【摘要】 本文借助目前国内外较先进的信号记录仪器及其专用数据处理设备,应用随机函数理论分析了孔流坝面压力脉动的幅、频特性,并按开度、水头等不同水力条件比较了坝面水流压力脉动的变化规律与分布规律。更多还原

【Abstract】 The pressure fluctuation of the flow over a dam. Can increase the instantaneous load and cause vibration of the dam body and cavitation damage on the surface of a dam. Therefore it is significant in engineering to make sure the amplitude and frequency characteristics and mechanism of the pressure fluctuation on a dam.Surface pressure fluctuations on a model overflow spillway flush-mounted with micro pressure transducers were systematically measured by using a signal recorder. Based on the theory of random function, the amplitude and frequency spectrum characteristics of the pressure fluctuations are analysed using data and signal processor. The smoothness and normal probabilistic distribution of pressure fluctuation process of the flow over a dam are checked using alternate method, x2-f itted optimization method and so on. It is assumed that the pressure fluctuation of the flow through a gate opening on a dam. Can be looked on as a smooth stochastic process subjected to ergodic law, the amplitude distribution of which is basically normal. In the domain of amplitude, the characteristics of the flow-fluctuation intensity on a dam and its structure of amplitude distribution are analysed by computing and comparing the standard deviation σx, Coefficient of variation CT, maximum deviation K and probabilistic distribution function. In the domain of frequency, it is shown that the pressure fluctuations of the flow through a gate opening on a dam have the same frequency characteristics of low frequency and large amplitude with open channel flow, multi-peak, and the same spectrum property as the narrow-random noise. Furthermore the change rule of the pressure fluctuations of the flow on a dam and its distributional regular pattern are compared according to the gate opening, head and flow conditions. The results showed that the amplitude and density of frequency vary mainly in proportion with the gate opening, and the upstream head. There is a turning point in the change of the amplitude with the head which varies with the gate opening. The distribution of amplitude and power spectrum density of the pressure fluctuations along the surface of a dam are closely related to the flow pattern and boundary conditions. Compared systematically, the pressure fluctuation. showed a tendency similar to that of a time-averaged pressure distribution in some degree. The effect of centrifugal force takes an important role in making the underpressure and pressure fluctuation increase by se-parating the nappe. The development of vortex of the overflow gate opening is the major energy source of the pressure fluctuations. The time-averaged pressure only depends on the boundary conditions near the measured point and the mean flow field. In addition to the above influences, the pressure fluctuations depend on the longitudinal motion of the upstream vortex.The impulse effect of the nappe on the surface of a dam is related to mean curvature of streamlines as the flow with vortex enters the overflow gate opening. Generally speaking, the impulse and fluctuations will shift backward along the dam surface as the gate opening increases and the mean curvature of inflow streamlines decreases and forward with rising of the upstream head and increasing of the mean curvature of inflow streamlines. The unstable state of nappe along the flow due to rapid contraction of bottom boundary and unbalance between the gravity force and the centrifugal force are the main cause of rising and falling of the time-averaged pressured and pressured fluctuations along the surface of a dam.更多还原