Vena Contracta

Vena contracta is a narrowest point of the flow cross-section where the fluid velocity is maximum, and pressure is minimum.  It is a phenomenon that occurs when a fluid flows through a restricted opening, such as an orifice plate or a valve, and the streamlines converge and form a contracted jet.  The vena contracta occurs downstream of the orifice, where the fluid flow rate is the highest and the pressure is the lowest, leading to a reduction in kinetic energy and an increase in potential energy.  It is important to measure the vena contracta to accurately calculate the flow rate and head loss in a fluid system.

For example, this happens just downstream of a stream flowing through an orifice.