Research On Transition Flow Between Un-full And Full State In Conveyance Tunnel

In practical engineering, there are various hydraulic phenomena about flow transition from free-surface to pressured state in different application, so study on this transition flow is great significant. At first, both the domestic and foreign research progress are summarized in this paper about the hydraulic phenomenon of flow transition for mixed free-surface-pressure in conveyance tunnel, with which the existing results and calculation methods can be seen on transition flow between un-full and full state of a tunnel at home and abroad. Based on this, the problems to be researched through this thesis are put forward. As a one-dimensional unsteady flow problem, unsteady flow theory both for open channel flow and pressure flow are detailed introduced, and Saint-Venant equations are derived. Before this, the commonly used equation of wave speed for water hammer in a pressure pipe, is dealt with simply, with which, a foundation is made for the numerical simulation of transition flow for mixed free-surface-pressure flow.Then the method of Preissmann slot which is used commonly in the research of transition flow from open channel to pressure is introduced, and its principle is introduced in detail. Based on this, the Preissmann 4-point eccentric scheme for space and time computational grid,commonly used,is introduced and boundary conditions and initial conditions are discussed with complete derivation, which are applied to an example with dramatic changes of upstream and downstream boundary conditions. After the results of numerical calculation are analyzed, it is concluded that Preissmann four implicit methods is unstable for upstream and downstream boundary conditions in the severe situation. Then, to study the hydraulic phenomena of transient mixed free-surface-pressure flow, the characteristic implicit method has been simply introduced, which is expressed as the flow variables of depth and discharge, simply proposed by Fan Honggang, et al. This method borrows the idea of characteristics and uses the upwind difference scheme. The computational results, applied to the same example as one computed by Preissmann scheme, are analyzed. And it is concluded that the method can be stable in calculation process until to the final steady flow situation, but the results of simulation may be in distortion. To solve this problem, we try to put forward the improvement for the method, in which the depth h and velocity v are used as flow variables for the method of characteristic implicit scheme, still with applying the idea of characteristics and using the upwind difference schemes; and the common upstream and downstream boundary conditions and initial conditions are also introduced. Then the same example as mentioned above is used in numerical simulation. Through analysis to the simulation results, it can be concluded that the method has better stability than the above old method, and the computer time used is lesser. Even under the flow situation with rapid changes of upstream and downstream boundary conditions for, it can be effectively simulated by the new method. But through further numerical experiments it finds that if the time step is too large, it will lead to distortion of simulation, so we need to combine the results of numerical experiment and analyze to choose the appropriate time step.To explore the applicability of method that is termed by the depth h and the velocity v as the flow variables for the characteristic implicit scheme, several examples with different type are computed to analyze its applicability. It is concluded that the method can be applied to a variety of practical engineering, and can guarantee the calculation accuracy.