3. Turbulence

3.1 Turbulence in nature

 

This is a simple experiment to demonstrate the laminar flow: Go to your kitchen sink and open the faucet. The stream of water that emerges from your faucet is very smooth and very regular. The flow of water is smooth because all the water molecules move, at more or less the same speed, in the same direction. This is called a laminar flow. Furthermore, if you did not open the faucet too much, the water will also flow down the drain in a laminar flow.

What is a flow? A flow is the continuous movement of a fluid, either a liquid or a gas, from one place to another. Basically there exist two types of flows, namely laminar flows and turbulent flows. Forces Roughly speaking we can say that a laminar flow is a 'simple' flow while a turbulent flow is a 'complicated' flow. We will illustrate what we mean by 'simple' and 'complicated' using the following, simple experiments.

Another example of turbulence you can easily observe at home is in a cup of hot water when you put the tea bag or the flow of water in a boiling pot of water. Put water in a pot and heat it up on your electric cooker. If you wait for a short while, the water will start to move in a laminar way, in a very regular way. If you wait a bit longer bubbles will start to rise from the bottom to the surface and the motions of the water become very complicated or turbulent. In this particular case the turbulence is due to convection.

Convection refers to the movement of molecules within fluids (liquids and gases). Convection is one of the major modes of heat transfer and mass transfer.

A faucet and a cup

 

Now we can define the laminar flow:
Laminar flow is that state of fluid motion which is characterized by same direction, same speed, the move being more or less smoothly.

 

After you have done the previous experiment, try this simple experiment to demonstrate the turbulent flow:
Place a cup under the stream of water emerging from the faucet. Although the stream is still laminar, the flow pattern of the water in the sink has become very complicated. This is due to the fact that now the water molecules tend to move in different directions at different speeds. Such a flow is called turbulent.

 

Now we can define the turbulent flow:
Turbulence is that state of fluid motion which is characterized by apparently random and chaotic three-dimensional velocity. When turbulence is present, it usually dominates all other flow phenomena and results in increased energy dissipation, mixing, heat transfer, and drag. If there is no three-dimensional velocity, there is no real turbulence. The reasons for this will become clear later; but briefly, it is ability to generate new velocity from old velocity that is essential to turbulence.

 

Try this simple experiment to visualize the turbulent flow of the smoke generated by a perfumed stick. For the first few centimeters, the flow remains laminar, and then becomes unstable and turbulent as the rising hot air accelerates upwards. After that you can place a pen or an eraser in the smoke (in the first few centimeters when the flow is still laminar) and observe what is happening. The smoke will avoid the obstacle and will start to flow in very different directions and different speeds, it will be very unstable and turbulent.

Where does turbulence occur?

Smoke Cup of tea River boulders

Turbulent motions are very common in nature.

Turbulence occurs nearly everywhere: in the oceans, in the atmosphere, in rivers, even the flow of blood in arteries, oil transport in pipelines, lava flow, flow through pumps and turbines, and the flow in boat wakes and around aircraft wing tips, even in stars and galaxies. In fact it is easier to find a turbulent flow than a flow that is really laminar.

Here are a few examples of turbulent flows: smoke rising from a perfumed stick; similarly, the car exhaust fumes and the dispersion of pollutants in the atmosphere are governed by turbulent processes; the wake of a ship or submarine is turbulent; the rain water on the road; the swirls and eddies in a fast flowing river are turbulent.

How is turbulence generated ?

How easily a fluid becomes turbulent depends to a large extend on its viscosity. Simply speaking, viscosity is the resistance of a fluid (either a liquid or a gas) to movement. The more viscous a fluid is the less likely it is to become turbulent.

Thus, water and air which have a low viscosity can become turbulent relative easily, while honey or syrup, which are very viscous, tend not to become turbulent.

There are many ways in which a fluid can become turbulent.

1. Heating: If you heat a fluid at the bottom and cool it at the top the fluid becomes turbulent due to convection. This is what happens in a boiling pot of water.

2. Pressure: The water stream that emerges from the faucet in the picture above is laminar. This is because the faucet is not fully open and the pressure in the pipe is fairly low. If you open the faucet to its full extend the water will shoot out in a very wild manner. When the faucet is fully open the pressure in the pipe is very large.

3. An obstacle introduced in the flow. For example, a river may flow smoothly until it hits a boulder, at which point the water around the obstacle will become turbulent as it moves around or over it. In the air, turbulence can be caused by things such as the collision of two weather fronts, or by the formation of a storm. Air turbulence can also be caused by obstacles on the ground, ranging from mountains to buildings. The agitated, irregular motion usually involves movement at various rates of speed, and a number of factors can influence the movements of liquids and gases. This is why turbulence on an aircraft can be difficult to predict.

3.2 Turbulent flow around aircraft

Turbulences and Inclination Wing

Turbulence on a wing

All solid objects traveling through a fluid (or alternatively a stationary object exposed to a moving fluid) acquire a boundary layer of fluid around them where viscous forces occur in the layer of fluid close to the solid surface. Boundary layers can be either laminar or turbulent. A reasonable assessment of whether the boundary layer will be laminar or turbulent can be made by calculating the Reynolds number of the local flow conditions. The flow can become unstable, and it can experience transition to a turbulent state where large variations in the velocity field can be maintained. If the disturbances are very small, as in the case where the surface is very smooth, or if the wavelength of the disturbance is not near the point of resonance, the transition to turbulence will occur at a higher Reynolds number than the critical value. So the point of transition does not correspond to a single Reynolds number, and it is possible to delay transition to relatively large values by controlling the disturbance environment. At very high Reynolds numbers, however, it is not possible to maintain laminar flow since under these conditions even minute disturbances will be amplified into turbulence. The area of separation is called separation bubble.

 

The boundary layer is that layer of fluid in the immediate vicinity of a boundaing surface. The Reynolds number, Re is a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces.

3.3 What is the turbulence wake?

Turbulences and Shapes

Turbulence on a Shape

Turbulences and Inclination Wing

Turbulence wake is the turbulence that forms behind an aircraft as it passes through the air. All aircraft produce wake turbulence, which consists of wake vortices formed any time an aerofoil is producing lift.

Lift is generated by the difference in pressure over the wing surfaces. The lowest pressure occurs over the upper surface and the highest pressure under the wing.

Air will want to move towards the area of lower pressure. This causes the air to move outwards under the wing and curl up and over the upper surface of the wing. This starts the wake vortex. The pressure differential also causes the air to move inwards over the wing. Small trailing edge vortices, formed by outward and inward moving streams of air meeting at the trailing edge, move outwards to the wingtip and join the large wingtip vortex.

When a solid body is placed in the wind, the air will start flowing differently, creating turbulence.

 

Angle of attack (a) is a term used in fluid dynamics to describe the angle bewteen a reference line on a lifting body (often the chord line of an airfoil) and the vector representing the relative motion between the lifting body and the fluid through which it is moving.

Turbulences of a Plane

Turbulence on a landing plane

Simulation of Turbulences

The laser visualization in INCAS wind tunnel

The degree of turbulence is also influenced by the angle of attack. Angle of attack is the angle between the lifting body's reference line and the oncoming flow.

The air flow from the wing of this agricultural plane is made visible by a technique that uses colored smoke rising from the ground. The swirl at the wingtip traces the aircraft's wake vortex (the turbulence that forms behind an aircraft as it passes through the air), which exerts a powerful influence on the flow field behind the plane.

The turbulence generated by plane’s wings can be made visible in the wind tunnels, using laser visualizations. (See the laser visualization in INCAS wind tunnel)

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Dr. Corieri Patricia

von Karman Institute for Fluid Dynamics
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