A moving liquid exerts a pressure, and the pressure exerted will depend on the velocity of the liquid. A liquid flowing into a pipe will tend to exert a larger pressure than a liquid flowing past a pipe opening. Using this difference we can find the speed of a fluid using the apparatus below.
The fluid exerts a static pressure – the sum of any external (often atmospheric) pressure and pressure due to the depth of liquid at a point – and dynamic pressure due to the velocity of the liquid.
From Bernoulli's equation
where
is the density of the liquid.
Subtracting the external pressure then gives
The speed v may then be found by rearrangement:
From the diagram above, the dynamic pressure is
so