Research On Hydraulic Characteristics Of Energy Dissipation In Bottom Flow Of Sag Sills With Different Heights

At present,many of the water conservancy and hydropower projects built in to be built in our country are with large flow,high head and high power.Therefore,in order to ensure the safe discharge of flood with huge energy,energy dissipation and erosion control should be done well for the safety of the downstream and the impact of the construction project on the ecological environment.More and more energy dissipation methods are studied and come into being.In this paper,the basic energy dissipation methods are compared first.Although the energy dissipation rate of underflow energy dissipator is high,the erosion of bank slope is slight,and the atomization range is small,the large flow velocity and fluctuat ing pressure near the bottom have become the main factors restricting its development.In order to solve this problem,a new type of energy dissipator,the drop sill-stilling basin,appears.This form can not only undertake the energy-dissipating task of high head and large unit width discharge,but also effectively solve the problem of high near bottom velocity.However,in the current research,the velocity of the opposite wall,the velocity change of the whole section,some dimensionless hydraulic parameters and fluctuating pressure are not comprehensive enough.Therefore,in order to further understand the hydraulic characteristics of the plunge bucket stilling basin,this paper adopts the methods of model test,theoretical analysis and control variables,The hydraulic characteristics of stil ing basin without drop sill and stilling basin with drop sill depth of 3cm,6cm and 9cm are studied under five stage flow regime.The comparison and analysis are carried out according to the data of water surface height,velocity,Froude number fr,energy dissipation rate,time average pressure and fluctuating pressure in stil ing basin.The main research results and conclusions are as follows:(1)The flow pattern and velocity characteristics of four stilling basins are studied by using the control variable method,and the relationship between the four stilling basins and the single width discharge and the depth of the drop sill is compared and analyzed,The results show that the change of water surface line and flow velocity in each flow pattern,and when the single width flow rate is fixed,the relationship between the velocity drop rate at the bottom of the flow with the change of the depth of the sill is: 6cm drop > 3cm drop > 9cm drop > no drop,and the maximum velocity at the bottom decreases with the increase of the depth of the sill.(2)The variation of Froude number fr along the path of four stil ing basins is analyzed.It is found that the Froude number fr along the path is the most stable point when the depth of the plunge bucket is small.After comparing the energy dissipation rate,the more chaotic the flow pattern in the stilling basin,the more sensitive the energy dissipation efficiency is to the change of the depth of the drop sill,and the greater the positive effect of the depth of the drop sill on the energy dissipation effect.(3)The amplitude characteristics,probability density distribution and spectrum density of time average pressure and fluctuating pressure of four kinds of stilling basin floor and side wall are analyzed.the amplitude of the fluctuating pressure of the bottom plate and the side wall increases with the increase of the flow rate;Under the condition of constant single width discharge,the fluctuating pressure amplitude of the bottom plate and side wall of four kinds of underflow energy dissipators increases first and then decreases gradually;When analyzing the frequency spectrum characteristics of the fluctuating pressure of the bottom plate,the dominant frequencies of the four types of drop bucket are concentrated between 4Hz and 9hz,In the side wal,the dominant frequencies of the four types of drop sill are concentrated between 6Hz and 10 Hz,With the increase of the depth of the drop sill,the change of amplitude shows a decreasing trend,and the energy carried in the dominant frequency gradually weakens;Generally speaking,when the depth of drop bucket is 6cm,the effect of fluctuating pressure on the boundary of stilling basin is the least.The research results can provide useful theoretical and experimental basis for the plunge bucket-stilling basin.