Experimental Study On Hydraulic Characteristics Of Reverse Arc Stepped Spillway

As a water conservancy hub integrating discharge and energy dissipation,the stepped spillway has the advantages of simple structure,easy construction and maintenance,and low construction cost.Compared with the smooth spillway,the stepped spillway can reduce cavitation damage while improving the energy dissipation rate.However,in the case of large flow rate and high head,the energy dissipation rate and aeration capacity of the stepped spillway with high flow rate and large unit width flow rate decrease,especially the upstream ladder is more vulnerable to cavitation erosion.In order to further improve the energy dissipation rate of stepped overflow dam under large unit width flow,improve the flow pattern,and reduce the cavitation and cavitation erosion problems,an anti-arc stepped overflow dam is proposed for the first time in this thesis.The flow pattern,initial aeration position and energy dissipation rate on the surface of anti-arc stepped and right angle stepped overflow dam are analyzed by physical model test and theoretical analysis.The main research results are as follows:(1)The flow pattern of counter-arc stepped spillway can be divided into four types:Drop flow,flip flow,transition flow and sliding flow.When the flow rate per unit width is small,the flow freely drops from the upper step to the lower step.With the increase of flow rate,the free drop tongue is gradually triggered by the ejection of the counter-arc step surface,and the jet flow and drop flow appear alternately on the step surface before the complete jet flow is formed.When the flow rate increases to a certain extent,the water depth increases,and the thickness of the dam surface flow increases.Affected by gravity,the pick-up height and pick distance of the pick-up tongue gradually decrease,and gradually fit with the anti-arc ladder surface,and gradually develop into sliding flow.Through the analysis of the experimental data on the critical condition of flow state transition of right-angle stepped spillway and counter-arc stepped spillway,it is concluded that although the flow state development process of right-angle staircase and right-angle stepped spillway is slightly different,the upper limit of transition flow is basically within the range of 0.96?1,and the flow limit of counter-arc stepped spillway is slightly less than that of right-angle stepped spillway.The relative critical water depth(y_c/a)of the two steps entering the transition flow is about 0.09-0.11.(2)Through the study on the location of aeration,it is concluded that with the increase of single-width flow rate,the location of aeration in anti-arc stepped and right-angle stepped overflow dam shows an increasing trend,but under the same single-width flow rate,the location of aeration in anti-arc stepped overflow dam is obviously smaller than that in right-angle stepped overflow dam,which indicates that anti-arc stepped can occur aeration earlier to a certain extent and improve the cavitation erosion resistance of dam surface.According to the experimental data,the empirical formulas for the occurrence positions of the counter-arc step and right-angle step aeration at the spillway slope of ?=26.6°are fitted respectively.(3)The energy dissipation characteristics of anti-arc stepped spillway are studied.It is pointed out that the energy dissipation rates of anti-arc and right-angle stepped spillway decrease with the increase of single-width flow rate.The energy dissipation rate of counter-arc stepped spillway is greater than that of right-angle stepped spillway,and the change of step size has no significant effect on the functional relationship between the energy dissipation rate and the height of relative discharge building.Under the experimental conditions in this thesis,the energy dissipation rate of counter-arc stepped spillway is 14.04%higher than that of right-angle stepped spillway.Under large unit-width flow rate,the decrease of the energy dissipation rate of the right-angle ladder is greater than that of the anti-arc ladder.According to the experimental data,the empirical formulas of energy dissipation rate of anti-arc ladder and right-angle ladder atq=26.6°spillway slope are fitted respectively.