Numerical Simulation Study On Destruction Mechanism Of The Step Behind The X-shaped Flaring Gate Piers

The combined energy dissipator of the X-shaped flaring gate pier and stepped spillway was invented under the specific condition of flood discharge and energy dissipation in Suofengying hydro-plant of China,the mechanism of which has been deeply studied by model test.However,varying degrees of erosion destruction occured on the step surface in the later popularization and application process in A-Hai Hydro-plant and Ludila Hydro-plant.In recent years,many researches about the energy dissipation characteristics of the X-shaped flaring gate pier-step dissipator at home and abroad.But a good result about the reasons for steps damage problems especially the cavitation effect has not been achieved.In order to study the destruction mechanism of the step behind the X-shaped flaring gate piers,a hydro-plant on Jinsha River was investigated as an example.Flow 3D was selected as the CFD software.The parameters such as air concentration,cavitation potential,cavitation gas volume fraction were calculated through the air entrainment model and cavitation model.The parameters of water nearby the step surface,such as fluctuating pressure,impact pressure,fluid velocity were calculated and analyzed in the meanwhile.The simulated results were in good agreement with the model test results and prototype measurement results before.The following are the main results obtained in this paper:(1)The Flow rate and Flow velocity in the six typical working condition were calculated.The results were close to the designed values,and the pressure distribution on the facade and the level of the steps were consistent with the previous research results.The Flow pattern and velocity distribution near the step surface were also consistent with the general law,which indicates that the selected software,mesh and model parameters were accurate and the calculation result is effective.(2)This paper focuses on the analysis of the air concentration near the convex corner and concave corner of the steps in the six typical working condition.The results show that the aerators can ensure that the first 30 steps are fully aerated,but the entrained air near the 30?40 steps is not enough(less than 8%).(3)This paper took the opening sluice gate working condition at the normal water level as an example,using the tool of probe and baffle in Flow 3D to analyze the impact pressure and fluctuation pressure near the step surface and ANSYS to simply calculate the stress of the step.The results show that the impact pressure and the fluctuation pressure on the step surface is not enough to cause breakage to the step structure,while the low aeration region on the step surface is basically in coincidence with the destruction region,which means it is the lack of aeration that results in the erosion destruction on the step surface.(4)In this paper,the active cavitation model in Flow 3D was used to simulate the cavities of the steps for the case where gate is fully opened at the normal water level(1223m),and the results of cavitation gas volume fraction and cavitation potential were obtained.The results show that cavitation is likely to occur near the convex corner in the vicinity of the steps.At the same time,the mathematical model of the model scale(1:50)under the same working condition and the same calculation condition is also calculated and analyzed,and the dissimilarity characteristics of aeration and cavitation were found.