Study On Local Unsteady-state Flow In Steady Divergent Pressure Flow

Divergent current is one common flow phenomenon in hydraulic structures, for example the flow movement in intakes/outlets of a pumped-storage power station, outlets of pressure discharge tunnel and so on. Under given boundary condition, flow separation would occur near the boundary. Once the flow separates from the boundary vortex would appear, and under the action of unsteady displacement of the vortex or other little disturbance the major flow would develop to unstable lateral swings. The phenomenon of main flow swinging laterally in divergent segment is called local unsteady-state flow. Due to the region of local unsteady-state flow being not localized near the boundary, the harmful effects of the flow to structures would be evident. However, compared with its importance, the study on the local unsteady-state flow is scarce.The forming condition, forming mechanism and movement rules of the phonomenon of local unstable-state flow in divergent segment are systemically studied in this paper, according to physical test, numerical simulation and theoretical analysis.(1) The model experiment study on intakes/outlets of one pumped-storage power station. In the model test of intakes/outlets of one pumped-storage power station, ADV 3-D velocity metre is used to measure the velocity at the outlets under the pumping condition, and through frequency spectrum analysis of data gained, the velocity characters of each point is discussed, the phenomenon of local unsteady-state flow existing in the pressure divergent flow is revealed.(2) The experimental study on local unsteady-state flow in pressure divergent segment. Based on the pressure divergent segment experiment of different divergent angle, the motion characters of local unsteady-state flow appeared in divergent segment under given boundary condition is studied. The flow pattern is observed by silk thread affixed to the wall, and the velocity duration course of characteristic points recorded by ADV 3-D velocity metre is analysed. The forming condition, forming mechanism and movement rules of the local unsteady-state flow are discussed.(3) The numerical simulation study on local unsteady-state flow in pressure divergent segment. The movement and development of whole flow field in pressure divergent segment are given through large eddy simulation, flow characters are analysed by spectrum analysis, and the results are compared with the experiment result.(4) The theoretical analysis of local unsteady-state flow in pressure divergent segment. The results of theoretical analysis on results of experiment and numerical simulation show the main flow of divergent flow would separate from the boundary, and form the unstable vortex region near the boundary. Under the effect of these unstable vortexes, the main flow would swing with the irregular movement of vortex, the phenomenon is namely local unsteady flow in stable flow. In other words, there are low-frequency and pseodu-order unstable swings of main flow, which is caused by the transport and motion of large eddy, besides stochastic flow fluctuation. In common cases, compared with the flow stochastic fluctuation, the swing of main flow shows the lower-frequency and bigger swing range and follows some regularity.