In order to study the flow in the completely symmetric structure of the converging-diverging tube such as Venturi

the incompressible unsteady flow numerical models are established respectively for two dimensional channels and three dimensional tubes

and the numerical simulation method is adopted in the study. This paper confirms the flow bifurcation phenomenon

and mainly discusses the asymmetric flow in the symmetric structure. The simulation results show that

when Reynolds number reaches a critical value

the flow field in the symmetric Venturi tube will appear the asymmetric phenomenon

and produce irregular recirculation zones in the extended corner. In addition

the study indicates that the flow characteristics in the symmetric converging-diverging tube are affected by Reynolds number and geometry shape of tube

and the variation rules which flow asymmetric characteristics change with Reynolds number and geometry shape are obtained.