Vena Contracta is the point in a fluid stream where kinetic energy is maximum and pressure energy is minimum.
At Vena Contracta fluid stream where velocity is maximum and the diameter of stream is minimum.
in orificemeter maximum contraction or Vena Contracta takes place at a section slightly down the stream of the flow, at that section where the jet is more or less horizontal.
This phenomenon happens because fluid streamlines cannot abruptly change direction. Streamlines are unable to follow the sharp angles in orifices, venturis and everywhere where it encounters sharp turn or sharp angle change.
Stream follows a curve when it reaches a sharp turn converging in a smooth part even if it reaches the orifice. From the start of the sharp edge it starts to converge and converges to the minimum cross section in downstream of flow and then converges again . This minimum cross section area is called Vena Contracta.
From the above figure we can clearly see flow seperation or turbulence Zone in the river near sharp turn.