圆形月池流激振荡实验研究

EXPERIMENTAL RESEARCH ON FLOW INDUCED OSCILLATION OF CIRCLE MOONPOOL

采用基于经验模态分解(EMD)的本征模态函数(IMF)分析方法和对比研究的实验方法,实验研究了圆形月池在均匀流条件下的流激振荡特性．通过实验研究发现,尽管腔内的流体在不同来流的条件下运动趋势有所不同,但归纳起来对于月池结构,其内部存在两类振荡源,一类是腔内流体的固有振荡,该频率与腔体结构形式、尺寸、腔内流体深度有关,与来流无关;另一类则是流体流经腔口时产生的流体动力振荡,该频率与来流特性、速度有关．当二者频率相近时将会激起腔内流体的共振,即会有“活塞(piston)”现象产生．实验中,还在腔体侧壁上布置了压力传感器,用来分析腔体侧壁受到的流体压力特性及其变化趋势．

Moonpool is just like a cave-in cavity with a free surface. With the help of empirical mode decomposition （EMD） method this experiment was conducted in a towing tank. The key part of the EMD method is that complicated data set can be decomposed into a finite and often small number of ＇intrinsic mode functions （IMF）＇. By a comparative method, the data were collected with and without the moonpool in all conditions separately. With the empirical mode decomposition, the intrinsic mode functions of the data, and then the PSD spectrum of each IMF are obtained. From the difference of each PSD spectrum between the results with and without moonpool, the data for the flow over the moonpool can be obtained. For the moonpool, the fluctuating force consists of the water oscillation and the shear layer self sustained oscillation. The frequency of the water oscillation is determined by the shape and size of the moonpool. The frequcney of the shear layer self sustained oscillation is also related with the flow velocity over the moonpool, which will change with the velocity. When the two frequencies are close to each other, a resonance will happen. In this experiment, when the Froude number falls within the range of 0.26 to 0.39, the resonance phenomenon is observed. It appears that the water in the moonpool moves up and down in the vertical direction. This is so-called the ＂Piston＂ phenomenon. When the Froude number is greater than 0.52, there is no resonance phenomenon any more. In this paper, under some assumptions and the potential flow theory, the resonance frequency of the water in the moonpool is calculated, which is very close to the experiment result. In a resonance, the fluid fluctuating force in the moonpool is a harmonic force. The frequency is the same as the resonance frequency and the amplitude is great. When there is no resonance, the fluctuating force can be obtained with the EMD method, which consists of the shear layer oscillation and the water oscillation. The wall pressure is also measured in this investigation. The wall pressure of the moonpool is found quite different not only in value but also in distribution in the circumferential direction and the vertical direction, whether in resonance or not.