| With the potential hydroelectric exploit more and more, we have less and less high water head resource. Therefore, more and more people pay attention to the development about low water head hydroelectric. In such circumstances, the horizontal type light bulb set has already adopted broadly for it has many technology and economic advantages such as high efficiency, big unit rate of flow, high unit rotation rate, small dimension, less investing of soil builds, well running function and so on. At the same time, because of being confined to landform geology condition, it is arranged to the surface-bottom outlet pattern. And such pattern can resolve conditional restricting of landform, and also save a great quantity of fund.In order to establish theory and apply technique basis for the flow field of discharge from surface-bottom outlet on low water head and big flux, making it can have vast application and dissemination and consult value in electricity construction project of the low water head in our country , this paper makes summarize study on home and abroad relevant field, through the hydraulic model test and 2D numerical simulation , systematic studies this allies self with flood-releasing and energy-dissipation form about flow condition and subarea, critical hydraulic characteristics, the distribution of velocity and time-pressure etc.The study results indicate that when the spillway discharges the flood, the bottom hole which simulate the power station flowing is passing water or not, the factors of affecting flux is different. While the bottom hole does not pass water, the rate of flow is connected with upstream water level only; When the bottom hole passes water, the rate of flow is connected with both upstream and downstream water level. The downstream flow field of this type can be divided into four kinds of fundamental fluid state, and they are affected by downstream water level remarkably. When the bottom hole passes water, the outlet flow field which formed is superposed with the bottom backflow which formed by surface water tongue slipping intobottom, comparing with surface hole discharge flow singly, the bottom area of rotation flow decreases and move back. The velocity of bottom outlet diminishes bottom hole outlet flow pulsation, and is beneficial to normal operation of the power station.In the field of velocity distribution, the downstream velocity concentrates water surface mainly, the distributed characteristic is that the bottom velocity is small, the surface velocity is big, and gradually equally along the lengths. The larger the flow rate of bottom hole is, the more obvious the distribution of the velocity of the area round the bottom outlet is, the bigger the range of bottom backflow is; In the field of time-pressure, the distribution of surface hole is according with the law of broad crested weir and flip bucket. The pressure of downstream bottom board is affected mainly by downstream water level, and makes direct proportion with depth. The wall pressure of the bottom hole is connected with bottom flux, and it assumes the creasing trend from up to down.On basis of model test, through the fluent fluid calculation software, the paper carries out numerical simulation when the bottom hole is passing water and not in various fluid state, ascertains the calculation area and divided the structural mesh, chooses the k-ε turbulence model and VOF to calculate. By the contrast of test and calculation results, it fits in most area fairly good such as pressure and velocity and so on, The calculation results show that the distribution of k and ε along different cross and vertical section, which indicates that the bottom swirling flow field goes back, and the bottom hole outlet flow pulsation diminishes, so as to be beneficial to normal operation of the power station. The results are compatible with theoretical analysis, the conclusion is more rational, which indicates that calculating this fluid field by numerical simulation is feasible.
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