Flow Tank

Choose a shape and place it the middle of a flowing, thick and shiny fluid. Watch how the fluid flows into intricate vortices and patterns as you angle the shape. Which shapes are the most aerodynamic and create the least turbulence?

Explanation

The fluid in this tank contains shining particles that reflect the light, showing you how it is flowing.

As it flows over the shapes, it forms small areas of turbulence that you can see as swirls.

Some shapes create more turbulence than others. This turbulence results in a low-pressure region, which creates a drag force on the shape. Shapes that have sudden ‘drops’—relatively large surfaces that are perpendicular to the flow of the fluid around them—experience the most turbulence and drag.

You might have felt this turbulence if you’ve tried to shelter from a strong wind behind a flat, vertical wall. The wind swirls over the top of the wall and around its edges, then curls around and forms vortices right where you stand! You will actually experience more turbulent wind this way.

This is why windbreaks tend to consist of irregular shapes. They are so called because they break up the wind and reduce its force instead of trying to block it out entirely. Trees make good wind breaks.

As you play with Flow tank, keep in mind that you’re conducting flow visualisation experiments, a beautiful field of science! Flow visualisation examines flow patterns around an object and its surface.

Extras for Experts

Have you ever wondered why a golf ball has a dimpled surface? It’s all explained by fluid dynamics!

When you hit a golf ball, it flies through the air and the air flowing over its surface acts as a fluid. If the ball’s surface is perfectly smooth, the air flows around the front face of the ball, partially follows the ball’s curve, then separates from the ball a little after it has curved halfway around the ball. The separated air then swirls behind the ball, creating a low pressure region. This low pressure region creates a drag force on the ball, slowing it down and reducing the distance it can fly.

If we could move the region of air separation to a point further back on the ball, the low pressure region and hence the drag force on the ball would be smaller. The ball could then fly further!

The tiny dimples on the golf ball create a small turbulent layer just at the surface of the ball. This turbulent layer adds energy to the air as it flows over the ball, keeping it close to the ball’s surface for longer. This pushes the region of air separation further back on the ball’s surface, reducing the drag.

Why aren’t cars and planes dimpled? This is because these shapes are designed to keep the air flow close to the surface of the vehicle until it reaches the trailing edge. Drag is reduced by using the shape of the vehicle itself. The spoilers on cars are there to reduce the sudden dip between a car’s bonnet and its trunk, interrupting the region of air flow separation and thus reducing drag. This helps the car move forward and increases its fuel efficiency.

Questions to Ask

What do you think is important when designing a new car? How do you think you could make it use less petrol and help the environment?

Further Reading

• Aerospace web on golf balls
  http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml

Flow Tank
As the fluid flows around the various shapes, intricate vortices and patterns are formed.

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