Study On The Influence Of Recessed Steps On The Hydraulic Characteristics Of Stepped Spillway

Southwest China has a high terrain and a large gap,which contains abundant water resources.Therefore,a large number of high-head water conservancy projects have been built.However,high water head is easy to produce high-speed discharge flow during flood discharge,which will cause erosion and damage to the downstream of the dam.Many scholars have put forward various ways of flood discharge and energy dissipation.Compared with the traditional smooth spillway,the stepped spillway is widely used in high-head discharge structures due to its high energy dissipation rate.However,with the excessive head and unit width flow,the stepped spillway surface is also vulnerable to cavitation damage.Some scholars have proposed various steps to reduce the damage of water flow to the steps.Inspired by the energy dissipation technology of stilling basin,this paper adds grooves on the surface of the step to adjust the discharge flow,and studies its influence on the discharge flow characteristics,in order to find the best groove form that can meet the improvement of energy dissipation rate and controllable cavitation phenomenon at the same time,and provide reference and new ideas for the design and application of stepped spillway with large unit width flow.In this paper,the effects of different types of grooved steps on the flow pattern,water surface line,velocity field,pressure field,energy dissipation and other hydraulic characteristics of stepped spillway are studied by numerical simulation and model test.In addition,the cavitation erosion characteristics are analyzed by combining the velocity and negative pressure distribution.The main research results are as follows :1 The influence of different shapes of grooved stepped spillway on hydraulic characteristics is studied.The results show that the change trend of the water surface line of the stepped spillway with different shapes is basically the same,and all of them are higher than the traditional type,and the flow pattern is still the sliding flow pattern.The pressure distribution in the step is similar,but it will relatively reduce the negative pressure area and the negative pressure extreme value,and the maximum positive pressure position in the step will be fixed on the back wall of the groove.The velocity distribution in the step is roughly the same,but it will reduce the flow velocity.It can increase the turbulent kinetic energy and turbulent kinetic energy dissipation rate in the step,thereby increasing the energy dissipation rate;reduce the negative pressure extreme value and negative pressure area in the step and near the wall,and reduce the risk of cavitation erosion.2 The influence of hydraulic characteristics of stepped spillway with different depths is studied.The results show that with the increase of groove depth,the flow pattern in each spillway is sliding flow pattern,and there is no adverse flow pattern.The groove depth has little effect on the height of water surface line.The pressure distribution in the step with different groove depths is similar,and the blocking effect of the groove on the water flow increases with the increase of depth.The negative pressure area and extreme value at the concave corner of the step decrease first and then increase,and the positive pressure extreme value at the convex corner gradually increases,but it is still at the angle between the back wall of the groove and the level.Appropriately increasing the groove depth will reduce the negative pressure area and extreme value in the step,but it cannot be too deep,otherwise it will increase the pressure in the step and increase the deposition in the groove.The maximum velocity position of the groove step with different depths remains unchanged,but the large velocity area decreases with the deepening of the groove,and the decrease amplitude gradually weakens.The increase of the depth of the groove makes the vortex in the groove fully extend,and the extreme value of the turbulent energy dissipation rate and the range of the high turbulent energy dissipation rate in the step show a decreasing trend,which leads to the continuous decrease of the energy dissipation rate.3 The influence of different widths of grooved steps on the hydraulic characteristics of stepped spillway is studied.The results show that the wider the groove is,the more significant the aeration effect is,but the flow pattern of the stepped spillway with different widths does not change much.The groove widening weakens its hindering effect on the water flow to a certain extent,and has little effect on the negative pressure area and the negative pressure extreme value near the wall of the horizontal plane.The appropriate groove width can reduce the negative pressure area and the extreme value near the vertical wall.The area of the large flow velocity region with the smallest groove width is the smallest,and the vortex of the water flow at the groove extends more obviously with the increase of the groove width.The generated rolling phenomenon is more intense,which makes the low flow velocity region larger,but its influence on the extension degree is less than the groove depth.The increase of groove width makes the turbulent kinetic energy and turbulent dissipation rate at the concave corner of each step decrease continuously,and the turbulent kinetic energy and turbulent dissipation rate in the groove increase gradually,so that the energy dissipation rate of spillway increases gradually.4 The influence of different groove steps on the hydraulic characteristics of stepped spillway is studied.The results show that the water surface line in the upstream of the stage decreases with the decrease of the distance between the groove and the step angle,but the decrease is not large.The water surface line in the downstream of the stage is almost the same,and the change of the groove position will not produce unfavorable flow pattern and are all sliding flow.The closer the groove is to the concave angle of the step,the negative pressure area in the step changes little,and the negative pressure area range and negative pressure extreme value of the horizontal and vertical near-wall pressure are smaller.In the stage of the stage,the area of large flow velocity becomes smaller as the groove is close to the upstream.The vortex center position at the concave angle of each step gradually shifts to the concave angle of the step,and the flow velocity in the step gradually decreases.As the groove gradually moves to the concave corner of the step,the turbulent kinetic energy at the concave corner of each step gradually decreases and the gradient becomes larger,the extreme value of turbulent kinetic energy in the groove gradually increases,the extreme value of turbulent kinetic energy dissipation rate at the concave corner of the step gradually decreases and the gradient becomes larger,and the extreme value of turbulent kinetic energy dissipation rate in the groove has little difference.The energy dissipation rate of the spillway gradually increases with the left shift of the groove position,but the increment does not exceed 1 %.5 The effects of different shapes,depths,widths and positions of grooved steps on cavitation erosion are studied.By comparing the negative pressure areas and negative pressure extreme values of horizontal near-wall and vertical near-wall in different types of grooved steps,it is found that the addition of grooves has a good effect on reducing the negative pressure extreme values of horizontal and vertical near-wall in the upper,middle and lower reaches of the stage,which can help reduce the cavitation damage of the step surface.In addition,combined with the reduction of negative pressure extreme value,it can be concluded that the working conditions with better corrosion reduction effect are A3,A5 and A7.Therefore,when the groove form is rectangular,position D = 23.7mm(1/4l),width L =12.6mm(2/15l),depth H = 10mm(2/5h),this type of groove stepped spillway can achieve the best engineering effect in practical application.