Research On The Hydraulic Properties Of An Orifice Spillway Tunnel

The hydropower in our country has been developing rapidly, a lot of hydropower projects have been established, and many achievements were made, especially, the study of the spillway and energy dissipation of hydraulic projects for the high head and large scale discharge has reached the world's advanced levels in the past fifty years. A lot of planned hydropower projects that the height of its dams are up to 200 to 300 meters and the spillway discharges are 20,000 to 50,000 m3/s, will be founded in the 21st century, this will challenge the high dam hydraulics for some new problems. Therefore, it is inevitable to develop the different dissipaters. Among these dissipaters is to rebuild the diversion tunnel to the spillway tunnel, which is a great economic-profit project but having many difficulties, so it is necessary to research on it.Since the orifice spillway tunnel which is a new kind of dissipater, was applied to Xiaolangdi hydraulic project in China, it was highly concerned and a number of model tests were made and many useful data were obtained. However, the strong vibration was occurred during the test of No.l orifice spillway tunnel under the emergency operating condition at site. Why did the phenomenon appear? Weather to threaten the security of the tunnel? So the detailed research on the new dissipater is carried out herein by using both numerical and experimental techniques.Briefly, the following achievements are made:1. The hydraulic properties of the orifice spillway tunnel under the emergency operating condition were obtained by physical model tests. A computational model of the water-air two-phase and transient flow was developed firstly, and the hydraulic properties of the tunnel under the emergency operating condition were calculated successfully with this computational model. The computational model can be applied to computing not only the water-air two-phase flow but also single-phase flow.2. It is indicated that the suction of air from the emergency ventilating shaft is the reason to cause the harmful hydraulic vibration. Lower the level of the reservoir is, earlier the ventilating shaft begins to suck the air, longer the enduring time of the water-air two-phase flow is, and stronger the vibration is. On the contrary, higher the level of the reservoir is, although more the static pressure is, shorter the enduring time of the water-air two-phase flow is, and weaker the harmful hydraulic vibration is.3. A large eddy simulation approach based on the weakly compressible hydrodynamic equation has been developed and successfully employed in simulating 3-D unsteady viscous flow through the orifice spillway tunnel.4. It is showed that the total head lose through three orifices is about 40% of the total head and the dissipating' rate is satisfactory. The maximum fluctuating pressure is occurred near the 0.5D location at downstream, the main frequency is less than 2.0Hz and it belongs to the low frequency and large amplitude.5. The non-constant density computational model for cavitation flow is developed and applied to simulating the cavitation inception of the orifice spillway tunnel, the computational result is basically fitted with the test result. It is indicated that the easily cavitated locations are at the top of orifice, the comer at downstream of orifice and the center of the backflow. The cavitation is mainly located at the inside of the flow, so the threaten to the spillway tunnel is small.6. The distribution of the enstrophy in the computational domain of the orifice spillway tunnel are calculated firstly, which indicates the dissipating rate and implies the physical conception more clearly than the dissipating rate calculatedby the k-e model.