The Determination Of The Thickness Of The Shear Zone Of The Water Flow In The Water Cushion Pond And The Study Of The Gas Exchange Process In The Water Flow Structure Area

At present,the dam body flood discharge and energy dissipation form is widely used in the construction of high dams in high mountain and gorge areas in China.Practice shows that flood discharge through dam body is the most economical and effective flood discharge and energy dissipation form for high arch dams in gorge areas.There are many high dam and super-high dam projects under construction or completed in China,which have very high water head,so the discharge flow is huge and has extremely high flood discharge power,which is quite different from the engineering characteristics of aerated energy dissipation of middle and low dams.In practical engineering,the water discharged from the dam at high speed will carry a large amount of gas and collide with the water body inside the cushion pond,resulting in severe turbulent shear.During the energy dissipation along the way,a large amount of aerated water body diffuses to the downstream river,which is harmful to the water flow In the process of basin development,a large amount of water is aerated in cascade projects due to flood discharge and energy dissipation,and gas supersaturation will lead to cascade gas barriers in rivers,which will not only destroy the continuity of the basin,but also adversely affect the water environment ecology and fish health of the whole basin.Therefore,studying the water environment problems caused by flood discharge and energy dissipation and aerated water flow is also one of the key points in the construction and development of water conservancy projects.At present,China has not systematically studied the gas supersaturation of river water in the back stage of flood discharge and energy dissipation downstream of high dams,and has not formed an environmental assessment system focusing on the high solubility of gas.Studying the bubble exchange process in each water flow structure area in plunge pool and the gas migration and diffusion characteristics in the pool can provide an important basis for the study of gas movement mechanism.Under the energy dissipator of plunge pool,the high-speed jet sucked gas down into plunge pool to form aerated water flow,and the main jet and the water body in the pool had a strong turbulent shear action to form a water flow shear zone.Bubbles continued to coalesce,diffuse and float along the way,and the gas exchange process among the water flow structural areas was realized through the water flow shear zone.The migration and diffusion of bubbles in plunge pool is accompanied by the transformation and dissipation of different energies.The water depth of plunge pool will increase when a large amount of gas is mixed in water flow.Meanwhile,the migration and diffusion of bubbles have a certain impact on the shear strength,velocity distribution and pressure gradient of water flow.Therefore,the study of aerated flow is of great significance to the shape optimization design of energy dissipators in plunge pool and the protection of cavitation erosion.In this paper,the hydraulic model test and theoretical analysis are combined to divide the flow structure of cushion pond,and the flow velocity and aeration concentration in each flow structure area of cushion pond with different inflow are measured.According to the flow velocity,the time-averaged kinetic energy and time-averaged kinetic energy dissipation rate at the measured cross-section of each water flow structure area in the water cushion pond are calculated,and the thickness of the water flow shear zone is determined by combining with the law of velocity gradient attenuation.The water flow shear zone is defined as between 50%and 30% of the maximum flow velocity in the structure area.Through the gradient change of aeration concentration in the shear zone of water flow,the bubble exchange process among different water flow structural zones is analyzed.It is concluded that the contribution to energy dissipation of plunge pool is turbulent shear > absorption in rolling area > bubble consumption.