A calibrated orifice is designed to restrict the flow of liquid or gas in a pipeline.
With a certain pressure drop across the orifice, the flow of gas/liquid through the orifice depends on its geometry.
Due to the sharp change in geometry – the change in the cross-section of the channel – the flow starts to curve as it approaches the calibrated orifice, as can be seen in the flow line visualization.
Such a sharp change in the channel diameter also contributes to the flow detaching from the wall inside the channel, which narrows the flow within it and reduces its throughput capacity.
In this example, the pressure drop was 0.5 Bar, and for water as the fluid, the ideal average outflow velocity from the orifice under this pressure drop is ~ 11 m/s.
However, due to the sharp change in the channel's cross-section and friction, the actual average volumetric velocity was only 8 m/s, and the discharge coefficient is approximately 0.76.