What is Fluid Flow?
Fluid Flow is a part of fluid mechanics and deals with fluid dynamics. It involves the motion of a fluid subjected to unbalanced forces. This motion continues as long as unbalanced forces are applied.
For example, if you are pouring water from a mug, the velocity of water is very high over the lip, moderately high approaching the lip, and very low at the bottom of the mug. The unbalanced force is gravity, and the flow continues as long as the water is available and the mug is tilted.
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Types of Fluids
Ideal fluid
A fluid is said to be ideal when it cannot be compressed and the viscosity doesn’t fall in the category of an ideal fluid. It is an imaginary fluid which doesn’t exist in reality.
Real fluid
All the fluids are real as all the fluids possess viscosity.
Newtonian fluid
When the fluid obeys Newton’s law of viscosity, it is known as a Newtonian fluid.
Non-Newtonian fluid
When the fluid doesn’t obey Newton’s law of viscosity, it is known as Non-Newtonian fluid.
Ideal plastic fluid
When the shear stress is proportional to the velocity gradient and shear stress is more than the yield value, it is known as ideal plastic fluid.
Incompressible fluid
When the density of the fluid doesn’t change with the application of external force, it is known as an incompressible fluid.
Compressible fluid
When the density of the fluid changes with the application of external force, it is known as compressible fluid.
The table below represents the density and viscosity of different types of fluids
| Types of fluid | Density | Viscosity |
| Ideal fluid | Constant | Zero |
| Real fluid | Variable | Non-zero |
| Newtonian fluid | Constant/ Variable | \(\begin{array}{l}T=u(\frac{\mathrm{d} u}{\mathrm{d} y})\end{array} \) |
| Non-Newtonian fluid | Constant/ Variable | \(\begin{array}{l}T\neq u(\frac{\mathrm{d} u}{\mathrm{d} y})\end{array} \) |
| Incompressible fluid | Constant | Non-zero/ zero |
| Compressible fluid | Variable | Non-zero/ zero |
Interested to learn more about other concepts related to fluid, below are the links:
Classification of flows on the basis of Mach number
- Incompressible flow has M<0.3.
- Compressible subsonic flow has M between 0.3 to 1.
The different types of fluid flow are represented in the graph below:
Types of Fluid Flow
Fluid flow has all kinds of aspects — steady or unsteady, compressible or incompressible, viscous or non-viscous, and rotational or irrotational, to name a few. Some of these characteristics reflect the properties of the liquid itself, and others focus on how the fluid is moving.
Steady or Unsteady Flow
Fluid flow can be steady or unsteady, depending on the fluid’s velocity:
- Steady: In steady fluid flow, the velocity of the fluid is constant at any point.
- Unsteady: When the flow is unsteady, the fluid’s velocity can differ between any two points.
Viscous or Non-viscous Flow
Liquid flow can be viscous or non-viscous.
Viscosity is a measure of the thickness of a fluid, and very gloppy fluids such as motor oil or shampoo are called viscous fluids.
Fluid Flow Equation
Mass flow rate is the rate of movement of a massive fluid through a unit area. In simple words it is the movement of mass per unit time. The formula for mass flow rate is given as follows:
\(\begin{array}{l}Mass\;flow\;rate = \rho \: A\; V\end{array} \)
From the equation, we can see that mass flow rate depends on the density, velocity and the area of cross-section of the fluid.
Solved Example
A fluid moves through a tube of 15 m/s, the tube has a transverse area of 0.4 m2. If the density of the fluid is ρ = 1.5 grams/m3, what is the amount of mass flowing through the tube?
To calculate the total mass of fluid flowing through the tube, we use
Substituting the values in the above equation, we get
Fluid Flow Through a Pipe
The general capacity of the pipes varies on its size. The table given below shows the capacity of the flow of fluid based on its size.
| Pipe size (in inch) | Maximum Flow (in gal/min) | Velocity (in ft/s) | Head Loss in (ft / 100 ft) |
| 2 | 45 | 4.3 | 3.9 |
| 2.5 | 75 | 5.0 | 4.1 |
| 3 | 130 | 5.6 | 3.9 |
| 4 | 260 | 6.6 | 4.0 |
| 6 | 800 | 8.9 | 4.0 |
| 8 | 1600 | 10.3 | 3.8 |
| 10 | 3000 | 12.2 | 4.0 |
| 12 | 4700 | 13.4 | 4.0 |
| 14 | 6000 | 14.2 | 4.0 |
| 16 | 8000 | 14.5 | 3.5 |
| 18 | 10000 | 14.3 | 3.0 |
| 20 | 12000 | 13.8 | 2.4 |
| 24 | 18000 | 14.4 | 2.1 |
Frequently Asked Questions – FAQs
What is fluid flow?
Define ideal fluids.
What are Newtonian and Non-Newtonian fluids?
What is an ideal plastic fluid?
Why do fluids exert pressure?
Name the different types of fluid
State TRUE or FALSE: The Mach number for incompressible flow is less than 0.3.
Define incompressible fluid.
Write the formula for mass flow rate.
Mass flow rate = ρAV
Give two examples of non-newtonian fluids.
Put your understanding of this concept to test by answering a few MCQs. Click ‘Start Quiz’ to begin!
Select the correct answer and click on the “Finish” button
Check your score and answers at the end of the quiz
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