Vena contracta
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Vena contracta is the point in a fluid stream where the diameter of the stream is the least, such as in the case of a stream issuing out of a nozzle, (orifice). (Evangelista Torricelli, 1643).
The maximum contraction takes place at a section slightly on the downstream side of the orifice, where the jet is more or less horizontal.
The effect is also observed in flow from a tank into a pipe, or a sudden constriction in pipe diameter. Streamlines will converge just downstream of the diameter change, and a region of separated flow occurs from the sharp corner of the diameter change and extends past the vena contracta.
The reason for this phenomenon is that fluid streamlines cannot abruptly change direction. In the case of both the free jet and the sudden pipe diameter change, the streamlines are unable to closely follow the sharp angle in the pipe/tank wall. The converging streamlines follow a smooth path, which results in the narrowing of the jet (or primary pipe flow) observed.
Measurement of the vena contracta is useful in echocardiography, where it describes the smallest area of the blood flow jet as it exits a heart valve. This corresponds to the Effective Orifice Area calculated for heart valves using the continuity equation.
[edit] Coefficent of contraction
It is the ratio between the area of the jet at the vena contracta to the area of the orifice.
Cc = area at vena contracta/ area of orifice
The typical value may be taken as 0.64
