| The free overfall has obvious characteristics of three-dimensionality and turbulence,and the water tongue is liable to cause serious erosion damage to the downstream riverbed during the fall.Therefore,the study of free overfall and related problems has always been a difficult and hot topic for domestic and foreign experts.Firstly,this paper reviews and summarizes the theoretical research,experimental research,numerical simulation,and the research status of water cushion energy dissipation during overfall.Then based on the deficiencies of floor scour due to free overfall and the low efficiency of energy dissipation,the concept of energy dissipation of slope water cushion was proposed.Finally,based on a practical project,the theoretical analysis,model test and numerical simulation method were used to study the hydraulic characteristics of the slope water cushion.The main research contents and conclusions of this paper are as follows:(1)Several sets of water model tests for sloped waterslides were performed,and the test results with more obvious phenomena were selected to analyze the flow characteristics.It was found that due to the action of the slope of the floor,the impingement jet phenomenon of the water tongue striking the floor after the water impinges on the floor is obvious,but the backwater upstream phenomenon is effectively controlled.With the increase of the flow rate,a more pronounced large-scale backspin phenomenon is formed in the cushion area.(2)By analyzing the flow characteristics of two model tests with or without drop sill,it is found that there are obvious differences in the flow patterns of the two models at the same conditions.When there is no drop sill,a large number of bubbles are generated on the bottom plate when the water tongue enters the water,and violent welter phenomenon occurs in the water cushion area.When there is a drop sill,the bubbles generated after the water tongue enters the water do not directly hit the bottom plate,and a large number of bubbles create a “soft water cushion” effect at the bottom.The bottom of the water cushion generates a concave bottom surface of the air bubbles,which rapidly rises upwards to cause a violent welter phenomenon,and forms a clear recirculation in the cushion area.(3)From the analysis of energy dissipation mechanism of the slope water cushion,a reasonable calculation formula of energy dissipation efficiency was chosen to calculate the energy dissipation efficiency at different working conditions.The results show that the energy dissipation efficiency of the water cushion at different slopes is better,and the energy dissipation efficiency is higher than 85%.When the flow rate is small,the energy dissipation rate is as high as 95%,and the energy dissipation rate is increased by about 30% compared to the case without water cushion.(4)The k-? dual-equation turbulence model and the VOF model were used to simulate the water cushions at different flow rates.The calculated results are in good agreement with the model tests.Through the analysis of the numerical simulation results,we can find that the pressure and the bottom flow velocity of the floor of the water cushion area increase with the flow rate.The maximum pressure occurs at the bottom of the water tongue and at the bottom of the tail sill of the cushion area,and the downstream flow due to the effect of the water pad is stable,and the maximum flow rate is 0.3 m/s.(5)At the same conditions,the water cushions with dip and slope were simulated.The results show that the maximum value of the pressure on the floor slabs in the water cushion area with falling slopes is less than 20%.And the maximum value appears at the bottom of the padding.In the case of falling undulations,the streamline of a large number of water bodies is concave,and it rises upward before reaching the bottom of the water cushion area.A dynamic water cushion effect is formed at the bottom of the water cushion area,which greatly reduces the impact of the water tongue at the bottom of the water cushion area,and the flow rate at the bottom is greatly reduced,indicating obvious effects of the “soft water cushion”. |
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