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A fluid flowing through a closed channel such as pipe or between two flat plates is either laminar flow or turbulent flow, depending on the velocity, pipe size (or on the Reynolds number), and fluid viscosity.
Laminar Flow
Laminar flow is the movement of fluid particles along well-defined paths or streamlines, where all the streamlines are straight and parallel. Hence, the particles move in laminar or layers gliding smoothly over the adjacent layer.
Laminar flow occurs in small diameter pipes in which fluid flows at lower velocities and high viscosity. This type of flow is also called streamline flow or viscous Flow.
Laminar flow is subdivided into:
- Unidirectional laminar flow
- Pulsatile laminar flow
- Oscillatory laminar flow
The typical examples of laminar flow are oil flow through a thin tube, blood flow through capillaries, and smoke rising in a straight path from the incense stick. However, the smoke changes into the turbulent flow after rising to a small height as it eddies from its regular path.
Turbulent Flow
Turbulent flow is defined as the flow in which the fluid particles move in a zigzag way. Due to the movement of fluid particles in a zigzag way, the formation of eddies takes place, which is responsible for high energy loss.
In turbulent flow, the speed of the fluid at a point continuously changes in both magnitude and direction. Turbulent flow tends to occur in large diameter pipes in which fluid flows with high velocity.
The main tool available for the analysis of turbulent flow is CFD analysis. CFD is a branch of fluid mechanics that uses algorithms and numerical analysis to analyze and solve problems that involve turbulent fluid flows.
It is widely accepted that the Navier-Stokes equation or simplified Reynolds-averaged Navier-Stokes equations are the basis for essentially all CFD codes.
The type of flow is determined by a non-dimensional number called the Reynolds number for a pipe flow.
Re = (VD/v)
Where,
D = Diameter of pipe
V = Mean velocity of the flow in pipe
v = Kinematic viscosity of fluid
Result-
- If the Reynolds number is less than 2000, then the flow is called laminar flow.
- If the Reynolds number is more than 4000, then the flow is called turbulent flow.
- If the Reynolds number is between 2000 and 4000, the flow may be laminar or turbulent flow.
The common examples of turbulent flow are blood flow in arteries, oil transport in pipelines, lava flow, atmosphere and ocean currents, the flow through pumps and turbines, the flow in boat wakes and around aircraft wingtips.
Difference between Laminar and Turbulent Flow
The laminar flow can be visualized by an experiment using ink in a cylindrical tube. The ink is injected into the middle of a glass tube through which water flows. When the speed of the water is still slow, the ink does not appear to mix with water; the streamlines are parallel and are called laminar flow.
A sudden change will occur when the speed of the water increases. Then, the flow completely disrupts, and the water turns homogenous through the ink. Thus, the streamlines are chaotic, not linear, and are called turbulent flow.
How to Avoid Turbulent Flow?
- Try to prevent restrictions in the process, such as valves, adapters, and elbow couplings.
- Do not mount the flow meter directly behind a restriction, such as a valve. However, if this cannot be arranged differently, a turbulence filter is used between the valve and flow meter.
- Use of an elbow coupling close to a flow meter must be limited as much as possible.
- Limit the speed of flow by using the correct pipe length.
- Use the pipe length of 10X the pipe diameter at the inlet of the instrument.
- Use the pipe length of 4X the pipe diameter at the outlet of the instrument (flow meters only).
- Use the pipe length for gas flow rates > 100 l/min; it is common to use a 12 mm or ½” pipe as a minimum.
- Using a turbulence filter in the flow process will filter the flow before reaching the sensor and making it laminar flow. Flow meters such as Bronkhorst low delta P are used widely nowadays.
FAQs
A fluid flowing through a closed channel such as pipe or between two flat plates is either laminar flow or turbulent flow, depending on the velocity, pipe size (or on the Reynolds number), and fluid viscosity.
The laminar flow can be visualized by an experiment using ink in a cylindrical tube. The ink is injected into the middle of a glass tube through which water flows. When the speed of the water is still slow, the ink does not appear to mix with water; the streamlines are parallel and are called laminar flow.
A sudden change will occur when the speed of the water increases. Then, the flow completely disrupts, and the water turns homogenous through the ink. Thus, the streamlines are chaotic, not linear, and are called turbulent flow.
The equation to determine the Reynolds number for a flow is-
Re = (VD/v)
Where,
D = Diameter of pipe
V = Mean velocity of the flow in pipe
v = Kinematic viscosity of fluid
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Chezy’s and Manning’s Formula in Open Channel Flow
