Fluid Dynamics

Fluid Dynamics: Have a go at building your own mini engineering project!

Today, you will learn how engineers use their understanding of the physics of fluids to manipulate them and to control their flow in order to solve real-world problems. So, what is a fluid? Let’s start by considering the three states of matter we commonly encounter and work out whether or not they are fluids.

Solids have strong intermolecular bonds which hold them together. Chunks of solid placed in a container will not deform into the shape of the container, and they are not easily compressible. It typically takes a lot of force to change the shape and volume of a solid.

Liquids have weaker intermolecular bonds which allow individual molecules to move freely within the liquid. This is why liquids placed in a container will take on the shape of that container: think about how if you pour a bottle of water into a jug, it changes shape to fit the jug. But like solids, it requires a fair amount of force to compress a liquid.

Gases have negligible intermolecular bonds which allow molecules to move freely throughout the available space and to have much larger space between them. A gas placed in a container will spread out to fill the entire space available in the container. Conversely, if you make the container smaller or exert force on the gas, it is relatively easy to compress.

Both liquids and gases are considered to be fluids in the field of Fluid Mechanics, because they are deformed under a force of any magnitude. You wouldn’t be able to swim through water if it didn’t move and change shape when you push against it!

Today we have two sets of activities for you to choose from. If you’re new to engineering and want to learn about the science behind how water moves through pipes, choose the first activity to watch a fun demonstration and then have a go yourself.

If you’ve learnt about engineering before, or you’re feeling more confident, you might like to take a look at the second activity which challenges you to put these principles into practice in a real-world scenario under the streets of London!

Activity 1 – Click this bar to view

Pipe Flow

One of the most common everyday examples of fluids in motion is in the flow of liquids through pipes. When you turn on the tap or the shower, you expect a steady flow of water, but how does it get there? You might have used a garden hose before, which is a good example of pipe flow, with the amount by which you turn on the tap, the length, diameter, material and shape of the hose all affecting the rate of flow of water at the other end.

In the video below, Grady Hillhouse builds a pipe system and experiments with how these factors alter the loss of pressure in a pipe. He uses the Hazen-Williams equation to explain his results, and describes how engineers use a Hydraulic Grade Line to chart changes in pressure across the entire length of a pipe.

Grady used the term headloss, which is a way of describing the difference in pressure at the start and end of a run of pipe. In a fluid, pressure can be exchanged for potential energy – in the form of height – or kinetic energy, in the form of increased velocity. In almost all practical cases, some of this energy will be lost due to friction with the side of the pipe. Engineers want to minimise this loss to make the flow more efficient and to reduce the amount of energy needed to pump fluid through the pipe.

Activity: Juice VS Gravity

In this activity you will have a go at building your very own basic pipe system. The “pipe” will be a drinking straw! You will need:

  • A cup
  • Some drinking straws
  • Sticky tape and scissors
  • Something to drink – we recommend a coloured liquid like squash, since it’ll be easier to spot if your pipe leaks!

If you don’t have these items on hand right now, feel free to come back and try this activity later.

Start by just drinking normally through a straw. It should be pretty easy to drink through. Our team member Alfie is demonstrating this for us!

Of course, real pipe systems are normally a lot longer than this. Use your tape and scissors to combine two straws carefully. Make sure the ends of the two straws are lined up and that they are sealed together well to avoid any leakages.!

Keep adding more straws and experiment with how different lengths of pipe make it harder or easier to drink the juice. What force do you think is changing this? Is it friction with the wall of the pipe, leakage, minor losses through poor quality joins between the straws, or something else?

By the time your pipe gets a lot longer, you’ll probably find it really hard to drink through! We’d love to see some photos of your creations and your ideas about what forces are at work here. We had lots of fun experimenting with really long straws!

Going Further: In a theoretical pipe system with no losses, Bernoulli’s equation can be applied to understand how energy is conserved throughout a pipe. Click here if you’d like to go further and learn more about Bernoulli’s equation

Activity 2 – Click this bar to view


Ventilation systems are one way in which engineers manipulate gases (usually air) in order to keep people comfortable and safe. Ventilation is especially important in environments where vast amounts of heat and potentially toxic gases could quickly create a dangerous situation. In such a situation, if there is insufficient fresh air, people would be unable to breathe, and the excess heat could cause critical systems to fail and increase the risk of fire.

An example of an environment like this is an underground railway, of which the Elizabeth Line, which opened earlier this year in London, is a good example. When the engineers for this project (also called “Crossrail”) were thinking about the need to manage the flow of air underground through ventilation systems, they identified four key requirements of a ventilation system

  • To relieve the air pressure resulting from the movement of trains
  • To remove the heat generated by running trains from the tunnels
  • To keep trains stopped in the tunnel cool
  • To remove smoke from the tunnels and keep evacuation routes clear in an emergency

Watch the video below which shows some practical examples of how the Crossrail project tackled the issue of ventilating the tunnels and trains, and how the system the engineers developed was capable of adapting to different requirements according to the conditions. Think about how the system they developed addresses the four requirements we mentioned above.

Did you notice one of the engineers commented that “during normal operations, there’s no need for forced ventilation” (1:44)? Let’s take a closer look at how the ventilation system handles the flow of air in the Crossrail tunnels during normal operation, during periods of congestion or delay, and during an emergency situation.

Normal Running

As the engineer outlined in the video, the trains running through the tunnel create a draught by forcing the air in front of and above them forwards as they move through the tunnel. This helps air flow through the tunnel to keep it cool but it also increases the air pressure by forcing air into a smaller space. Remember what we said about fluids being easily deformed and compressed? Fluctuations in air pressure can cause discomfort for passengers and maintenance workers in the tunnel. The body isn’t designed to withstand such changes. If you’ve ever felt your ears pop or felt funny in a tunnel or on a plane, you’ve experienced this. The increased pressure also increases the resistance acting against the train body which means it needs to draw more power to run.

To relieve the pressure created by the train displacing air, and to remove some of the warm air from the trains, the engineers installed Draught Relief Shafts. These are a form of passive cooling, since they are outlets for the air but do not require energy to be exerted by forcing the air to move artificially through fans or other equipment. The diagram below shows how this passive cooling through draught relief shafts works using the draught generated by train movement.

A block shape diagram shows two shafts labelled “Forced Vent Shaft” connecting to an underground tunnel and with a fan at the top of each. One shaft is labelled with a blue arrow representing cool air being drawn in towards a train in the tunnel marked with the word “stationary”, and the other shaft is labelled with a red arrow showing warm air being drawn upwards out of the tunnel.

Congestion and Emergency Situations

This passive cooling is helpful, but is not sufficient to cool trains stopped in the tunnel, since trains standing still would not be creating the draught necessary to make the draught relief shafts useful.

The Crossrail engineers calculated that after about 6 minutes stopped in the tunnel, a train would require cooling to ensure the temperature in the tunnel did not become dangerously high, causing discomfort to passengers and meaning that the systems onboard the trains would not be able to operate safely. Without the benefit of the passive cooling, trains in this scenario require active cooling, achieved through the installation of forced vent shafts. These shafts use huge powered fans to force large volumes of air into motion and to cool trains stopped in tunnels, and are remotely controlled by the railway controllers. One of the benefits of this active cooling is that the fans can either push air into or draw it out of the tunnel depending on the situation. When a train is stopped in the tunnel, one fan is configured to draw fresh air in, and another to expel the air heated by the train.

The forced vent shafts are also useful for dealing with emergency situations and can be used to extract smoke rapidly from the tunnels to allow a train to be evacuated if there is a fire. In the challenge below you will be asked to think about how you would deal with this situation.

Challenge: Fire in the tunnel!

A train has caught fire and is stranded deep beneath the surface! The tunnel is rapidly filling with deadly smoke. To save the passengers and make it safe for rescue workers to get them out, you need to make use of your knowledge of the ventilation systems to extract smoke from the tunnels. Remember, you can use the fans in the forced vent shafts either to draw in or to extract air. There are several possible combinations for how you could operate the fans, but which will move hot air and smoke most efficiently? Draw out your own version of the diagram above. How will you react to save the day? Add arrows to show the movement of air. What else needs to be done to respond to the situation? Make some bullet points that the train controllers can follow in this situation.

When you’re done, click this bar to see which configurations we came up with.

The simplest approach is to configure both forced vent shafts to draw air out of the tunnel. This will efficiently draw smoke out of the tunnel, but it won’t actively cool the tunnel since no cool air is being drawn in. Moreover, if the fire is in the middle of the train, the fans will draw it through the rest of the train, increasing the number of people at risk of smoke inhalation.

The solution used by the Crossrail engineers is shown below. One forced vent shaft draws in fresh, cool air to cool down the train and to provide air the passengers and rescue workers can breathe. This also creates a draft which forces the air towards the other forced vent shaft which draws the smoke out of the tunnel leaving it clear and ready for emergency workers.

Going Further

If you have enjoyed learning about how engineers use their understanding of fluids to manage public infrastructure, you might like to think about how the same principles can be used to limit damage in a natural disaster.

Click here to watch another video from Practical Engineering in which Grady discusses the methods engineers use to control flooding.

The same principles of fluids apply, and the way you divert water to avoid a flood in one area can have consequences elsewhere.

Click here to play a game provided by the UN Office for Disaster Risk Reduction in which you will be tasked with budgeting for and constructing structures to mitigate the impact of natural disasters.

This might prompt you to think about how engineers have to weigh up the cost and potential benefit of their proposed measures, and how education can sometimes be just as effective as physical engineering interventions.

Emmet O'Leary
Emmet O’Leary

Emmet O’Leary studied History at St John’s College, Oxford. Whilst studying he spent time as JCR Class Rep, promoting the interests of students from under-represented socioeconomic groups and later as President of the Junior Common Room (JCR). His undergraduate thesis explored the impact of Famine in Ireland, and its demographic shockwaves, on cultural and religious practice in the nineteenth century. He has wide-ranging historical interests including the writings of the Enlightenment and the social upheavals of the European Reformations and is also keenly interested in constitutional law and its connection to current affairs. Emmet is currently Access and Outreach Intern at St John’s.