Numerical Simulation Study On Hydraulic Characteristics Of Pre-aerator Stepped Spillway

With the rapid development of high dam construction,in order to reduce the erosion damage to the downstream of the dam by the discharge flow,the stepped spillway is widely used in the discharge of high dam due to its high energy dissipation rate.Due to the high head and large single wide flow rate,the step surface and side walls of the spillway are susceptible to cavitation and cavitation damage.Some scholars put forward adding flip bucket to aerate in front of the stepped to reduce the damage caused by water flow.Therefore,the study of its hydraulic characteristics is of great significance for the application of this new aerator in stepped spillway.In this paper,physical model test,numerical simulation method,prototype observation data and other methods are used to study the hydraulic characteristics of the stepped spillway with front bucket and the traditional stepped spillway.Combined with a high dam stepped spillway,the ICEM software is used to divide the structured grid of the two stepped spillways.RNG k-? model in fluent software and the VOF method to trace the free water surface are used to carry out the three-dimensional numerical simulation calculation of the spillway with different single width discharge.The results obtained by the simulation are basically in good agreement with the experiment.On this basis,the flow pattern,water surface profile,velocity,pressure,energy dissipation and other hydraulic characteristics are studied in detail.In addition,the cavitation erosion characteristics are analyzed based on the prototype observation results.The research conclusion of this article is shown as follows:(1)Pre-aerator has a certain effect on the water surface line after picking.The water surface line after picking decreases first and then increases,but has little effect on the water surface line of the step section and the downstream outlet section.With the increase of single-wide flow,the water surface line gradually rises along the stepped spillway.(2)The mainstream flow velocity along the pre-aerator stepped spillway is greater than that of the traditional stepped spillway.As the flow rate increases,the mainstream flow rate increases.The water flow vortices in the first step of the front spur type are different from the traditional distribution pattern,but they have basically no effect on the internal vortex distribution in the second half of the step with stable aeration.The side wall has a certain effect on the flow velocity of the near wall.Since the flow is away from the side wall,the main flow velocity gradually increases,and the clockwise vortex distribution in the step gradually becomes clear.(3)The negative pressure area along the stepped spillway is mainly concentrated in the previous steps and the middle and downstream areas of the ladder.The absolute value of the horizontal negative pressure along the front side of the pre-aerator type is higher than that of the traditional type.The minimum pressure of the stepped horizontal plane near the wall is 0.3 times the step length,and the highest pressure is 0.7 to 0.8 times the step length.The minimum pressure of the vertical wall near the wall is 0.9 times the step height,the pressure of the preaerator type in the downstream section is lower than that of the traditional type,and the risk of cavitation damage on the vertical step surface increases.The side wall has an influence on the pressure distribution in the step.Since the water flow is away from the side wall,the negative pressure range of the step surface is transferred from the concave angle of the step to the middle of the step.(4)As the flow increases,the energy dissipation rate of the stepped spillway decreases.Compared with the traditional stepped spillway,the energy dissipation rate of the pre-aerator stepped spillway is slightly reduced,but the difference is not large,and the high energy dissipation rate is still maintained.the pre-aerator has a small effect on the energy dissipation rate.(5)The aeration of pre-aerator stepped spillway is significantly larger than the traditional stepped spillway in the front step of the ladder section,and the aeration in the middle and lower steps is very small.The scope of the aeration is relatively limited,and it only affects the front step.As the flow rate increases,the aeration decreases and the aeration distance increases Although aeration reduces the step pressure value,a certain concentration of aeration rate can effectively reduce cavitation damage on the wall.(6)The cavitation position of the step is in the upper range of the triangle area inside the water level and the inner step,that is,the common area of the mainstream area with the negative pressure area and the higher velocity;combined with the prototype observation results of a ladder spillway,the cavitation damage location mainly exists in the middle and downstream areas.The level of cavitation erosion is in the lower part of the cavitation zone.