Streamline Flow

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When you look at a stream flowing, you first observe a few things about it like the speed of the water in it, its width, amount of water flowing, etc. One of the primary things that characterize a stream is its flow which is also termed as streamflow. 


A liquid or fluid motion happens when shear stress acts on it. This shear stress or force is acting parallel to the surface of the liquid. In the case of a stream flowing, the gravitational force of the earth is acting upon it and pulling it down.


Streamflow is described by a single atom's path in a fluid and is of two types: streamline flow and turbulent flow. A flow that is steady, smooth, and predictable is called streamline flow. Sometimes the flow of a stream could be marked by chaotic changes that we call a turbulent flow. Let us learn about streamline flow definition and distinguish between streamline and turbulent flow in this article.

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What is Streamline Flow

A streamline flow is also called laminar flow, and in this flow, there are no major velocity fluctuations. A streamline is a path of imaginary particles within the fluid that are carried along with it. The streamlines are fixed in a steady flow, and fluid travels in a smooth and regular path. This means that the flow properties like velocity, pressure, etc. at each point remain constant. To define streamline flow, think of the laminar flow consisting of laminae or thin layers, all parallel to each other. In a streamline motion, these layers of water are flowing on top of each other at different speeds, and there is no mixing between layers.

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If you draw a tangent to the streamline at any point in space in a streamline flow, it will be aligned with the instantaneous velocity vector at that point. These streamlines cannot cross each other at any one instant in time. The fluid in contact with the surface is stationary, and the other layers are sliding over it.


Turbulent Flow

A turbulent motion in a fluid is an irregular motion caused either by high velocities or abrupt changes in velocities. If you imagine a ball in a river stream, can you predict its direction of motion? It is probably not because water is splashing all around and the ball could go any which way every second. It is similar to the turbulent flow of fluids where there are random and unpredictable fluid particles' motions. In a turbulent flow, fluid is not passing in parallel layers, and there is a high level of lateral mixing and disruption between layers.

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The three main characteristics of a turbulent flow are:

  • Eddies

  • Recirculation

  • Apparent randomness

At any given point in the fluid which is undergoing turbulent flow, there is a continuous change in magnitude and the direction of flow. In our bodies, blood flow is generally streamline or laminar. But in certain high flow conditions, laminar flow can get disrupted and result in a turbulent flow. The flow in large arteries at the branch points are also turbulent flows. 


The streamline flow vs turbulent flow table below sums up the difference between streamline and turbulent flow:


Distinguish Between Streamline Flow and Turbulent Flow

Streamline Flow

Turbulent Flow

It is a steady motion where liquid flows in parallel layers

It is a chaotic motion where the liquid is not travelling in parallel layers and there is high lateral mixing of liquid

Each particle in this flow follows the path of its preceding particles and all have the same velocity (same magnitude and direction)

The particles in a turbulent motion of liquid have different speeds and directions at different points.

The velocity of liquid in a streamline flow is less than the critical velocity of the fluid

In a turbulent flow, the liquid has a velocity greater than its critical velocity


Reynold’s Number

Reynold’s number is a dimensionless number that has a vital role in predicting patterns in a fluid’s behaviour. Reynold’s number is represented as Re, and it is used to determine whether fluid has a laminar or turbulent flow. Re is the ratio of forces of inertia (forces that tend to resist motion) to simple viscous forces (the intermolecular glue which holds the fluid together). At high values of Re, there is turbulence.

Re = inertial force/viscous force = (ρ * v * L)/𝜇

Where ρ is the density, v is the velocity, L is the diameter of the tube, and 𝜇 is viscosity.

It could also be expressed as:

Re =  fluid & flow properties/fluid properties. 

So if there is a glass of water in a jar at rest then flow properties are 0. Thus, the numerator in the above equation is 0, which means that fluid at rest is independent of Reynold’s number. When the jar is tilted, and water starts flowing, we can predict water flow using Reynold’s number.

FAQ (Frequently Asked Questions)

Q1. What is Meant by Viscosity?

Ans. Viscosity is the measure of the resistance of a fluid to change its shape, i.e. resistance of relative movement of portions close to each other in a fluid. In other words, viscosity is the opposition to flow. Fluidity is the opposite of viscosity, and it measures the ease of flow. For instance, honey is a lot denser than water. VIscosity could be seen as internal friction between molecules that oppose the development of velocity differences within a fluid. When fluids are used in lubrication, viscosity is one of the major factors determining the forces that have to be overcome for the fluid to be transported into pipelines. In certain processes like spraying, surface coating, and injection molding, viscosity controls fluid flow. Viscosity has the following properties:

  • When there is a temperature rise, the viscosity of liquid decreases rapidly.

  • Gases become more viscous as the temperature increases.

  • Upon heating, the fluidity of liquids increases, and that of gases decreases.

Q2. What is the Definition of an Ideal Fluid and What is its Use?

Ans. An ideal fluid is also called a perfect fluid, and it has the below-mentioned properties:

  • It can not be compressed.

  • It has a constant density.

  • It has no viscosity.

An ideal fluid does not exist in nature, but it is used in fluid flow problems to simplify them and understand what would happen to an ideal fluid in a particular situation.