| Compared with the traditional smooth spillway,the stepped spillway is convenient in construction,greatly shortens the construction period,improves the energy dissipation rate,reduces the length of the downstream stilling basin,and saves costs.However,when the discharge is large,the wa ter depth of the cross-section will increase,which will lead to the problem of insufficient air entrainment and reduced energy dissipation rate.The energy dissipation method of step spillway and wide tail pier can meet the condition of large single wide overflow,but the step increases the unevenness of the overflow surface,which makes the flow pattern near the bottom complex,and the negative pressure is easy to appear in the step,especially when the flow velocity at the bottom is high,the air entrainment on the step surface is reduced,and the risk of cavitation damage is more likely to occur.In order to explore the possible failure mechanism of the near bottom flow on the stepped surface,this paper studies the hydraulic characteristics of the near bottom flow of the stepped spillway combined with the X-type wide tail pier by using the numerical simulation method on the basis of the model test.The main conclusions are as follows:(1)Along the normal direction of the step surface,the pressure of the flow section is affected by the rolling water flow.From the concave angle,the pressure first decreases in the water depth range of 0-0.4m,and then increases in the range of 0.4m-1.15 m.Above 1.15 m,it is basically separated from the impact of the rolling water flow.Then the pressure slowly decreases,which is manifested as the distribution of hydrostatic pressure,which is basically the same as the pressure distribution law of the smooth open channel water flow tending to be stable.(2)For stepped spillway,slip flow can be divided into three parts: rolling area,main flow area and transition area.The maximum point of section pressure is the boundary of sliding flow between the transition area and the main flow area,and the line between the intersection point of water flow on the vertical plane of the same step and the separation point of water flow on the horizontal plane is the boundary of transition area and rolling.The transition area is the transition area between the rolling flow and the sliding flow in the main flow area.(3)The thickness of transition zone is basically the same along the path,and the change of single width discharge has no too much influence on the thickness.The macro flow pattern of transition area is sliding flow,but the actual flow pattern is more complex than that of main flow area.The flow velocity in the transition area fluctuates along the flow direction,and the closer it is to the step surface,the stronger the fluctuation is,and the greater the flow energy consumption is.When it is far away from the step surface,the fluctuation gradually weakens,and the fluctuation disappears at the main flow area.Within a certain range above the virtual floor,there is a bottom layer with no increase of flow velocity,and its thickness is about 1 / 3 of the thickness of the transition area.(4)The pressure on the horizontal surface and the vertical surface of the steps in the rolling area are wave like.The water flow is affected by the flow around the convex angle of the step.There is a negative pressure near the vertical Y / h = 0.9 of the step.After passing through the convex angle,it flows downward along the vertical lead surface,and converges with the water flowing upward along the vertical lead surface of the rolling area,forming a phenomenon of pressure increase near the vertical Y / h = 0.7,which is the water flow intersection.When the horizontal plane X / b = 0.7,the flow impinges on the wall to produce a large pressure,which is the separation point of the flow.The line between the separation point and the intersection is the boundary of the rolling flow. |
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