What Causes Turbulent Flow? Key Factors and Physical Principles
Do you know what turbulent means? Well! Turbulent Flow is the rough or a chaotic movement of fluid through a region.
Chamoli Glacier Burst led to a haphazard flow of water inside the city; this haphazard flow of water is the turbulent flow. So, the turbulent water is full of fluctuations whose speed varies at every point in terms of magnitude and direction.
However, in Physics, we will understand what is turbulent flow and the turbulent flow equation as well.
What is Turbulent Flow?
In the brief introduction to the meaning of turbulent flow, we understand that turbulent water or fluid turbulence is an irregular fluctuation or mixing of two or more liquids.
Here, the fluid movement is in such a way that at every point, the speed of the liquid undergoes continuous variations in both magnitude and direction.
So, the rough movement of the fluid at every point is the cause of these fluctuations, that’s why we call the water turbulent water.
Point to Note
One must note that for a turbulent flow, the speed of water should be high. If it gets low, the flow becomes a smooth or non-turbulent flow.
A Must Note
The turbulence concept is considered the last unsolved problem of Classical Mathematical Physics.
What is Turbulent in Fluid Mechanics?
The concept of turbulence flow is straight-forward. Just look at the image below to understand it visually:
(Image to be added soon)
This image clearly shows the difference between a turbulent flow and a non-turbulent flow.
There is a term called Reynold’s number that helps us to elaborate on the turbulent flow equation.
So, What is Reynolds Number?
A Reynolds number helps us determine whether the flow is laminar or turbulent. It is the ratio of internal forces to the viscous force.
Now, let’s understand the Reynolds number concept through the Turbulent Flow Equation:
Turbulent Flow Equation
The equation is as follows:
Re = (⍴𝜈D) /𝜇
Here,
⍴ = Fluid Density in Kgm-3
𝜈 = Kinematic viscosity in m2s-1
D = Characteristic Linear Dimension
𝜇 = Dynamic viscosity in Pa.s
A low value of Reynolds number (Re) means that if the viscous forces of liquid are more than internal forces in a fluid, i.e., a liquid carries a low Reynolds Number or Re, the liquid is sufficient to align all its particles inline, and therefore, the flow becomes laminar. However, a higher Reynolds number means the flow is turbulent.
We can rewrite the equation (1) in the following manner:
Re = (⍴𝜈D) /𝜇 = VD/𝜇
Here,
V = Flow velocity in m/s
𝜈 = Kinematic viscosity, which is given by
𝜈 = 𝜇/⍴
Transition From Laminar Flow to Turbulent Flow
For turning the laminar flow of fluid to turbulent, Reynolds number with respect to x should exceed Rex ∼ 5,00,000.
The turbulence may occur before the above range, depending on the roughness of the surface or a region through which water flows. Also, the turbulent boundary layer thickens quickly than the laminar boundary layer because of the increased shear stress at the body surface.
Turbulent Velocity
In a turbulent flow, there is varying empirical velocity at every point. The simplest and the best-known velocity profile is the power-law velocity profile.
The velocity profile in turbulent water flow is flatter in the central part of the pipe, which is the turbulent core than in the laminar flow of the fluid. The flow velocity drops sharply when extremely close to the walls. This is because of the diffusivity of the turbulent flow. It has the following graph:
(Image to be added soon)
Here, you can see that the rise in the velocity of the fluid shows turbulent flow, while the drop in velocity shows that the flow is laminar.
Turbulent Flow in Everyday Life
If you have seen the windmills in your area, then you are close to the concept of turbulent flow.
A high-speed turbulent airflow leads to the movement of windmills, which, in turn, generates electricity that we get in our homes.
A turbulent water flow leads to the rotation of turbines and this rotatory motion produces electricity. The turbulent flow concept has helped us in performing daily chores during nights.
Oil transport in pipelines is a turbulent flow.
The pipeline gas system has relieved us from standing in a queue for purchasing cylinders. This is possible because of the turbulent flow of gas to our kitchens.
Blood flow in our arteries is a turbulent flow.
We have all heard a story since our childhood that magma is generated by the internal heat of the planet or moon and it erupts as lava at volcanoes. Here, the lava flow is turbulent.
An aircraft lifts its wings because of the dynamic lift. Here, dynamic lift occurs because of the pressure difference between the upper and lower airflow, which is the turbulent airflow.
Smoke emitting from a cigarette is a turbulent flow.
FAQs on Turbulence Flow Explained: Definition, Examples & Equations
1. What is meant by turbulent flow in physics?
Turbulent flow is a type of fluid motion characterised by chaotic, irregular, and unpredictable changes in pressure and velocity. Unlike the smooth, orderly movement seen in laminar flow, particles in turbulent flow move in erratic paths, creating swirls, eddies, and vortices. This behaviour typically occurs when a fluid moves at high speeds, has low viscosity, or flows past an obstacle.
2. What are the main characteristics of turbulent flow?
Turbulent flow is defined by several key characteristics:
- Irregularity: The flow is disordered and random, making its path difficult to predict deterministically. It is often analysed using statistical methods.
- Diffusivity: It exhibits high rates of mixing and diffusion of mass, momentum, and energy. This is why smoke from a chimney quickly spreads out in the wind.
- Rotationality: The flow contains numerous small, swirling regions of fluid known as vortices or eddies.
- Energy Dissipation: A significant amount of kinetic energy is converted into internal energy (heat) due to viscous shear stresses, leading to greater energy loss compared to laminar flow.
3. What is the difference between laminar and turbulent flow?
The primary difference lies in the fluid's behaviour. In laminar flow, fluid particles move in smooth, parallel layers (or streamlines) that do not cross. Think of honey flowing slowly from a spoon. In contrast, turbulent flow involves chaotic, swirling motion where fluid layers mix extensively, like a rapidly flowing river or smoke billowing from a fire. Laminar flow occurs at low velocities and high viscosities, while turbulent flow occurs at high velocities and low viscosities.
4. How does the Reynolds number help in identifying turbulent flow?
The Reynolds number (Re) is a dimensionless quantity that predicts the flow pattern in different situations. It represents the ratio of inertial forces to viscous forces within a fluid. A low Reynolds number (typically Re < 2000) indicates that viscous forces are dominant, leading to smooth, laminar flow. A high Reynolds number (typically Re > 4000) signifies that inertial forces dominate, causing chaotic, turbulent flow. The range between these values is considered transitional.
5. How does a fluid's viscosity influence its tendency to become turbulent?
Viscosity is a measure of a fluid's resistance to flow. A fluid with high viscosity, like syrup, has strong internal friction that dampens disturbances and resists changes in motion, thus promoting smooth, laminar flow. Conversely, a fluid with low viscosity, like air or water, has weaker internal friction. This allows small disturbances to amplify and grow, quickly leading to the chaotic eddies and swirls characteristic of turbulent flow.
6. Why is turbulent flow so common in everyday phenomena like rivers and smoke?
Turbulent flow is common in nature because most natural fluid systems, such as rivers, ocean currents, and atmospheric winds, involve large-scale movements at high velocities. These conditions result in very high Reynolds numbers, favouring turbulence over laminar flow. Additionally, the presence of irregular boundaries, like a riverbed or obstacles on the ground, introduces disturbances that disrupt smooth flow and trigger the transition to a chaotic, turbulent state.
7. What are the practical consequences of turbulent flow in engineering systems?
Turbulent flow has significant practical consequences, both negative and positive. On the negative side, it increases drag on vehicles like cars and airplanes, requiring more energy to overcome. It also causes greater energy loss in pipelines, necessitating more powerful pumps. However, the high mixing rate of turbulence is beneficial for enhancing heat transfer in radiators and cooling systems, and it is essential for achieving efficient combustion in engines by rapidly mixing fuel and air.
8. Is turbulent flow always an undesirable effect to be avoided?
No, turbulent flow is not always undesirable and is often intentionally created in many applications. While it increases drag and energy loss, its property of enhanced mixing is crucial. For example:
- Heat Exchangers: Turbulence in a car radiator helps to transfer heat away from the engine coolant more effectively.
- Combustion: In an internal combustion engine, turbulence ensures that fuel and air are thoroughly mixed for complete and efficient burning.
- Aerodynamics: The dimples on a golf ball are designed to create a thin turbulent boundary layer, which paradoxically reduces the overall pressure drag and allows the ball to travel farther.
