云状空化的非定常流体动力特性

Unsteady hydrodynamics of cloud cavitating flows in a convergent-divergent channel

基于高速全流场显示和动态压力测量技术,实现了非定常空化空穴形态和壁面压力的同步测量。采用该方法,同步观察和测量了收缩—扩张流道内云状空化形态的准周期性变化及其对应的壁面压力波动特性,分析了压力信号频谱,讨论了非定常空穴形态变化和壁面压力波动的关系。结果表明:扩张段内云状空穴的发展过程呈现出附着型空穴生长,附着型空穴断裂、脱落以及脱落型空穴聚合、生长、溃灭的准周期过程。壁面压力波动的主导频率约为21 Hz,对应附着型空穴的准周期生长、断裂和大尺度脱落型空穴生长、溃灭;次级频率约为42Hz,对应小尺度脱落型空穴非定常变化。同时发现空穴变化和压力波动存在如下关系:在附着型空穴的生长阶段,其空穴前部稳定附着区域的压力波动较小,其尾部非稳定区域的压力波动较大;当附着型空穴发展至最大长度时,其尾部空泡的非定常波动会使得相应区域压力波动更加剧烈。伴随着反向射流的发展,附着型空穴断裂、脱落产生许多小尺度脱落型空穴,其生长、溃灭等非定常变化会使得相应区域的压力波动变得复杂。小尺度脱落型空穴逐渐聚合、生长形成单一的大尺度脱落型空穴,伴随着其不断的向下游移动,相关区域被其覆盖时压力波动减小,脱离其覆盖或者经历其溃灭过程时压力波动显著增大。

A simultaneous experiment for unsteady cavitating flow observations and hydrodynamic measurements in a convergent-divergent channel is conducted by combining a high speed visualization and a dynamic pressure measurement set ups. Both the cloud cavitation images and wall-pressure are addressed experimently. Spectral analysis is conducted for pressure signals, and the correlation between the unsteady cavity behaviors and the pressure fluctuations is discussed. The results show that a typical quasi-periodic process of cloud cavitation development is characterized by the process of attached cavity growth, attached cavity shedding, and the collapse of cloud cavities. The pressure signal ＇s dominant frequency is 21 Hz,which corresponds to the quasi-periodic cycle of growth and shedding of the attached cavities. The pressure signal＇s secondary frequency is about 42 Hz, which corresponds to the events of growth and collapse of small cloud cavity in the rear of the attached cavity. The correlation between cavity behavior and pressure fluctuation is summarized as follows. During the attached cavity growth stage, pressure fluctuation in the region covered by stable attached cavity is small, while the pressure fluctuation at the cavity closure region is strong. When attached cavity obtains its maximum length, it is rear part becomes quite unstable, and the pressure fluctuation in the rear region of attached cavity becomes more stronger. After that, the attached cavity is broken up with the development of re-entrant jet, and the pressure fluctuation becomes complex due to the growth and collapse of the small cloud cavities. Finally, during the process of its convection to the downstream, a large cloud cavity is formed by the coalescence and growth of small cloud cavities, while a decrease in pressure fluctuation is detected.