A fluid passing through smoothly varying constrictions experience changes in velocity and pressure, as described by Bernoulli's principle. A Venturi is a system for speeding the flow of the fluid, by constricting it in a cone-shaped tube. They are found in many applications where the speed of the fluid is important, and form the basis of devices like a carburetor.
Venturis are also used to measure the speed of a fluid, by measuring pressure changes from one point to another along the venturi. Placing a liquid in a U-shaped tube and connecting the ends of the tubes to both ends of a venturi is all that is needed. When the fluid flows though the venturi the pressure in the two ends of the tube will differ, forcing the liquid to the "low pressure" side. The amount of that move can be calibrated to the speed of the fluid flow.
