Q:

Venturi effect and velocity of the liquid (or air)

I know that based on the Venturi effect, the velocity of a liquid (or air) going from a large sectional tube to a narrow sectional tube will increase. So, what happens if the liquid (or air) goes from a narrow sectional tube to a large sectional tube ?

A:

That depends on the flow speed in the narrow section:

If it is subsonic, the flow will slow down while its pressure increases. Depending on the pressure gradient, the flow might separate from the tube wall and show a chaotic flow behavior outside of the core flow.
If the speed of sound is reached in the narrow section, a widening tube downstream will cause the flow to accelerate further while pressure continues to drop. Now the details depend on the pressure downstream: If it is high enough, the flow will suddenly decelerate in a straight shock.

