可压缩空化流动空穴演化及压力脉动特性实验研究

空化流动具有高度的压缩性,空化流动非定常特性及其流体动力与压缩性密切相关.为研究可压缩空化流动空泡脱落的回射流和激波机制下周期性空穴结构演化及其诱导流体动力特性,本文采用多场同步测试方法对典型云状空化流动进行了实验研究,获得了文丘里管扩张段内部云状空化空穴形态演化及其诱导同步壁面压力脉动信号.并基于数字图像处理技术,对附着型片状空穴和脱落型云状空穴结构演化进行了精细化定量分析.结果表明:可压缩空化流动回射流机制下,空穴演化呈现附着型空穴生长$\!$-$\!$-$\!$回射流产生及发展$\!$-$\!$-$\!$附着型空穴失稳断裂及大尺度空泡云团产生脱落的非定常过程,激波机制下空穴演化具有附着型空穴生长$\!$-$\!$-$\!$激波产生及传播$\!$-$\!$-$\!$附着型空穴失稳断裂及大尺度空泡云团脱落的非定常特征,激波传播时间占空穴脱落周期小于回射流推进.激波与空穴相互作用导致空穴内部含气率瞬间大范围大幅度下降,诱导复杂流体动力.激波传播过程中,空泡内部压力脉动大幅增加,激波前缘诱导压力脉冲,而回射流推进过程中,壁面压力脉动相对平稳,回射流头部存在小幅增加. 不同机制下空穴结构存在显著差异,具有不同的相间质量传输过程. 

EXPERIMENTAL INVESTIGATION OF CAVITATION CHARACTERISTICS AND DYNAMICS IN COMPRESSIBLE TURBULENT CAVITATING FLOWS 1)

Cavitation is the strongly compressible flows, where the cavitation compressibility effects could be closely associated with the cavity instabilities and dynamics. To investigate the cavity structure evolution and dynamics under both the re-entrant jet and shock wave mechanisms in compressible cloud cavitating flows, experiments were conducted in the divergent section with 10° in a venturi, using the simultaneous sampling technique to synchronize the observations of transient cavity behaviors with the wall-pressure signals measurements in cloud cavitating flows. A novel image processing algorithm is developed to analyze the transient cavity structures in detail. Based on the developed image processing method, the attached-type sheet cavity and shedding cloud-like cavity structures can be studied quantitatively. Results showed that unsteady cavity behaviors under re-entrant jet mechanism (RJM) can be divided into three stages: 1) the attached cavity growth, 2) the re-entrant jet formation and movement, and 3) the attached cavity breakup and cavity cloud shedding, collapse. The transient cavity structures under shock wave mechanism (SWM) present periodic behaviors: 1) the attached cavity growing, 2) the shock wave generating and propagating, and 3) the attached cavity collapsing and shedding. The period of re-entrant jet development is larger than that during the shock wave propagation. The interactions between shock wave and cavity cause significant local void fraction reduction, which induces complex cavity dynamics. During the shock wave propagation, both large pressure fluctuations and pressure peaks are captured, while in the process of the re-entrant jet movement, relatively smooth pressure fluctuations are measured, with small pressure fluctuations increase at the re-entrant jet head. Furthermore, the behaviors of attached-type sheet cavity, and shedding cloud-like cavity are totally different under re-entrant jet and shock wave mechanisms,indicating different mass transfer processes in different cavity shedding mechanisms.