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.
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.