• What is Venturi’s effect?

    Or how to apply physics principles to cool down your soup.

Venturi’s effect happens when a fluid goes through the narrowing of a conduct; as it gets nearer to the narrowing, its speed increases, whereas static pressure goes down. On the contrary, as it moves away from the narrowing, speed decreases and static pressure increases.

This effect was proven by the Italian physicist Giovanni Battista Venturi in 1797 and is applied to many everyday actions that we present to you as examples, further on in this post.

For now, let’s study Venturi’s effect theory a little, as we did with our post about Bernoulli’s Principle. Read on!

Theory and formula of the Venturi effect

Venturi’s effect is the most common application of Bernoulli’s principle so it forms part of its explanation; although we could also explain Venturi’s effect by the principle of continuity of mass.

Also through the conservation of energy theorem, the sum of the three forces (speed, static pressure and dynamic pressure) must be kept constant, therefore, the modification of any of them, inevitably carries a variation of the others.

This relation is explained by the following formula:

Where:

  • V = velocity of fluid in the studied section
  • P = pressure at the studied point
  • g = gravitational acceleration
  • γ = specific weight
  • z = vertical height
  • Subindexes = values at point 1 and 2
CONVERGENTDIVERGENT
SPEEDIncreasesDecreases
DYNAMIC PRESSUREIncreasesDecreases
STATIC PRESSUREDecreasesIncreases
TOTAL PRESSUREConstantConstant

Another of the factors to keep in mind is the fluid’s temperature, which decreases significantly in the narrowing.

Additionally, so that the table above is easier to understand, we should explain, a conduct is convergent when it becomes close to the gorge of the narrowing and divergent once it has gone past it.

Applications of Venturi’s effect in aviation

The Venturi effect is the main agent responsible for the wings of aeroplanes generating sustentation and it is very likely you have noticed that their wings aren’t symmetrical.

The top part of the wing, known as extrados, is more curved than the bottom part, termed intrados.

This difference in the curvature is designed to produce Venturi’s effect on the wing, as air circulates through the extrados must increase its speed to reach the trailing edge of the wing at the same time as the air that circulates through the intrados.

When speed increases, static pressure decreases at the extrados and this is one of the reasons airplanes fly.

Another application of Venturi’s effect is in carburettors. Carburettors mix fuel and air in the most homogeneous possible way and achieve this by narrowing, so that the mix increases its speed and becomes more homogenous.

Furthermore, Venturi’s effect is used in gyroscopic instruments or those requiring a vacuum for operating.

Did you know…

Did you know formula 1 cars use Venturi’s effect in the opposite way aeroplanes do?

In aviation, aeroplanes are made to fly but in car racing, the Venturi effect is applied to achieve more adherence to the tracks, increasing speed at bends.

This is achieved leaving the least possible space between chassis and ground, making the air that circulates over the car push it down.

Applications of Venturi’s effect in our day-to-day lives

Let’s do the fun part!

Blowing with your lips

Returning to this post’s title, where we warned you Venturi’s effect can cool your soup down, we’d like to ask you to press your lips together and blow. Is the air cool? Now try with your lips apart and you’ll see it’s warm.

This is due to your lips creating the narrowing which causes a simple Venturi. See how we use physics for the most basic things?

Tongue effect with a sheet of paper

Now, if you’d like to carry on doing experiments, let’s grab a plain sheet of paper and cut it into two halves at its length. Hold the piece bringing the narrowest end right under your lips. Blow hard and you’ll see the sheet rise rather than go down.

By doing this, you’re accelerating air at the top part of the sheet of paper, decreasing static pressure and making the piece go up as if it were a wing.

Venturi with a double tongue

And one more! Take the two halves of the sheet you’ve just cut and hold them vertically from the narrowest part, separate them 3 to 4 cm and keep them parallel to each other. Now blow hard in the space between them and watch how they separate instead of coming closer together.

What has happened is that you have increased the velocity in the airflow between the two sheets of paper, causing the external pressure (on the other side of each sheet) to increase and push the sheets inwards.