Research On Turbulence Characteristics And Energy Dissipation Mechanism Of Multiform Stepped Spillways

In the water conservancy project,the water release structure plays an important role.The water flow carries huge energy through the water release structure.If the energy dissipation is not sufficient,it will cause scour damage to the dam body,dam foundation and downstream river bed,and the safety of hydraulic structures is greatly threatened.Therefore,the good energy dissipation effect of the water release structure is one of the key factors for the safe operation of the water conservancy project.Stepped spillway,as a new type of spillway structure,has the advantages of reducing the construction period and energy dissipation effect.It has been used more and more in water conservancy projects.Because of the complex boundary conditions,many hydraulic characteristics of stepped spillway need to be further studied,such as the classification and boundary of flow pattern,the development of boundary layer,the velocity field and pressure field of stepped spillway,aeration characteristics,turbulence intensity,etc.;in addition,the research on the factors affecting the energy dissipation rate of stepped spillway The research is still not comprehensive.In this thesis,the turbulent characteristics and energy dissipation mechanism of stepped spillway with a gradient of 0.8H: 1.0V are systematically studied by model test.The variation laws of hydraulic characteristics such as flow pattern,boundary layer development,velocity field and pressure field of stepped surface,aeration characteristics,turbulence intensity,autocorrelation scale and energy dissipation rate are analyzed.It is found that the turbulence on the step surface of practical weir is violent in practical weir inflow conditions,and the energy dissipation effect is better;the reduction of roughness makes the location of incepotion point of aeration move upstream,and the aeration concentration on the step surface increases,which improves the energy dissipation rate;in the application scheme of stepped spillway combined with flaring gate pier,the jet flow increases the turbulence and aeration concentration,greatly improves the energy dissipation rate,and increases the applicable flow discharge range of stepped spillway.The main contents of this thesis are as follows:(1)Considering the factors that may affect the flow characteristics of the stepped surface,a variety of stepped spillways including different inflow modes(broad crested weir and practical weir inflow),different roughness and the application of combined flaring gate pier were set up.Through the flow turbulence characteristics and energy dissipation performance above the virtual floor of the stepped surface,the sensitivity of the stepped surface flow to various factors was analyzed.(2)The critical values of nappe flow,transition flow and skimming flow were determined by observing and recording the flow pattern.The nappe flow and transition flow are divided into two types according to whether there is jet flow in the upstream step(such as the second step).(3)According to whether the flow on the step surface is aerated or not,it can be divided into boundary layer developing area and aerated area.The water depth,velocity and thickness of boundary layer were analyzed,and the critical point of free surface aeration was determined.By comparing the free surface fluctuation with the bottom roughness,the correlation between the turbulent characteristics and the macro roughness of stepped spillway flow was revealed.The specific energy,average energy,head loss coefficient and Darcy-Weisbach friction coefficient of the boundary layer developing area were calculated,and the energy dissipation effect of the boundary layer development area was analyzed from many aspects.(4)The position and water depth of the inception point of aeration were measured and recorded,and the relevant empirical formula was put forward.The air concentration distribution of the flow downstream of the inception point of aeration was measured with a conductivity probe,and the aerating effect of all stepped spillways was evaluated.(5)The flow velocity along the water depth direction in the aerated area is measured intensively.It is found that the flow velocity from the virtual bottom plate to the 50% aerated concentration follows the law of power function distribution.The physical boundary point at the 50% aerated concentration is proposed.Compared with the traditional scale selection based on 90% aerated concentration,the water phase changes from continuous phase to discrete phase at this location,and there are extreme points or inflexion points in the flow velocity,pressure,turbulence intensity,autocorrelation scale,etc.as the scale variable,the impact of steps on the flow can be analyzed intuitively.(6)Based on the relevant parameters measured by the conductivity probe and micro pressure sensor and the turbulence statistical theory,the calculation formula of the turbulent intensity of the water phase in the aerated area was proposed.The turbulent characteristics such as the fluctuation of the total pressure and the autocorrelation scale were compared and analyzed,and the energy consumption characteristics of the stepped spillway were analyzed from the perspective of the flow turbulence.(7)The method of integral average was used to calculate the average energy and energy dissipation rate.The Darcy-Weisbach friction coefficient of aerated flow was calculated by friction slope,and the influence of bottom step on energy dissipation was analyzed.(8)This thesis studied the influence of flaring gate pier on the aerated concentration of stepped spillway,and revealed the energy dissipation mechanism and energy consumption characteristics of stepped spillway combined flaring gate pier from the aspects of flow velocity distribution,pressure fluctuation,turbulence intensity,autocorrelation scale,etc.