classical mechanics - Why does water gulp out of a water bottle with a narrow opening instead of a steady flow?

For example, take a water bottle. Fill it with water and then turn it upside down. Instead of flowing steadily downward, it gulps down in parts. Why?

Answer

The gulping you describe is due to air being sucked into the bottle and temporarily halting the flow through the nozzle. When the bottle is filled with water, it is at a particular pressure. When you turn it over and some water leaves, the pressure is now lower in the bottle.

Once the pressure in the bottle is lower than atmospheric pressure, air forces it's way back into the bottle. This equalizes the pressure and water flows again. Then the pressure drops, air gets sucked in, and so on. Eventually all the water is gone and the bottle is filled with air at the same pressure as the atmosphere.