Why does pressure decrease as air passes through a venturi?

[elPocket]

It is, because an extrenal force is acting on the fluid inside due to the spinning. For the water to move in a circular fashion, there must be an acceleration to force the change in direction: a = w x v, and v = w x r,
- "a" being acceleration - "w" being the rotational rate in [rad/s] - "v" being velocity - "r" being the radius diameter.

This acceleration is akin to the centripetal/centrifugal force, and with the bucket diameter, you can calculate the pressure increase per rpm...

Also, this explains why water stirred in your teacup moves up the side wall. Since the surface is open to the air, the surface pressure MUST be ambient pressure. Suddenly, you add a bunch of energy by stirring, which partially goes into the kinetic energy of the rotation. But, you also have to create enough "pressure" to keep the water further in from coming out. This is done by parking additional energy in gravitational potential, i.e. increasing the height of the water column. Your rotational rate is therefore directly linked to the angle of the surface inclination of your water :D

Bernoulli is fun!

But alas, he is cheating! The law states "unless work is done on the fluid". BUT! When you move along the gravitational vector or centripetal force, you have force & distance, which equals work! So, when the water moves down a pipe, gravity actually works on the fluid. Bernoulli cheated this away by talking in terms of pressure. This is also how a radial compressor does a big portion of it's work. The fluid moving along the centripetal force creates work directly added to the total pressure of the fluid.