Influences of Depth of the Falling-sill on the Flow State Conversion of Underflow Hydraulic Jump

In restricted topographic conditions,a falling-sill can be set at the end of the discharge chute for the spillway.The length of the dissipation pool is shortened by increasing the flow velocity attenuation along the way of the jet,or increasing the sudden expansion ratio of the stilling pool at the same time.The energy is further dissipated by increasing the internal turbulence of the water body through the plane reverse flow.Based on the existing mechanism of jet diffusion,the flow state conversions of underflow energy dissipation without falling-sill,sudden expansion underflow energy dissipation and energy dissipation of rectangular stilling well are studied.The results show that:①the forced water jump submerged criterion of underflow energy dissipation without falling-sill is between 1.00~1.20,and sudden expansion underflow energy dissipation and energy dissipation of rectangular dissipation well can be distinguished by the critical submerged criterion.②When the angle of the incident water flow remains unchanged,the Froude number (Fr) increases,and the energy dissipation of rectangular stilling well is transformed into sudden expansion underflow energy dissipation under the same depth of falling-sill;when the Froude number remains unchanged,the angle of the incident water flow (θ) increases,and the depth of falling-sill needs to be increased to maintain energy dissipation of rectangular stilling well.In other words,the change in depth of falling-sill can be applied to influence the flow state conversion and effectively reduce the length of the energy dissipater in the direction of water flow.