The Venturi Effect explained

  • There is a direct relationship between pressure and velocity which can be used to mix gases and different substances at a constant rate.
  • Everyday products such as the choke on an engine or the air pump on a fish tank use the Venturi Effect.
  • The Venturi Effect goes against the natural assumption that pushing fluid through a restricted pipe increases pressure when in reality the increase in velocity leads to a reduction in pressure.

Giovanni Battista VenturiThe Venturi Effect was discovered by Italian physicist Giovanni Battista Venturi who lived between 1746 and 1822. In practice there were a number of other physicists who were involved in the Venturi Effect but Giovanni Venturi is generally accepted as the first person to discover and explain the effect. So, what is the Venturi Effect and how does it affect practical everyday living?

A simple explanation of the Venturi Effect

In its simplest form the Venturi Effect is perfectly illustrated with fluid flowing through a pipe which then narrows. You might be mistaken for assuming that as the pipe narrows and the fluid is forced through the narrow section there would be a buildup of pressure because of the fluid behind pushing forward. This is where the Venturi Effect comes into play because while the water is forced through the narrow section of pipe it increases in velocity and there is a reduction in pressure. Once the pipe opens up again to the original size the fluid reduces in velocity and the pressure returns to the previous level.

The mathematical formula to explain this is known as the Bernoulli equation and while the formula is fairly technical the practical examples of the Venturi Effect are many.

Practical demonstrations of the Venturi Effect

One interesting example is that of a skyscraper when you stand at the base of the building and feel the wind blowing at what feels like a quicker rate than elsewhere. In this instance, where there are many buildings close together, the wind is channelled down a narrow path than further up the building hence the increase in velocity but the reduction in pressure. In this instance the principle of continuity and the principle of conservation of mechanical energy come into play. The Bernoulli principle states that where there is an increase in velocity there is a reduction in pressure and vice versa – a Venturimeter can be used to demonstrate core fluid mechanics concepts.

Interestingly, the Venturi Effect is also often described as the choke effect which is perfectly illustrated with the old-fashioned “choke” on a carburettor of a vehicle powered by an internal combustion engine. The choke works by restricting the area in which the air flows which reduces the pressure, increases the velocity and sucks in fuel from a fuel pump. The throttle valve works in a similar manner controlling the flow of air and fuel by restricting the size of the area through which they flow.

A traditional aquarium pump is also a perfect illustration of the Venturi Effect with the water tube connected to an adjustable air valve. As the water is circulated throughout the tank it goes through a restricted area of the tube which is connected to the air supply. As the pressure reduces when the water enters the restricted area, the water flow increase in velocity and drags in air from the air tube, very similar to the “choke” on an engine. The amount of air which is sucked in through the constricted pipe can be varied using the air valve.

The Venturi Effect is extremely commonplace in industry where a variety of different gases need to be mixed at controlled rates. The principle is obviously the same, the gas flows through a pipe which is then constricted, the pressure falls, the velocity increases and gas from a connected pipe is then sucked in and mixed together. As we are able to calculate the change in velocity for a specific pressure it is possible to create the perfect mix of different substances/gases.

Summary

For many people the Venturi Effect can be difficult to understand because you might expect the pressure to increase when a fluid is pushed through a restricted area. The fact that the increase in velocity is greater than any potential increase in pressure means that there is a net increase in velocity and a net reduction in pressure. The ability to mix-and-match certain fluids and gases via this process is relatively straightforward because the reduced pressure allows other substances to be sucked in through a connecting pipe at a rate of your choice.

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