非定常自然空泡脱落机理研究
首发时间:2017-05-11
摘要:本文采用实验和数值计算相结合的方法对非定常空泡的脱落机理进行了研究。实验中,应用高速摄像技术观察了不同空化阶段,空穴的发展过程及空泡的脱落细节,并采用动态应变仪捕捉到了由空泡脱落引起的动力信号变化频率。数值计算采用均质流模型,应用基于汽液相间质量传输的空化模型模拟空化流动现象,采用滤波器湍流模型(FBM)对空泡的脱落细节进行捕捉。研究结果表明:在不同的空化阶段,非定常空泡体现出不同的脱落机制:当处于相对稳定的片状空化时,由于空穴内部自身的小幅度压力波动,造成了小尺度空泡团的频繁、无规则的脱落;当空化流场处于片状空化向云状空化的过渡阶段,通过对升力信号进行频谱分析,发现了空泡脱落存在两个不同的特征频率,其一是由于空穴自身的不稳定脉动特性,其二体现出类似空化云的脱落特性;当空化数继续减小,空化流场体现出明显的大尺度空泡团的周期性脱落现象,云状空化尾部的高压区域是引起反向流动的直接因素,随着反向流动逐步向前缘推进,形成了大尺度空泡团的脱落现象。
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Physical investigation of the unstaedy cavity shedding mechanism
Abstract:The objective of this paper is to investigate the unsteady cavity shedding mechanism via combined experimental and numerical methods. The high speed camera is used to observe the instantaneous cavity structures and the dynamic strain indicator is applied to obtain the dynamic signal caused by the cavity shedding. As for the numerical methods, the Filter Based turbulence model (FBM) and the transport equation based cavitation model are used to simulate the unsteady cavitating flow. The results showed that there are different cavity shedding mechanism in different cavitation stage: During the sheet cavitation stage with a relative steady cavity shape, the small-scale cavity sheds frequently and irregularly, which is due to the small-amplitude pressure fluctuation. During the sheet to cloud transition stage, two characteristic frequencies are obtained, one of which is induced by the unsteady characteristics of the cavity, the other is bacause of the shedding of the cloud cavity. With further decreasing of the cavitation number, the periodic shedding of the large-scale cloud cavity can be observed. The high pressure distribution in the rear region of the cloud cavity is the main reason of the re-entrant jet, which plays the dominant role in such a breakoff phenomenon.
Keywords: Fluid Mechanics Unsteady cavitation Cavity shedding
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