Study on Unstable Structure of Three-dimensional Vortices and its Oscillation induced by Cavitation Interaction

旋转空化、空化喘振等空化不稳定性问题是实现小型高速涡轮泵需要解决的重要问题之一。了解内部流动中的复杂涡结构和非定常空腔行为是重要的，这两者在空化不稳定中起主导作用。在本研究中，我们重点研究了螺旋诱导管内的空化喘振，其中出现了包括强进口回流在内的复杂涡旋流结构。已知空化喘振与进口回流密切相关，在诱导体上游设置障碍板可有效抑制空化喘振。在本研究中，我们在FY2005中测量了不同叶尖角诱导的进口流量，并研究了阻止叶尖附近回流的进口环板对进口回流的影响以及对空化浪涌发生区域的影响。结果发现，安装进口板增加了回流中的角动量，从而增加了叶尖附近的静压，抑制了空化浪涌。在2006财年，我们对诱导体进行了数值流动分析，支持了之前的实验结果。其次，我们测量了由于进口挡板在周向部分阻挡回流而导致的进口流动畸变。同时，我们还开发了多摄像头观测系统，使我们能够同时观察叶片与叶片之间的空腔模式。在2007财年，我们通过流量测量、空腔观测和数值模拟，阐明了安装上述两种障碍板对空化浪涌的抑制机理。

Cavitation instabilities such as rotating cavitation and cavitation surge are one of the most important problems to be solved for realization of small-sized high-speed turbopumps. It is important to understand complex vortex structures in the internal flow and the unsteady cavity behavior, both of which play dominant roles in the cavitation instabilities. In the present study, we focused on the cavitation surge in helical inducers, in which the complex vortical flow structures including strong inlet back flow appear.It is known that the cavitation surge is strongly associated with the inlet back flow and the installation of the obstacle plate upstream of inducer is effective for the suppression of the cavitation surge. In the present study, in FY2005, we measured the inlet flow of inducers with the different blade tip angle, and we investigated the effect of the inlet ring plate, which blocks the back flow near the tip region, on the inlet back flow as well as on the onset region of the cavitation surge. As a result, it was found that the angular momentum in the back flow is increased by installing the inlet plate, resulting in the increase of the static pressure near the tip region and the suppression of the cavitation surge. In FY2006, we carried out the numerical flow analysis of the inducer, which supported the previous experimental results. Next, we measured the inlet flow distortion due to the inlet baffle plate which blocks the back flow partially in the circumferential direction. And also we developed the multi-cameras observation system which enables us to observe the simultaneous cavity pattern from blade to blade. In FY2007, we clarified the suppression mechanism of the cavitaion surge by installing the above two kinds of obstacle plate through the flow measurement, the cavity observation and the numerical simulation.