the time they will be treated as incompressible since in this case the density change due to velocity is only more or less about 5-6 percent, and in that case, the fluid is usually treated as incompressible.

The compression of flow can be applied to many different applications such as high-speed aircraft, jet engines, rocket engines, high-speed entry into planetary atmospheres, gas pipeline systems, and abrasive blasting applications, among others.

Examples of these applications are:

  • high-speed aircraft,
  • jet engines,
  • rocket engines, and even
  • rockets.

Explaining with a diagram why the liquid transfers into gasses

It is well known that fluids, like water-skiers who skim over the surface of a lake or butter that is spread on a slice of bread, will continuously deform when they are subjected to tangential or shear forces, much as these forces are exerted when water-skiers skim over the surface of a lake or when a slice of bread is spread with butter.

  • There are many factors that determine the rate at which fluids deform continuously, much like the force applied to the fluid, but there is also a property of the fluid called viscosity that also affects the rate at which fluids deform continuously.
  • As the solid is sheared, it will also deform, but a position of equilibrium is soon reached in which the elasticity forces induced by the shearing force exactly counterbalance the bending force applied to the solid, and a further deformation does not occur. 
  • There are two general categories of fluids, which are liquids and gases, and each of these categories can be broadly classified. There is a certain density, viscosity, and close proximity between molecules that characterize liquids.
  • The volumes of liquids remain basically constant over time, regardless of pressure, temperature, and the size of the vessel in which they are stored. A gas, on the other hand, is extremely light in weight, and has a relatively low density and viscosity; it possesses molecules that are extremely close together; in general, if gas is placed in a container, it will completely fill the entire container.
Figure 1: Liquid-changed-into-gasses-diagram

(Figure1: Explaining through the diagram how the liquid turns into gasses)

Taking a look at the pressure/temperature diagram shown in the above figure, we can readily see that an initially gaseous fluid at point G becomes a liquid at point H. The reason for this is quite obvious. A decrease in temperature along with an increase in pressure will result in the vapor-pressure curve being crossed quickly as well as the fluid condensing and apparently transforming into a liquid at the point L, thus thereby achieving the desired result.

The fluid is returned to its original state of being a gas by constantly adjusting the pressure and temperature in a clockwise manner while circumnavigating the critical point C through the process, and in this way, the fluid is converted back into a gas once more. However, if you want to know where the transition from liquid at L to gas at G occurs, then the answer is that the change between the two occurs continuously and gradually, as they pass through an entire spectrum of intermediate states on their way to becoming gas at G.

What is the concept of Fluid Mechanics?

It is important to understand the aspect of fluid mechanics is an important subject in Civil, Mechanical, and Chemical Engineering and that it is one of the most important branches of physics and pertains to liquids as well as gases in a state of rest or motion.

It is a branch of mechanics that is relatively easy to learn. The subject of fluid statics, for example, is one area within the field, whereas the area of fluid dynamics is another area within the field.

  • Fluids are substances that flow with the flow of water. All substances that are liquid or gaseous are considered to be fluids since they are both liquid and gaseous. As we all know, water, oil, and various other substances play a very important role in our day-to-day lives since they are used for so many different purposes.
  • In most hydroelectric power plants and thermal power plants, the heat in the water during the generation of electricity is used to generate electricity, and the electricity is generated from the heat produced by the water during the generation of electricity.
  • The cooling of nuclear power plant or reactors is performed using water as a coolant, and the lubrication of the wheels is performed by oil in automobiles during their use.
  • There is a broad field of science called Fluid Mechanics, which examines the behavior of fluids in motion or at rest and the relationship between those states. It does not matter whether the fluid is at rest or in motion because it is exposed to different forces and different climatic conditions and it responds to these conditions based on the characteristics it possesses as fluid behaves under these conditions. As far as fluid mechanics are concerned, there are two main aspects of fluids to be considered:
  1. Fluid Static concept, and
  2. Fluid Dynamics concept

Explaining the two main concepts behind fluid mechanics:-

Fluid Static Concept

As a branch of fluid mechanics, fluid statics usually refers to the study of fluids at rest, also known as hydrostatics. Fluid dynamics is the study of fluids in motion, as opposed to fluid equilibrium, the study of fluids at rest in stable equilibrium; it encompasses the study of the conditions under which fluids are at rest in stable equilibrium.

Applied hydrostatics is the fundamental science that provides explanations for many phenomena that occur in daily life, including the changing atmospheric pressure with altitude; the floating of wood and oil on water, and the fact that water always forms a level surface no matter what the shape of the container is.

A fundamental element of hydraulics is the principle of hydrostatics, which is the design of equipment for storing, transporting, and utilizing fluids. There are many aspects of the theory that can be applied to other fields besides the study of atmospheric physics, meteorology, medicine, and many more.

Fluid Dynamic Concept

There is a unique sub-discipline of fluid dynamics called fluid dynamics which is the study of fluid flow, which is the study of liquids and gases moving in space. In addition to offering a systematic structure to these practical disciplines, fluid dynamics provides the whole framework.

  • This framework for laws is derived from empirical and semi-empirical flow measurements that can be applied to solve practical problems.
  • There are many different properties of fluid that need to be taken into consideration when completing a fluid dynamics problem, and these properties must be calculated as a function of space and time in order to come up with the correct solution to the problem. A liquid’s properties are usually velocity, pressure, density, and temperature, which correspond to the fluid’s characteristics.
  • Aerodynamics and hydrodynamics are two sub-disciplines of this field, each with its own specific sub-disciplines. An important part of fluid dynamics is its ability to calculate forces and movements on aircraft, determine the mass flow rate of petroleum through pipelines, predict changing weather patterns, understand nebulae in interstellar space, and model explosions in a wide range of situations.
  • As part of traffic engineering and crowd dynamics, certain fluid-dynamical principles are incorporated into the design

General Applications of Fluids

In Automobiles

Fluid is one of the most important components of an automobile. A fluid in an automobile performs three very crucial functions: it generates power, it lubricates the engine, and it cools the engine. It is the combustion of fuel in an engine that generates power, whether it is petrol or diesel.

Fuel is a term that is commonly used to refer to this substance. Vehicle moving parts, including the engine and gearbox, are lubricated with oil. The engine of larger automobiles, such as cars, buses, and trucks, is cooled by water.

In Thermal Power Plants

The working fluid in thermal power plants is water, which is used as a source of power. When water is heated in a boiler, it is converted into superheated steam which then passes through the blades of turbines, causing them to rotate as the steam passes through them, forcing them to get hotter.

Electricity is produced by the rotation of the shaft of the turbine in the generator, where the shaft of the turbine is located. The water that is used as a fluid in thermal power plants works as one of the most important components of these plants, which is why these plants are one of the world’s major sources of power.

For the operation of various instruments

A wide range of instruments and automatic valves are operated by compressed air, which is used in the operation of these instruments and valves. With the application of compressed air pressure, these valves can be activated and deactivated by adjusting the pressure applied to them. A pneumatic tool is one that works with compressed air and can be used to perform a variety of tasks such as grinding, screwing, and unscrewing different parts of a machine, etc.

In Hydraulic Machines

It is called hydraulic machines when they operate with fluids such as water and oil as their source of power. Fluid has the capability of lifting heavy loads and exerting extremely high pressures and has the ability to lift heavy loads. Different types of machining operations can be performed with the help of hydraulic machines.

As a matter of fact, the fluid used in the majority of these machines is oil. A large amount of energy is transferred to the fluid through the hydraulic motor that passes the oil through. It is also possible to apply large forces and lift heavy loads using this high-energy fluid when it is injected into a piston and cylinder arrangement.

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