When fluid is forced through a narrower passage it speeds up, and its pressure drops. On this colour-coded pipe, orange marks the high pressure at the wide ends and cyan the low pressure at the throat, while the manometer columns and pressure arrows shrink as the flow accelerates.
The Venturi effect is the drop in pressure that happens when a fluid — a liquid or a gas — flows through a narrow section of a pipe. The pipe narrows smoothly (a converging section), reaches its tightest point (the throat), and then widens out again (a diverging section).
Because the same amount of fluid has to get through a smaller opening, it speeds up at the throat. That extra speed is paid for out of the fluid's pressure, so the static pressure falls. When the pipe opens up again the fluid slows and the pressure comes back.
The pipe is colour-coded by pressure: orange where it is wide and the pressure is high, cyan at the narrow throat where the pressure is low. The three manometer tubes tell the same story with liquid columns — tall at the inlet and outlet, short at the throat — and the pressure arrows are long at the wide ends and short in the middle.
The effect is completely passive — only the shape of the passage does the work. That makes it useful: a Venturi can measure flow rate from the pressure difference, or use the low-pressure throat to draw in a second fluid, which is how a carburettor pulls fuel into the airflow.
At the inlet the pipe is full-width and coloured orange. The fluid moves at its slowest here, the static pressure is at its highest, the manometer column is tall, and the orange pressure arrows are long.
The walls curve inward smoothly. As the opening shrinks, the same flow is squeezed into a smaller area, so the fluid begins to speed up and the colour shifts from orange towards cyan.
At the throat — the narrowest point — the pipe is coloured cyan and the fluid is moving fastest. Because energy has shifted from pressure into speed, the pressure is at its lowest here: the manometer shows a short liquid column and the cyan pressure arrows are short.
Past the throat the passage opens back up. The fluid spreads out into the wider area and slows down, and the colour warms back towards orange.
As the fluid slows, the pressure rises back towards its original value. The columns at the inlet and outlet end up the same tall height and the arrows are long once more, showing that the pressure has recovered.
In a steady flow the same amount of fluid passes every point each second. Where the pipe narrows, the fluid must move faster to get through the smaller area, and the energy spent on speed is taken out of pressure — so the pressure falls.
The throat is the narrowest part of the passage, in the middle of the pipe where it pinches in. It is where the fluid moves fastest and the static pressure is lowest.
The pipe is colour-coded by pressure — orange where the pressure is high and cyan at the low-pressure throat. The manometer columns give the same reading: tall liquid columns at the inlet and outlet show high pressure, and the short column at the throat shows the pressure has dropped where the flow is fastest.
They are closely linked. Bernoulli's principle is the general rule that faster-moving fluid has lower pressure; the Venturi effect is that same rule seen in a pipe that narrows and then widens again.
It only drops by as much as the fluid speeds up, and it cannot fall below the fluid's vapour pressure. When the throat pressure does get very low, the higher pressure around it can push fluid or air into that low-pressure zone — which is exactly how a Venturi draws in a second fluid.
In carburettors that draw fuel into the airflow, in Venturi flow meters that measure flow rate from the pressure difference, in aspirators and garden sprayers, and in the steam nozzles of espresso machines.