Study On Numerical Simulation Of Water Hammer In Spillway Tunnel

Spillway tunnel,as the main drainage structure of water conservancy project,plays an important role in guaranteeing the safe operation of hydraulic project.Working gate is located at the end of spillway tunnel,and the safe operation of working gate is necessary for spillway tunnel discharging water.Water hammer generated when working gate is running may make gate vibrate,which is damaged for working gate.The method of characteristics was employed to simulate water hammer in spillway tunnel,and the research results can provide reference for designing working gate and working out running ways of working gate.The characteristic of spillway tunnel with high water head and large water flow is that the flow velocity is high,therefore,the impact of flow velocity on water hammer was considered in this paper.The main research work and results are as follows:(1)When working gate is running at a constant speed,water hammer is still generated although the running speed of working gate is slow.Water hammer pressure changes periodically and the reciprocating pressure acting on working gate lead to lots of noise and strong vibration of working gate,which could destroy working gate.Water hammer is important for the safe operation of spillway tunnel,so water hammer in spillway tunnel should be studied.(2)Keeping the flow pressure in front of working gate and the running time of working gate unchanged,simulating water hammer in 4 kinds of body length(namely 100 m,300m,600 m and 900m).By analyzing and comparing research results,some conclusions were gained.When working gate is running at a constant speed,water hammer is developed in different lengths of spillway tunnel.More specifically,the amplitude and the duration of water hammer both increase with the increasing of spillway tunnel length.Oscillating frequency,however,decreases with the increase of spillway tunnel length.If oscillating frequency of water hammer pressure is close to the natural vibrating frequency of working gate and spillway tunnel is long,it is more likely to cause the vibration of working gate.The impact of body length on water hammer should be considered when designing working gate structure and working out running ways of working gate.(3)The pulsating pressure of typical measuring points on working gate of spillway tunnel of Sanhekou Water Conservancy Project in Shaanxi Province was measured through hydraulic model test,and then water hammer in spillway tunnel was simulated by the method of characteristics,finally water hammer characteristics was compared with pulsating pressure characteristics.The results show that the oscillating frequency of water hammer pressure is greater than the frequency of pulsating pressure,and the amplitude of water hammer pressure is greater than that of pulsating pressure,namely the intensity of water hammer pressure is greater than that of pulsating pressure.When oscillating frequency of water hammer is close to the natural vibrating frequency of working gate,water hammer is more likely to cause working gate to vibrate than pulsating pressure because the transient load of water hammer acting on working gate is greater than that of pulsating pressure.When designing working gate of spillway tunnel of Sanhekou Water Conservancy Project,water hammer pressure should be considered particularly rather than pulsating pressure,and the natural vibrating frequency of working gate should not be equal to the oscillating frequency of water hammer(5.39Hz),in order to avoid the resonant failure of working gate,which could guarantee the safe discharge of spillway tunnel.(4)Doing numerical simulation on water hammer in spillway tunnel of Ciha Gorge Hydropower Station when working gate is opening or closing,and then computing water hammer in different running ways of working gate.The results show that when working gate is opening,water hammer is developed at the initial stage of the opening process,and the amplitude of water hammer decreases by logarithmic law;when working gate is closing,water hammer is generated after working gate closes completely,and the amplitude of water hammer decreases exponentially;the oscillating time of water hammer when working gate is closing is greater than that when working gate is opening.Consequently,the natural vibrating frequency of working gate of spillway tunnel in Ciha Gorge Hydropower Station should not be equal to the oscillating frequency of water hammer(0.42Hz),otherwise the working gate would be damaged resonantly.Although oscillating frequency of water hammer pressure has nothing to do with operating ways of working gate,the amplitude of water hammer pressure is closely related to running ways of working gate.The running way that working gate opens and closes at a non-constant speed has an effect on unsteady flow in spillway tunnel.If working gate opens faster and faster and the closing speed is slower and slower,water hammer pressure is reduced effectively,and the impact of water hammer on gate may even be neglected,which is conducive to the safe operation of working gate.Therefore,the running way that working gate opens at a accelerated speed and closes at a decelerated speed is recommended to be the operating way of working gate,for example,when the working gate of spillway tunnel of Ciha Gorge Hydropower Station opens at the acceleration of 0.000016m/s2 and closes at the acceleration of-0.000016m/s2,the impact of water hammer on working gate could be reduced effectively.