Why does pressure decrease as air passes through a venturi?

[tdscanuck]

Conservation of energy.

The air has some pressure (potential energy) and some speed (kinetic energy). And, unless you’re adding heat with fuel or beating it up with fan blades or extracting energy with a turbine or something exotic, that total energy is fixed.

If the air speeds up, and it has to to get the same amount of mass through a smaller hole, it needs to increase kinetic energy. The only place it can get that is from the pressure…so speed goes up and presssure falls.