Fluid Flow - Definition, Types, Example, FAQs

Q: What is fluid mechanics?

Fluid mechanics definition can be given as the branch of physics that deals with fluid statics and fluid dynamics. The subject which deals with the behavioural study on fluid and its forces are known as fluid mechanics.

Q: What is meant by fluid?

Fluid is the matter of substances which do not have any fixed shape and are subjected to external pressure.They include liquids, gases, and plasmas.

Q: What is fluid dynamics or what is fluid flow meaning?

Fluid flow includes the fluid motion due to some unbalanced force. Until external unbalanced forces are applied to the motion of the fluid, the fluid will continue its motion. The mechanics that deal with the study of fluid flow properties is known as fluid dynamics.

Q: What is fluid statics?

The study of fluid at rest and fluid in equilibrium state is known as hydrostatics or fluid statics.

Q: What is the classification of fluids?

Based on their properties, they are different types of fluid. They are listed below Ideal fluid, Real fluid, Newtonian Fluid, Non Newtonian Fluid, Ideal Plastic fluid, Incompressible fluid and compressible fluid.

Q: What is an incompressible fluid?

When the fluid is subjected to any kind of external force, the density properties of the fluid does not change and this kind of fluid is known as incompressible fluid. Incompressible fluid example: Water.

Q: Give some examples of compressible and incompressible fluids.

Incompressible fluid example: The liquids which have low compressibility like water, petrol, oil are examples of incompressible fluid. Compressible fluid example: Air and Vapour have higher compressibility than other liquids, so they are the best examples of compressible fluids.

Q: What is Newtonian fluid or what is Newtonian fluid definition?

A Newtonian fluid is defined as the fluid which obeys the laws of Newton of Viscosity. Some Newtonian fluid examples : Water and Gasoline.

Q: How does viscosity affect fluid flow?

Viscosity, which is a measure of a fluid's resistance to flow, significantly impacts fluid flow. Higher viscosity fluids flow more slowly and require more force to move, while lower viscosity fluids flow more easily. For example, honey (high viscosity) flows much more slowly than water (low viscosity) under the same conditions.

Q: How does pressure change affect fluid flow?

Pressure differences are a primary driver of fluid flow. Fluids naturally move from areas of high pressure to areas of low pressure. The greater the pressure difference, the faster the fluid will flow. This principle is utilized in many systems, from water distribution networks to the human circulatory system.

Fluid Flow - Definition, Types, Example, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 05:01 PM IST

Fluid flow is a topic of fluid mechanics that deals with the subject of fluid dynamics. Let us understand a fluid flow process by answering questions like what is fluid, what is the meaning of fluid, what is the meaning of flow, and what are the types of fluid and types of flow.

What is Fluid?

Fluid is the matter of substances that do not have any fixed shape and are subjected to external pressure. The main feature of a fluid is its flowing ability. Now, let's see What is Fluid Flow?

Define fluid flow

A Fluid flow is known as the movement of a fluid from one point to the other. Fluid flow includes a fluid motion due to some unbalanced force. Until external unbalanced forces are applied to the motion of a fluid, a fluid will continue its motion.

According to the natural properties of a fluid, there are different types of fluid. Let's discuss them in detail.

Ideal Fluid:

An ideal fluid is defined as a fluid that cannot be compressed and also does not contain any viscosity. This is an imaginary fluid used only for mathematical calculations. Other than that, this fluid does not exist in reality. They have a constant density. They possess irrotational motion which has a smooth flow. They do not have any internal friction and have a steady flow.

Real Fluid:

All a fluids of the real world which contain viscosity and can be compressed to some extent are known as real fluids. They are also called practical fluids.

Real Fluid examples- Water, oil.

Newtonian fluid:

A Newtonian fluid is defined as a fluid which obeys the laws of Newton of Viscosity, where the shear stress is directly proportional to the velocity gradient.

Newtonian fluid examples- Water and Gasoline.

Also read -

This Story also Contains

What is Fluid?
Define fluid flow
Classification of Fluid
Equation of fluid flow
Applications of Fluid Dynamics in Real Life

Fluid Flow - Definition, Types, Example, FAQs

Non-Newtonian fluid:

These fluids are opposite to Newtonian fluid. This fluid does not obey the laws of Newton of Viscosity. Theor viscosity changes with applied shear rate.

Non Newtonian fluid examples- Ketchup, toothpaste, blood.

Incompressible fluid:

When a fluid is subjected to any kind of external force, the density properties of a fluid does not change, this kind of fluid is known as incompressible fluid. Incompressible fluid example- Water, oil.

Compressible fluid:

When a fluid is subjected to any kind of external force, the density properties of a fluid will change and this kind of fluid is known as compressible fluid.

Compressible fluid examples- Natural gas, steam, air.

Superfluids:

It is a special phase of matter with zero viscosity that can flow without losing any energy. It has the ability to flow through tiny pores without resistance.

Superfluids examples- Liquid helium at very low temperatures

Plasma:

It is the fourth state of matter. An ionized gas where atoms are split into free electrons and ions, making it highly conductive.

Plasma examples- Stars (like the Sun), fluorescent lights, and plasma TVs.

Classification of Fluid

a fluid flow is categorized into different types considering different properties like steadiness, viscosity, compressibility, rotational property, and so on. They are categorized as below-

Based on their steadiness:

The velocity of a fluid decides the steadiness of a fluid flow.

Steady flow: When the velocity of a fluid is constant, a fluid flow is found to be steady.
Unsteady flow: When the velocity of a fluid varies from time to time, a fluid flow is found to be unsteady.

Based on their Viscosity:

a fluid is classified into viscous and non-viscous fluid.

Viscous fluid: The term Viscosity is known as the measurement of a fluid thickness and a fluid with the viscosity is known as viscous fluid.
Non-Viscous Fluid: A fluid having no viscosity is known as non viscous fluid.

Based on their compressibility:

On the basis of Mach number, flows are classified into compressible and incompressible fluid.

Incompressible fluid flow has the Mach number M < 0.3
Compressible fluid flow has the Mach number M between 0.3 to 1

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Equation of fluid flow

The mass flow rate of a fluid flow is defined as the rate of massive fluid movement through a unit area. It is considered to be movement of mass of fluid per unit time.

Mathematically, the mass flow rate of a fluid can be written as

$$
M=\rho A V,
$$

where,

$\rho$ denotes the density of a fluid
A denotes the area of a fluid cross-section
V represents the velocity of a fluid flow

Thus, the equation infers that the mass flow rate of fluid is proportional to density, area, and velocity of a fluid.

Applications of Fluid Statics in Real Life:

There are many applications of fluid statics in real life, the main application is it is used in the calculation and measurement of pressure of liquid in the field of hydrostatics like finding the pressure of floating and submerged bodies, pressure of water flow in water dams and gates, and water flow pressure in some liquid storage tanks of homes.

Applications of Fluid Dynamics in Real Life

There are huge applications in fluid dynamics, Let us discuss the few applications below:

It is used to calculate and find the different types of forces and moments in the field of aircraft,
It is used to determine the mass flow rate of petroleum or other gases through pipelines.
Even they are used to predict weather patterns,
It is used to understand and analyse the nebulae in interstellar space
The use of fluid dynamics also plays a role in modeling different fission weapon detonations.

Frequently Asked Questions (FAQs)

1. What is fluid mechanics?

Fluid mechanics definition can be given as the branch of physics that deals with fluid statics and fluid dynamics. The subject which deals with the behavioural study on fluid and its forces are known as fluid mechanics.

2. What is meant by fluid?

Fluid is the matter of substances which do not have any fixed shape and are subjected to external pressure.They include liquids, gases, and plasmas.

3. What is fluid dynamics or what is fluid flow meaning?

Fluid flow includes the fluid motion due to some unbalanced force. Until external unbalanced forces are applied to the motion of the fluid, the fluid will continue its motion. The mechanics that deal with the study of fluid flow properties is known as fluid dynamics.

4. What is fluid statics?

The study of fluid at rest and fluid in equilibrium state is known as hydrostatics or fluid statics.

5. What is the classification of fluids?

Based on their properties, they are different types of fluid. They are listed below

Ideal fluid,
Real fluid,
Newtonian Fluid,
Non Newtonian Fluid,
Ideal Plastic fluid,
Incompressible fluid and
compressible fluid.

6. What is an incompressible fluid?

When the fluid is subjected to any kind of external force, the density properties of the fluid does not change and this kind of fluid is known as incompressible fluid. Incompressible fluid example: Water.

7. Give some examples of compressible and incompressible fluids.

Incompressible fluid example: The liquids which have low compressibility like water, petrol, oil are examples of incompressible fluid.

Compressible fluid example: Air and Vapour have higher compressibility than other liquids, so they are the best examples of compressible fluids.

8. What is Newtonian fluid or what is Newtonian fluid definition?

A Newtonian fluid is defined as the fluid which obeys the laws of Newton of Viscosity. Some Newtonian fluid examples : Water and Gasoline.

9. How does viscosity affect fluid flow?

Viscosity, which is a measure of a fluid's resistance to flow, significantly impacts fluid flow. Higher viscosity fluids flow more slowly and require more force to move, while lower viscosity fluids flow more easily. For example, honey (high viscosity) flows much more slowly than water (low viscosity) under the same conditions.

10. How does pressure change affect fluid flow?

Pressure differences are a primary driver of fluid flow. Fluids naturally move from areas of high pressure to areas of low pressure. The greater the pressure difference, the faster the fluid will flow. This principle is utilized in many systems, from water distribution networks to the human circulatory system.

11. What is Bernoulli's principle, and how does it relate to fluid flow?

Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. In fluid flow, this principle explains why faster-moving fluids exert less pressure than slower-moving fluids. It's the reason why airplane wings generate lift and why shower curtains tend to move inward during a shower.

12. How does pipe diameter affect fluid flow?

Pipe diameter has a significant impact on fluid flow. As the diameter increases, the flow rate increases (assuming constant pressure), because there's less friction between the fluid and the pipe walls. Conversely, a smaller diameter results in higher fluid velocity but lower overall flow rate due to increased friction.

13. How does temperature affect fluid flow?

Temperature has a significant impact on fluid flow primarily through its effect on viscosity. Generally, as temperature increases, the viscosity of liquids decreases, making them flow more easily. Conversely, for gases, an increase in temperature typically increases viscosity. These changes in viscosity due to temperature can greatly influence flow characteristics and rates.

14. What is laminar flow?

Laminar flow is a type of fluid flow where the fluid moves in parallel layers without mixing. In laminar flow, fluid particles move in smooth paths, and the flow is predictable and steady. This type of flow typically occurs at low velocities or in fluids with high viscosity.

15. How does turbulent flow differ from laminar flow?

Turbulent flow is characterized by irregular fluctuations and mixing within the fluid. Unlike laminar flow, turbulent flow involves chaotic changes in pressure and velocity. It typically occurs at higher velocities or in fluids with lower viscosity. Turbulent flow is less predictable and often results in more energy loss due to friction.

16. What is the difference between compressible and incompressible flow?

Compressible flow refers to fluid flow where the density of the fluid changes significantly along the flow, typically seen in gases at high velocities. Incompressible flow, on the other hand, assumes that the density of the fluid remains constant throughout the flow. Most liquid flows and gas flows at low speeds are treated as incompressible for simplicity in calculations.

17. How do non-Newtonian fluids behave differently in terms of flow?

Non-Newtonian fluids, unlike Newtonian fluids, do not have a constant viscosity that is independent of stress. Their flow behavior can be shear-thinning (viscosity decreases with increased stress, like ketchup), shear-thickening (viscosity increases with stress, like cornstarch in water), or have a yield stress (require a minimum stress to begin flowing, like toothpaste). This variable viscosity leads to complex and sometimes counterintuitive flow behaviors.

18. What is the significance of the Mach number in fluid flow?

The Mach number is the ratio of the speed of a fluid to the speed of sound in that fluid. It's particularly important in compressible flow analysis, especially for gases. When the Mach number is less than 1 (subsonic flow), disturbances can propagate upstream. When it's greater than 1 (supersonic flow), shock waves can form. The Mach number helps in understanding and predicting fluid behavior in high-speed flows, such as in aircraft design.

19. What is fluid flow?

Fluid flow is the movement of liquids or gases from one point to another. It occurs when there's a difference in pressure, temperature, or elevation between two points, causing the fluid to move. This concept is fundamental in understanding how fluids behave in various systems, from blood circulation to water in pipes.

20. What is meant by 'streamline' in fluid flow?

A streamline is an imaginary line in a fluid flow that is tangent to the velocity vector of the fluid at every point along its length. In steady flow, streamlines represent the path that fluid particles follow. Streamlines never cross each other, and closer streamlines indicate higher fluid velocity. They are useful for visualizing and analyzing flow patterns.

21. How does surface tension affect fluid flow?

Surface tension, the tendency of liquid surfaces to shrink into the minimum surface area possible, can significantly affect fluid flow, especially at small scales or in thin films. It can cause capillary action, where liquids can move against gravity in narrow spaces. Surface tension also plays a role in droplet formation and can affect the behavior of fluids at interfaces.

22. How does fluid flow relate to heat transfer?

Fluid flow plays a crucial role in heat transfer, particularly in convection. As fluids flow, they can carry heat from one location to another. The rate of heat transfer is greatly influenced by the flow characteristics – turbulent flow generally results in more effective heat transfer than laminar flow due to increased mixing. This relationship is important in many applications, from cooling systems to weather patterns.

23. How does fluid flow in porous media differ from flow in pipes?

Fluid flow in porous media, such as soil or rock, is significantly different from flow in pipes. In porous media, the flow path is tortuous and variable, with fluid moving through interconnected pores. This flow is often described using Darcy's law, which relates flow rate to pressure gradient and permeability. Unlike pipe flow, where the entire cross-section is available for flow, only a fraction of the cross-sectional area in porous media contributes to flow, leading to generally slower flow rates.

24. What is the continuity equation in fluid dynamics?

The continuity equation is a fundamental principle in fluid dynamics that states that the mass flow rate of a fluid remains constant in a closed system. In simpler terms, the amount of fluid entering a system must equal the amount leaving it. This principle is expressed mathematically as A₁v₁ = A₂v₂, where A is the cross-sectional area and v is the velocity at two different points in the system.

25. What is meant by 'steady flow' in fluid dynamics?

Steady flow, also known as stationary flow, refers to fluid flow where the velocity, pressure, and cross-section of the flow do not change with time at any given point. While the fluid itself is moving, the overall flow characteristics remain constant. This is in contrast to unsteady or transient flow, where these properties can change over time.

26. What is the Reynolds number, and how does it relate to fluid flow?

The Reynolds number is a dimensionless quantity that helps predict flow patterns in different fluid flow situations. It's the ratio of inertial forces to viscous forces within a fluid. Low Reynolds numbers (typically less than 2300) indicate laminar flow, while high Reynolds numbers (usually greater than 4000) suggest turbulent flow. The transition between these two states occurs in the range of 2300-4000.

27. What is the no-slip condition in fluid dynamics?

The no-slip condition is a fundamental principle in fluid dynamics that states that fluid in direct contact with a solid boundary has zero velocity relative to the boundary. This means that at the surface of a pipe or container, the fluid is essentially "stuck" to the wall. This condition leads to the development of boundary layers and is crucial in understanding fluid behavior near surfaces.

28. How does gravity affect fluid flow?

Gravity plays a crucial role in fluid flow, especially in open channel flows and vertical pipe systems. It can either assist or resist flow depending on the direction. In downward flow, gravity accelerates the fluid, while in upward flow, it acts against the motion. Gravity is also responsible for phenomena like siphoning and is a key factor in the design of drainage systems.

29. How do fluids behave differently from solids when force is applied?

Unlike solids, fluids deform continuously under the application of shear stress, no matter how small. While solids can resist shear stress without continuous deformation, fluids cannot. This property of fluids is what allows them to flow and take the shape of their container. The rate of deformation in fluids is related to their viscosity.

30. What is the difference between Eulerian and Lagrangian descriptions of fluid flow?

The Eulerian description of fluid flow focuses on specific locations in the flow field and how fluid properties change at these fixed points over time. The Lagrangian description, in contrast, follows individual fluid particles as they move through space and time. While the Eulerian approach is often more practical for analysis, the Lagrangian approach can provide insights into particle trajectories and mixing processes.

31. What is the hydraulic jump phenomenon?

A hydraulic jump is an abrupt rise in fluid level that occurs when a fast-moving stream of liquid is forced to slow down suddenly. It's commonly seen in open channel flows, like rivers or spillways. During a hydraulic jump, the flow transitions from supercritical (fast and shallow) to subcritical (slower and deeper), often accompanied by turbulence and energy dissipation. This phenomenon is important in hydraulic engineering for energy dissipation and flow control.

32. What is cavitation, and how does it affect fluid flow?

Cavitation is the formation of vapor cavities in a liquid when local pressure drops below the liquid's vapor pressure. These cavities can collapse violently, causing damage to surrounding surfaces. In fluid flow, cavitation can occur in areas of rapid pressure change, like in pump impellers or around ship propellers. It can significantly affect flow characteristics, reduce efficiency, and cause erosion of materials. Understanding and preventing cavitation is crucial in the design of hydraulic systems.

33. How do boundary layers affect fluid flow?

Boundary layers are thin regions of fluid near a solid surface where viscous forces are significant. They develop due to the no-slip condition at the surface. Within the boundary layer, the fluid velocity changes from zero at the surface to the free stream velocity away from the surface. Boundary layers affect fluid flow by introducing drag, influencing heat transfer, and potentially leading to flow separation. Their behavior is crucial in aerodynamics and many other fluid flow applications.

34. What is the difference between forced and natural convection in fluid flow?

Forced convection occurs when fluid motion is induced by an external force, such as a pump or fan. Natural convection, on the other hand, results from density differences in the fluid caused by temperature variations in the presence of a gravitational field. In forced convection, flow patterns and heat transfer rates are primarily determined by the external force, while in natural convection, they depend on buoyancy effects. Understanding these differences is crucial in designing efficient heating, cooling, and ventilation systems.

35. How does the concept of vorticity relate to fluid flow?

Vorticity is a measure of the local rotation in a fluid flow. It's defined as the curl of the velocity field and indicates the tendency of fluid elements to spin. In a flow with vorticity, fluid particles rotate about an axis as they move along their path. Vorticity is important in understanding complex flow patterns, turbulence, and the formation of eddies and whirlpools. It plays a crucial role in many fluid dynamics phenomena, from weather systems to the lift generated by aircraft wings.

36. What is the significance of the Froude number in fluid flow?

The Froude number is a dimensionless number that describes the ratio of inertial to gravitational forces in a fluid flow. It's particularly important in free surface flows, such as in open channels or for ships moving through water. When the Froude number is less than 1, the flow is subcritical, and surface waves can propagate upstream. When it's greater than 1, the flow is supercritical, and disturbances cannot travel upstream. The Froude number is crucial in hydraulic engineering and naval architecture for predicting flow behavior and designing structures.

37. How does fluid flow change in a converging-diverging nozzle?

In a converging-diverging nozzle, fluid flow undergoes significant changes. In the converging section, the fluid accelerates and its pressure decreases. If the flow is compressible (like gas flow), it can reach sonic speed at the throat (narrowest part). In the diverging section, the behavior depends on the back pressure. For high back pressures, the flow decelerates subsonically. For sufficiently low back pressures, the flow can become supersonic, continuing to accelerate. This nozzle design is crucial in rocket engines and supersonic wind tunnels.

38. What is the role of dimensionless numbers in fluid flow analysis?

Dimensionless numbers, such as Reynolds number, Mach number, and Froude number, play a crucial role in fluid flow analysis. They allow for the comparison of flow behavior across different scales and conditions. By expressing the ratio of different forces or properties, these numbers help in predicting flow regimes, scaling experiments, and applying results from one system to another. They are essential tools in fluid dynamics for simplifying complex problems and generalizing findings.

39. How does surface roughness affect fluid flow in pipes?

Surface roughness has a significant impact on fluid flow in pipes, particularly on the flow resistance and the transition from laminar to turbulent flow. Rougher surfaces increase friction between the fluid and the pipe wall, leading to greater energy loss and pressure drop along the pipe. This increased friction can also promote earlier transition to turbulent flow. In turbulent flow, roughness affects the thickness and structure of the boundary layer, further influencing flow characteristics and heat transfer.

40. What is flow separation, and why is it important in fluid dynamics?

Flow separation occurs when a fluid flowing along a surface detaches from that surface. It typically happens in areas of adverse pressure gradient, where the fluid is moving from a region of high pressure to low pressure. Separation can lead to the formation of eddies, increased drag, and reduced lift in aerodynamic applications. Understanding and controlling flow separation is crucial in designing efficient airfoils, diffusers, and other fluid flow systems to minimize energy loss and optimize performance.

41. How does compressibility affect fluid flow at high velocities?

At high velocities, especially as fluid speeds approach the speed of sound, compressibility effects become significant. In compressible flow, changes in pressure lead to changes in density, which in turn affect the flow behavior. This can result in phenomena like shock waves, where abrupt changes in fluid properties occur. Compressibility effects are particularly important in aerodynamics, gas dynamics, and the design of high-speed fluid systems, as they can dramatically alter flow patterns and energy transfer.

42. What is the significance of the Navier-Stokes equations in fluid flow?

The Navier-Stokes equations are fundamental partial differential equations that describe the motion of viscous fluid substances. They are derived from applying Newton's second law to fluid motion, with the addition of viscous and pressure forces. These equations are crucial because they can, in principle, model and predict the velocity and pressure fields of any fluid flow scenario. While often too complex for analytical solutions, they form the basis for computational fluid dynamics (CFD) and are essential in understanding and simulating fluid behavior in various applications.

43. How does fluid flow in a rotating system differ from flow in a stationary system?

Fluid flow in a rotating system is subject to additional forces not present in stationary systems, primarily the Coriolis force and the centrifugal force. The Coriolis force causes moving particles to deflect relative to the rotating system, while the centrifugal force pushes particles outward from the axis of rotation. These forces can create complex flow patterns, such as cyclones and anticyclones, and are crucial in understanding large-scale atmospheric and oceanic circulation, as well as in the design of rotating machinery like centrifugal pumps and turbines.

44. What is the concept of drag in fluid flow, and how is it measured?

Drag is the force exerted on a body moving through a fluid, acting opposite to the direction of motion. It's composed of two main components: pressure drag (form drag) due to pressure differences around the body, and skin friction drag due to viscous effects at the surface. Drag is typically measured using wind tunnels or towing tanks, where forces on objects are directly measured. It's often expressed in terms of a dimensionless drag coefficient, which allows for comparison across different scales and flow conditions. Understanding and minimizing drag is crucial in many applications, from vehicle design to pipeline engineering.

45. How does fluid flow contribute to the lift force on an airfoil?

Lift on an airfoil is generated primarily due to the difference in fluid flow speeds over its upper and lower surfaces. The airfoil shape causes the air flowing over the top surface to move faster than the air below. According to Bernoulli's principle, this higher velocity results in lower pressure above the wing compared to below. This pressure difference creates an upward force known as lift. Additionally, the downward deflection of air by the airfoil (as described by Newton's third law) contributes to the lift force. Understanding this interplay between fluid flow and lift is fundamental to aircraft design and operation.