Further observations of the vortex pair in front of U-section prisms have been conducted by using smoke visualization technique. A number of models with different dimensions were systematically investigated. In this paper, the flow patterns are described and the physical mechanism is explained. A unified critical condition concerning the occurrence of the vortex pair is also determined. The influence of end plates is discussed in detail and the effect of Reynolds Number and turbulence is qualitatively considered. Also given in this paper is the scale and the number of vortex pairs related to dimensions. Velocity fluctuations in the vortex flow were detected using a hot-film anemometer.