The matter is classified as Solids, Liquids and Gases. Out of these liquids and gases are termed as fluid.Thus fluid is a substance that has the ability to flow in any direction. The shear force acting on any fluid due to the continuous relative motion between the fluid particles tends the particles to move. And when the fluid particles are not able to resist the shear force the fluid particles tend to move over each other. Thus a fluid flow takes place.
Fluid, as I said above, can be gas or liquid and the main difference between these two fluids is that liquid takes the shape of the container it is stored in but gas occupies the complete space.
Types Of Fluid Flow-
Considering the layers of fluid in a pipe, fluid flow is categorised into 3 types:
- Laminar flow
- Turbulent flow
- Transitional flow
Laminar Flow –
Laminar flow is otherwise called as Streamline flow. Particles of fluid are considered to travel in a smooth continuous path called streamlines. They can be curved or straight depending on the passway they are moving. In a laminar or streamline flow the fluid layers slide relative to each other. Any two layer doesn’t mix. For example, if any coloured fluid is introduced into the laminar flow, the coloured fluid remains in with the stream, so the fluid is steady. The laminar flow is represented with a set of straight or curved lines called streamlines or flow lines.
For a laminar flow, all the particles will follow a streamline and no two will intersect each other. The instantaneous velocity of the particle is in the direction of the tangent to the streamline.
As the streamlines get closer, the velocity of the fluid flow will increase, and when the streamlines flow somewhat far away from each other, the velocity stays low.
Turbulent flow is just the opposite of laminar flow. Vigorous mixing occurs as a result of which the flow pattern continually changes with respect to time. In turbulent flow, the fluid layers move very fast thereby mixing the fluid layers. The velocity of the fluid keeps on changing continuously.
A laminar flow changes to turbulent flow when the flow rate suddenly increases making the steady flow unstable.
Whenever there is any abrupt change in the boundary of the pipe, turbulent flow occurs. The volume flow rate dramatically decreases when the flow becomes turbulent.
Transitional flow is the combination of both laminar and turbulent flow. Considering a pipe with laminar flow, transitional flow is said as occurred when any disturbance is created and the fluid flow at the centre of the pipe is turbulent and at the edge of the pipe, it is laminar. This is basically the transformation of laminar flow to turbulent flow.
FACTORS DETERMINING THE LAMINAR OR TURBULENT FLOW.
Reynolds number: The fluid flow depends upon this dimensionless quantity. This number is used to know whether the fluid flow is laminar or turbulent.It is the ratio of force of inertia to the viscous force.
Inertial force is due to the momentum of fluid and is given as (ꝭv) v.Viscous force is the frictional force due the relative motion between the various layers of the fluid.
- Laminar flow occurs at low Reynolds number; because the flow is steady, hence viscous force is less. For a laminar flow, the Reynolds number is less than 2100.
- Turbulent flow occurs at high Reynolds number, as the flow is unsteady and velocity is high resulting in more inertial force. For a turbulent flow, the Reynolds number is greater than 4000.
- In the transition flow the Reynolds number ranges from 2100 to 4000.
|LAMINAR FLOW||TURBULENT FLOW||TRANSITIONAL FLOW|
|Particles travel in parallel layers||Particles do not travel in parallel layers||Particles move in mixed flow, laminar and turbulent|
|Layers do not mix with each other||Layers mix in random manner||It is the stage of changing of laminar flow to turbulent flow|
|Moves along the direction of flow.||Only average motion of flow is parallel to pipe axis||Flow transits from laminar to turbulent at the centre of pipe|
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