Study Of The Mechanism Of The Free Surface Vortex

The free surface vortex is a common hydraulic phenomenon in front of the intakes of hydraulic structures such as diversion pipeline of hydropower station, spillway, diversion tunnel, large scale pump station, and ship lock. The free surface vortex will evolve into air entrainment vortex when it develops to a certain extent, which may seriously endanger hydraulic structures or hydraulic equipments. The mechanism of the free surface vortex formation and evolution are greatly complex. It is an urgent problem in the practical engineering to research the vortex phenomenon and its evolution law, to reveal its flow mechanism and to seek the effective methods to control it. It also has an important academic value because many forward basic theories are involved in the study of the mechanism of the free surface vortex.In the thesis, experimental apparatus was designed and set up to study of the mechanism of the free surface vortex. The formation and evolution of the free surface vortex were observed in detail using flow visualization. The direction of rotation, position and shape of the free surface vortex in different development stage were investigated. Based on the observation results, the effect of the Coriolis force caused by the earth rotation to the formation and evolution of the free surface vortex were analyzed qualitatively. The critical submergence, which is an important factor that refelcts the free surface vortex development stage was obtained under different experimental conditions. The effects of the flow discharge, the incoming flow turbulence intensity, the incoming prerotation and the position of the outlet to the formation and evolution of the free surface vortex were analyzed.A particle image velocimetry(PIV) system was used to measure the whole flow field of the free surface vortex at different development stage. The evolution law and the changing law in space of the free surface vortex were obtained by analyzing the experimental data. In the evolution of the free surface vortex, with the radius increasing, the tangential velocity increases to a maximum value and then decreases. The radius corresponding to the maximum tangential velocity is defined as the vortex core radius. The maximum tangential velocity increases with the depth increasing and then decreases, and the maximum tangential velocity exists in a certain depth under the free surface. As the free surface vortex is a centripetal flow, the radial velocity increases with the radius decreasing. In the depth direction, the maximum radial velocity decreases with the depth increasing. The maximum radial velocity is in the free surface, which shows that most of the fluid sunk by the intake during the formation and evolution of the free surface vortex come from the top of it. So the free surface vortex can easily inhale floating debris on the free surface.Based on the experimental studies, the governing equations considering the Coriolis force caused by the earth rotation were established by theoretical analysis. By simplifying the governing equations, the flow law of the point sink flow under the Coriolis force was obtained, and the effect of the Coriolis force to the free surface vortex was analyzed. The theoretical analysis indicates that the Coriolis force caused by the earth rotation is one of the major reasons that cause the free surface vortex.A barrel with an outlet on the bottom was used as a physical model. Program was compiled to simulate the flow field in the barrel. The flow field in the barrel under different conditions were obtained by using FVM(Finite Volume Method) to discrete the governing equations and SIMPLE(Semi-Implicit Method for Pressure Linked Equation) to solve the discrete equations. The numerical simulation shows that vortex generated in the barrel under the effect of the Coriolis force. The changing law of the tangential velocity and the radial velocity in radius and depth are qualitatively consistent to that of the experiments. The errors between numerical calculation and experiment were analyzed. In the viewpoint of Lagrange, the relation of the remove acceleration, the viscosity force and the Coriolis force were analyzed. It shows that the particle acceleration was caused by the Coriolis force out of the vortex core during the vortex evolution. The effects of the magnitude of the Coriolis force and the flux to the vortex were also studied.Based on the program, the effect of the prerotation in the incoming flow together with the Coriolis force to the free surface vortex was studied. The calculations indicate that when the prerotation in the incoming flow is counterclockwise, the vortex generated by the prerotation and the Coriolis force is counterclockwise, and the rotation speed increase with the prertotation increasing. When the prerotation in the incoming flow is clockwise, parts of the prerotation was used to counteract the effect of the Coriolis force, and the remains was used to produce the clockwise vortex. The flow fields in the flume under different incoming flow conditions were numerically simulated using commercial software Fluent. The shape and position of the vortex obtained by numerical simulation agree well with that of the experiments.