Vena contracta

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.

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.

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

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