What is "vena contracta"

Vena \Ve"na\, n.; pl. Ven[ae]. [L. See Vein.] A vein.

Vena cava; pl. Ven[ae] cav[ae]. [L., literally, hollow vein.] (Anat.) Any one of the great systemic veins connected directly with the heart.

Vena contracta. [L., literally, contracted vein.] (Hydraulics) The contracted portion of a liquid jet at and near the orifice from which it issues.

Vena port[ae]; pl. Ven[AE] port[ae]. [L., literally, vein of the entrance.] (Anat.) The portal vein of the liver. See under Portal.

Vena contracta is the point in a fluid stream where the diameter of the stream is the least, and fluid velocity is at its maximum, such as in the case of a stream issuing out of a nozzle, ( orifice). ( Evangelista Torricelli, 1643). It is a place where the cross section area is minimum. The maximum contraction takes place at a section slightly downstream 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 contraction 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.