Type Of Flow

When a fluid is subjected to acceleration, its flow characteristics undergo significant changes, leading to different types of flow behaviour. Understanding these types of flow—such as laminar, turbulent, and transitional—becomes essential in various engineering and scientific applications. For instance, in real life, the streamlined flow of water through a hose when the pressure is low can be likened to laminar flow, while the chaotic splashing when the pressure increases represents turbulent flow. Similarly, the airflow over an aircraft wing is carefully managed to maintain an optimal flow type, ensuring smooth and efficient flight. These everyday examples highlight the importance of studying fluid flow in accelerated conditions, as it plays a crucial role in fields ranging from aerodynamics to hydraulics and even in the human circulatory system.

Type of Flow

"Type of Flow" refers to the different ways in which fluids (liquids and gases) move, governed by various factors such as velocity, viscosity, and the geometry of the flow path. The primary types of flow include laminar flow, where the fluid moves in smooth, orderly layers; turbulent flow, characterized by chaotic and irregular motion; and transitional flow, which exists between laminar and turbulent states. In everyday life, we encounter these flow types frequently: the gentle flow of honey from a spoon is an example of laminar flow, while the swirling patterns of water in a fast-moving river illustrate turbulent flow.

Now we will study about types of flow one by one

Steady Flow

Steady flow refers to a type of fluid flow where the velocity of the fluid at any given point does not change over time. In other words, the flow parameters, such as speed and direction, remain constant at any specific location, although they may vary from one location to another. This type of flow is crucial in many practical applications, as it simplifies the analysis and design of fluid systems.

In this type of flow fluid characteristics like Velocity, Pressure density etc at a Point do not change with time.

i.e dvdt=0,dpdt=0,dρdt=0

Unsteady Flow

Unsteady flow, also known as transient flow, occurs when the velocity and other characteristics of the fluid change with time at a given point. Unlike steady flow, where fluid properties remain constant at any location, unsteady flow is characterized by variations in speed, pressure, and direction over time. This type of flow is prevalent in many real-world scenarios where conditions are dynamic and subject to change.

In this type of flow fluid Characteristics like Velocity, Pressure density etc At a Point change with respect to time.

i.e dvdt≠0,dpdt≠0,dρdt≠0

Streamline flow

Streamline the flow of a liquid is the type of fluid flow in which each particle of the fluid passing through a point travels along the same path and with the same velocity as the preceding element passes through that point.

Or

Streamline flow is defined as the path (straight or curved), the tangent to which at any point gives the direction of the flow of liquid.

For the above figure path ABC is streamlined.

All the liquid particles passing through A, B, and C will have velocities as V1,V2 and V3 respectively.

Property of streamlined flow

The direction of velocity at any point on the flow line is along the tangent.

Two streamlines cannot cross each other.

Laminar Flow

If a liquid is flowing over a horizontal surface with a steady flow and moves in the form of infinitesimal parallel layers of different velocities which do not mix with each other, then the flow of liquid is called laminar flow.

This type of flow is also referred to as streamline flow.

In this flow, the velocity of liquid flow is always less than the critical

velocity of the liquid.

Turbulent Flow
When the velocity of liquid flow is greater than its critical velocity, then the motion of the particles of the liquid becomes disordered or irregular. Such a flow is called turbulent flow.

In turbulent flow, the velocity of the fluid at a point is continuously changing in both magnitude and direction.

Critical Velocity

Critical velocity is defined as the velocity of the liquid, flow up to which it's streamlined/laminar and above which its flow becomes turbulent.

Reynold's Number

Reynold's number is a number which determines the nature of the flow of liquid through a pipe.

Reynold's Number is defined as the Ratio of inertial force per unit area to the viscous force per unit Area for a flowing fluid.

It is denoted by R_e which is given by

Re=ρV⋅dη where ρ= density of fluid η= coefficient of viscosity V= velocity of fluid d= diameter of the pip

Range of Reynold's Number

If it lies Between 0 - 2000 then the flow of liquid is streamlined or laminar

If it lies Between 2000 - 3000 then the flow of liquid is unstable

(means it changing from streamlined to turbulent)

If it is Above - 3000 then the flow of liquid is turbulent.

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Solved Examples Based on Type of Flow

Example 1: A light cylindrical vessel is kept on a horizontal surface. The area of the base is A. A hole of cross-sectional area 'a' is made just at its bottom side. The minimum coefficient of friction necessary to prevent sliding the vessel due to the impact force of the emerging liquid is (a < < A) :

1) A2a
2) None of these
3) 2aA
4) aA

Solution

Reaction force due to the ejection of liquid

F=ρav2=ρa(2gh)→(1)fL=μN=μ(mg)=μ(ρAhg)→(2)

To just prevent the sliding of the vessel

F=fLρa(2gh)=μ(ρAgh)μ=2aA

Hence, the answer is the option (3).

Example 2:In streamline flow Velocity at a point is

1) Constant along its path

2) The direction of velocity is the same along the path

3) The direction of velocity is different at different points

4) None of the above

Solution:

Streamline flow

It is defined as the path, straight or cured, the tangent to which at any point gives the direction of the flow of liquid

wherein

The direction of velocity is along tangent, since tangent is different at different points hence its direction is different at each point.

Hence, the answer is the option (3).

Example 3: If the velocity of the liquid is v and its critical velocity is V_c then which of the following represents the condition for turbulent flow?

1) V=Vc
2) V>Vc
3) V<Vc
4) None of the above

Solution:

Turbulent Flow

When a liquid moves with a velocity greater than its critical velocity

The motion of the particles of liquid becomes irregular and disordered

For velocity greater than critical velocity the flow becomes turbulent.

Hence, the answer is the option (2).

Example 4: Which of the following represents the condition for steady flow?

1) ∂ρ∂t≠0
2) ∂v∂t≠0
3) ∂v∂α=0
4) ∂ρ∂t=0

Solution:

Steady flow

dvdt=0dpdt=0dρdt=0

i.e Velocity, Pressure, and density do not change with time

Hence, the answer is the option (4).

Summary

In fluid dynamics, understanding the different types of flow is essential for various applications. Steady flow maintains constant velocity, pressure, and density at a given point over time, while unsteady flow involves changes in these properties. Streamline flow ensures particles follow smooth, consistent paths, whereas laminar flow features orderly, parallel layers of fluid. In contrast, turbulent flow occurs when the velocity exceeds a critical threshold, leading to chaotic and irregular movement. Reynold's number helps determine the flow type by comparing inertial and viscous forces, with different ranges indicating laminar, transitional, or turbulent flow.