Unsteady Vortex Motion And Formation Mechanism

Unsteady vortexes phenomena are universal in the flow of hydraulic structures.Seperation flow often generates from the wall with unsteady vortexes in the seperation region when the diffusion angle is larger than a certain value in the outlet of a pumped storage station which have harmful effects on hydraulic structures. The vortexes have been observed frequently at intakes of hydraulic structures such as hydroelectric power stations, navigation locks, flow and level regulation and withdrawal from sumps in Emergency Core Cooling Systems (ECCS) of nuclear power stations. The presence of air-core vortexes at an intake can cause discharge reduction, increase the fluctuation of hydraulic structure, reduce pumping efficiency, and in extreme cases, completely disable the intake. Due to its randomicity, short-time persistence and motion complexity which contribute great difficultness to vortex research, At present, few literature about vortex motion and formation mechanism is found. Therefore, an analytical, computational, and experimental study was conducted to describe unsteady vortices and its formation mechanisms. The principal contents and achievements of this dissertation are as follows:(1) The numerical simulation study on local unsteady-state flow in pressure divergent segment. The motion and development of the whole flow field in pressure divergent segment are given. The flow characteristics were studied by large eddy simulation (LES) combined with frequency spectrum analysis. It is indicated that the flow is composed of traverse swing with lower frequency and fluctuation with high frequency, and the swing is caused by the transverse oscillation of eddies in the separation region.(2) The physical model test of flume was made to study the average velocity distributin, turbulence indencity, surface fluctuation and turbulence pressure. The hydraulic characteristics and influencing factor of unsteady vortexes were researched. Model test results shown that the history of near-intake unsteady votexes has the approximate feature with turbulent coherent structures of the flow in the flume.(3) The numerical simulation of the flow with unsteady vortexes near intake in the flume under different conditions was achieved by large eddy simulation. The flow field characteristics, vortex configuration motion rule vortex structure and their influencing factor were researched. Results showed that vortex movement was active near the intake and the number of air-core vortexes increased with the increase of turbulence intensity. Symmetric boundary conditions and forward-tilting breast wall may reduce the generation of air-core vortexes.(4) The theoretical analysis of unsteady vortexes at intakes was completed. The reason for vortex stretching at the bottom and the side intakes was explained. According to Rankine's combined vortex model and Cascade structure of energy, free surface vortex energy-balance process and the vortex forming, developing and varying were probed.