Ok after hours of contemplating I think I finally understood the concept(hopefully ). However I still need 1 quick clarification.

 

When water flows suddenly from a larger to a small dimater

Bernoulli says

 

1. Pressure decreases

2. Velocity of fluid increases

 

I'll put numbers just for easy understanding

 

This is according to Bernoulli assuming no loss

Energy in large diameter (Hydrostatic pressure=4 Kinetic energy=2)

Energy in small diameter(Hydrostatic pressure=2 Kinetic energy=4)

 

Ok in real life when you consider resistance in the smaller tube is it the K.E that is lost to create lower fluid velocity. If hydrostatic pressure is the pressure that exerts on the walls, why isn't that decreased instead. I mean resistance from walls only affect molecules hitting against it which is H.P. I just like to know what type of resistance decreases K.E in real life. Thanks a lot.

The average velocity has to increase to maintain steady state -- if the velocity didn't increase, there would have to be a leak or accumulation in the system.

 

And then, since the velocity increased -- the KE increases, and since the total energy remains constant, the energy that was in the pressure decreases.