Model Test And Turbulence Numerical Simulation On The Dragon-Raise-Head Free Flow Spillway Tunnel With High Head

In the hydraulic engineering diversion tunnel is often rebuilt to flood spillway tunnel,which linked dragon-raise-head sector to the diversion tunnel. The aerators are widely used in flood spillways tunnels to prevent the cavitation damage of the structures. The characteristics of the dragon-raise-head spillway tunnel are high head, large-unit-discharge, low Froude number and slight slope. Large-discharge and low Froude number imply high depth of water,the impact of the gravitations is notable. So there is easily backwater in the air chamber,which obviously affects on the hydraulic character of the aerator. On the condition of slight slope flows the bottom of air chamber tends to cause backwater flow and obstruct air current. In the view of research on configuration optimization of aeration device of the free flow spillway tunnel former scholars emphasis on two-dimension research,i.e. following flows direction variability and ignore the flows crossing run of vertical water direction. So for this flow condition the common aeration device doesn't meet the demand of protecting cavitation damage.A new V-type aerator, the three dimensional aerator has been put forward through model test. The main characteristic is that gravitations distribution of free jet nappe alternates not only in the directions of longitudinal and vertical, but also in the cross direction along vertical flowing direction. The velocity,jet-trajectory angle and water depth change lineally from the center to both sides. The range of downstream edge is continuous curvilinear. Thus backwater in the air chamber is eliminated. The reason of why the V-type is better has been analyzed theoretically. For this type, on the outlet of free jet the three-dimensional flow diffusion is more sufficiency than the traditional two-dimensional aerator. Therefore the air entrainment rate is larger, the length of air chamber is longer. It is mainly because that, the length of the air chamber is continuously varying along the lateral direction within a range and along downstream bilateral edge strong turbulence fluctuation further contributes to aerated capacity. Based on model experiment, numerical model for V-type aerator is built and theory formulas are put forward on the length and pressure of air chamber and air discharge. In order to study the hydraulic characteristics of the aeration device comprehensively the method of turbulence numerical simulation is used to study the free flow spillway tunnel. The fractional volume of fluid (VOF) model of the air-water two phase flow is introduced to the three-dimensional k-εtwo–equation turbulence model and combined with the geometry reconstruction scheme to solve the free water surface. The unstructured grid is used to treat the irregular boundary. By these methods, the turbulence flow field of the free flow spillway tunnel is simulated successfully. The free water surface and the distribution of velocity and pressure on the free flow spillway tunnel are obtained by the turbulence numerical simulation, which are analyzed and compared with the test results, and agreed well with each other.Behind the aerator of the anti-arc bottom self-aerated is not enough at the surface of flow and the aeration by force is not sufficiently diffuse,which causes a large un-airflow area in the sidewalls, at the same time influenced by centrifugal force and low pressure areas of sidewalls——all of these as we know it is easy to be destroyed by the cavitation under these conditions. In this paper an aerator with a sudden contraction and a vertical drop is used to bottom of anti-arc. The results of model tests indicate the sudden contraction ratio is suitable to 0.012~0.018 and grade of slope should be 1:40~1:20. The air entrainment quantity is obviously affected by lateral deflector. The lateral air chamber is an important aeration mechanism when the lateral air chamber is connected to the bottom cavity,which prevent cavitation damage of sidewalls and soleplates effectively.