Study And Simulation On The Mixed Flow In The Drainage Pipe

The mixed free-surface-pressure flow does exist in the urban sewerage system, when a storm exceeds the design event or some structure of the system failure occurs. This phenomenon will be more frequently with the cities'developing which increase the flood peak value, and with the design objective of the sewer networks that the storage capacity should be utilized. The study and simulate of the mixed free-surface-pressure flow in the sewerage system will be of great significance in both theoretics and practice. But the current design method of the drainage system is still mainly based on the assumption of the free surface steady flow. The domestic study on the unsteady flow simulation of urban drainage system just starts, and the study of mixed free-surface-pressure flow in drainage pipe network is very little, while most attention is paid on the phenomenon that occurs in hydraulic tunnels.The hydraulic calculation parts of the urban drainage system simulation models in and abroad is reviewed and evaluated, and the simulation methods on the mixed free-surface-pressure are summarized in this paper. The method of characteristics is selected to calculate the basic equations (Saint-Venant Equations) with full form to simulation the one-dimension unsteady flow in the pipe, and the shock fitting method is selected to simulation the interface between free surface flow and pressurized flow, by comparing the merits and drawbacks of various simulation methods.The interface types that may occur during the transition of free surface and pressurized flow, the condition on which the transition occurs and the characteristic of the various interfaces are studied and summarized. Numerical calculation methods for different types of interface are developed and four different experiment trails with different types are simulated. The simulation results are well agreed with the measured values.When multi-interfaces exist in the drainage pipe, an air pocket may be produced if air is closed between interfaces. The condition under which the air pocket can be formed is analyzed in this paper, when two interfaces exist in the pipe. The shock fitting method is used to simulate the movement of the air pocket, which is closed between interfaces. The numerical experiment result indicates that the volume of the air pocket decreases and the air pressure increases because of the air is pressurized while the air pocket moving in the pipe. So the formation of air pocket in the drainage pipe should be avoided by design or operate of the drainage system.