Impact Of The Type Of Transitional Step On The Hydraulic Characteristics Of Combined Dissipation

The widespread existence of flow problems of high velocity and large unit-width discharge caused by high head and large discharge and the lack of experimental data made it often encountered in actual projects that cavitation problems were encountered,but the engineering designer could only rely on engineering experience to solve the phenomenon.This paper was supported by the National Natural Science Foundation of China,“Research on the Influence of the Connection between the Stepped Spillway and the WES Surface on the Hydraulic Characteristics of Y-shape flaring gate pier,stepped spillway and stilling basin(51569010)”.This paper was based on the Ahai Hydropower station,using hydraulic model experiment whose geometric scale is 1:60,to study the influence of changing the type of transition step on hydraulic characteristics such as water depth,mean pressure,aeration status and energy dissipation rate of combined dissipation,in order to provide further experimental evidence for further improvement of the combined energy dissipation of Y-shape flaring gate pier,stepped spillway and stilling basin.The results of the study were as follows:(1)With different transitional step sizes,the changing trend of water depth,hourly pressure,and flow velocity was consistent.The overall depth of water depth showed a clear increase and then tended to be stable.Finally,there was a slight decline in free flow;The value of the mean pressure in the WES curve segment was very stable.At the stage of steps,the mean pressure fluctuation was small and the overall increase was slightly,but there was no certain regularity;The mean pressure of the ogee-section and the stilling basin increased sharply under the combined effect of high-speed flow,still water depth,and recirculation.Afterwards,due to the effect of water flow reflux,the flow velocity of the water decreased slightly,and the mean pressure decreased slightly.Finally,due to the impact of the energy dissipation of the stilling basin and the flaring gate pier of the stilling basin,the mean pressureincreased slowly.(2)The negative pressure distribution rules of the transitional steps of schemes were basically the same,and the negative pressure was mainly distributed on the vertical solid wall of the step.The step negative pressure gradually increased along the vertical wall of the step,and the maximum value of the negative pressure appeared at the bottom point of the vertical wall of the step.Among them,three types of transitional steps with a size of 25mm×12.5mm had the smallest negative pressure,which helped to reduce the maximum negative pressure value of the transitional steps and reduced the risk of cavitation in the step.(3)It was known through experiments that increasing the width and height of the transitional steps improved the aeration of the transitional steps.Among them,increase the height of the transitional step was the best for aerated condition,with an average aerated concentration of 84.73%.At the same time,when the width and height of the transitional steps were changed at the same time,the aeration condition was poor.It showed that the step size of the transitional step and the wide-tailed pier was too small,the stepped dam surface cannot be fully aerated,and the cavitation damage easily occurred.(4)Under the influence of the first large steps and the two large steps,the comparison of the sizes of the transitional steps had found that all the experimental programs were beneficial to improve the aeration of the stepped dam surface,reduce the negative pressure on the dam surface,and reduce the cavitation.(5)The increase or decrease of the overall energy dissipation effect of experimental programs were not significant,indicating that changing the size of the transitional step had little effect on the overall energy dissipation effect of the combined energy dissipation.(6)Through comparative analysis of experiments,it was found that the height of the transitional step had the greatest influence on the aeration of the dam surface when changing the size of the transitional step.At the same time,it wasn't better when the size of the steps was as large as possible and the number of large steps was as much as possible.When the type of step was appropriate,it was beneficial to reduce the possibility of cavitation damage of the step spillway.In summary,the hydraulic characteristics of the combined energy dissipation ofY-shape flaring gate pier,stepped spillway and stilling basin could be improved by reasonably selecting the transitional step type.Among them,the height of the transitional step had the greatest impact on it,and the transitional steps consisting of three 25 mm×12.5 mm steps was the best option.