The purpose of the present experimental study is to get a better understanding of the dynamics of the vapor phase spatiotemporal repartition in a cavitating backward facing step flow. We provide a refined data base of the use of the void fraction transport equation to model such flows. The backward facing step flow provides a well-known test case to compare vortex dynamics between single and two-phase flow. To evidence the vapor phase dynamics, the flow is probed by high-speed x-ray attenuation techniques and by pressure fluctuation measurements at the walls. Long-time dynamics are also visualized using conventional high-speed imaging synchronized with pressure measurements. Large vortex structures, free shear layer instability, wall interaction and reverse flow are observed. The two-phase structures are studied at different cavitation levels corresponding void fractions ranging from 1% to 50%. The topology of the mean and fluctuating void fraction maps is performed, leading to the establishment of three specific areas in the flow. These areas are distinguished by the underlying mechanisms happening within them: vaporization, transport, and condensation. The statistical analysis underlines the existence of extreme events associated with high void fraction levels and wave propagations. While these events are associated with topological changes from a shear layer to a wake mode that also exist in the single-phase case, they are associated with much lower frequency at high cavitation levels.

中文翻译：

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空化的后向步进流中的耦合X射线高速成像和压力测量

本实验研究的目的是为了更好地理解在空化后向步进流中气相时空重新分配的动力学。我们提供了一个改进的数据库，其中使用了空隙分数传输方程来对此类流动进行建模。向后的步进流提供了一个众所周知的测试案例，可以比较单相流和两相流之间的涡流动力学。为了证明气相动力学，通过高速X射线衰减技术和壁上的压力波动测量来探测流动。使用与压力测量同步的常规高速成像，还可以显示长期动态。观察到大涡结构，自由剪切层不稳定性，壁相互作用和逆流。在不同的空化水平下研究了两相结构，相应的空隙率范围为1％至50％。执行均值和波动的空隙率图的拓扑，从而在流中建立三个特定区域。这些区域的特征在于内部发生的潜在机制：汽化，传输和冷凝。统计分析强调了与高孔隙率水平和波传播有关的极端事件的存在。虽然这些事件与从剪切层到尾波模式的拓扑变化相关（单相情况下也存在），但它们与高空化水平下的较低频率相关。导致在流程中建立三个特定区域。这些区域的特征在于内部发生的潜在机制：汽化，传输和冷凝。统计分析强调了与高孔隙率水平和波传播有关的极端事件的存在。虽然这些事件与从剪切层到尾波模式的拓扑变化相关（单相情况下也存在），但它们与高空化水平下的较低频率相关。导致在流程中建立三个特定区域。这些区域的特征在于内部发生的潜在机制：汽化，传输和冷凝。统计分析强调了与高孔隙率水平和波传播有关的极端事件的存在。虽然这些事件与从剪切层到尾波模式的拓扑变化相关（在单相情况下也存在），但它们与高空化水平下的频率低得多相关。