Proposition of a rip-off model at the point of flow separation as a new mechanism of cavitation inception

1.通过对各种液压油流中分离点的显微观察，已经证实，第一个气穴始于分离点附近的壁上突然出现的微观空腔，其中流动是层流。一个空洞突然出现并迅速增长，例如在0.1毫秒内，其上游尖端附着在壁上，并从表面延伸约45度。这种初生空泡产生许多微小气泡，在下游的湍流中起所谓的空化核的作用。在液压油流中，强烈的空穴现象会产生光发射和电荷。当空化开始时，油和壁带上相反的电荷。光发射的光谱表明，当积累的电荷在近真空腔中释放时，它就会发生。分离流线在液压油流中自发地可视化。通过测量，证明了在分离点处产生热量，因此温度沿从分离点延伸的流线沿着变高。事实表明，由于这种温度分布产生的折射率差异，流线变得可见.本文最重要的研究结果是：空化的真实的原因是“流动分离点的奇异性”。当流速增加时，分离点处的固液界面突然被破坏，在那里产生裂缝。裂缝发展成一个初始的空洞，我们可以清楚地观察到在目前的研究。主要原因是什么？应力集中、生热还是拉应力？我们下一个研究目标就是弄清这些问题。

1. Trough microscopic observations on the point of separation in various hydraulic oil flows, it has been confirmed that the very first cavitation starts with a microscopic cavity which suddenly emerges on the wall near the point of separation, where the flow is laminar. A cavity suddenly emerges and rapidly grows, in 0.1 millisecond for example, with its upstream tip attached on the wall and elongates about 45 degrees away from the surface. This incipient cavity produces many minute bubbles, which act as so-called cavitation nuclei in the downstream turbulence.2. Light emission and electrical charges are always observed in vigorous cavitation in hydraulic oil flows. Oil and the wall put on charges with opposite signs when cavitation starts. The spectrum of light emission suggests that it occurs when accumulated electrical charges are discharged in a near-vacuum cavity.3. A separation streamline is spontaneously visualized in a hydraulic oil flow. Through measurements, it has proved that heat is generated at the point of separation and accordingly temperature becomes higher along the streamline extending from the separation point. The fact shows that a streamline becomes visible owing to differences of refractive index generated by this temperature distribution.4. The most important result of our research is as follows ; the real cause of cavitation is "the singularity of the point of flow separation". When a flow rate increases, the solid-liquid interface at the point of separation is suddenly destroyed, producing a rift there. The rift develops into an incipient cavity, which we could clearly observe in the present research.What is the destruction of the solid-liquid interface? What is the major cause of it? Stress concentration, heat generation or tensile stress? Our next target of research is to make clear these questions.