In the above sequence of streaklines for a flow around a square prism, 8 frames of streaklines are presented at 8 instants in a complete cycle of the lift force. For every frame, the corresponding lift force is drawn on its upper left-hand side panel, in which the black dot represents the instantaneous lift force value induced by the flow field corresponding to the streamlines shown.
This sequence of streaklines clearly demonstrates the process of vortex shedding. In Frame 1, the total lift force on the square is zero, and three vortices are indicated by A, B and C behind the square. A is already shed, preparing to exit out of the stream. B was originally formed from the lower side of the square and is still attached to the rear side of the square. In Frame 2, A moves further outside with the stream, and B is completely shed off the rear side surface, while C moves to the center of the rear side surface. In Frames 3 and 4, this trend continues. In Frame 5, half a cycle of the lift force is completed, and it may be concluded that the streaklines close to the square in Frame 5 are similar to those in Frame 1, but only reversed. In Frame 5, a new vortex, D, is formed at the lower side behind the square. In this frame, D is in a similar position as C was in Frame 1. In Frames 5 to 8, genesis of vortex shedding process is similar to that observed in Frames 1 through 4. According to this trend, in every full cycle of the lift force, two vortices are shed, one from the lower side and one from the upper side of the square prism.