Figure (a) shows a pipe (a venturi tube) that is uniform in diameter at both ends, but has a constriction between the ends. The cross-section A1 is greater than the cross-section A2 and it can be concluded that V2 is greater than V1, since the flow speed increases where the area decreases and the flow speed decreases where the area increases, as shown in figure (b). Figure (c) shows the streamline pattern in the venturi tube. In the area of the throat, the streamlines crowd closer together than in the wide part, indicating regions of relatively high-speed flow.

Credits - NASA