2003 Fiscal Year Final Research Report Summary
Proposition of a rip-off model at the point of flow separation as a new mechanism of cavitation inception
| Project/Area Number |
13450066
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
設計工学・機械要素・トライボロジー
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| Research Institution | Okayama University |
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Principal Investigator |
WASHIO Seiichi OKAYAMA UNIVERSITY, Faculty of Engineering, Professor, 工学部, 教授 (20026222)
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| Co-Investigator(Kenkyū-buntansha) |
YOSHIDA Atsumasa Osaka Prefecture University, Professor, 大学院・工学研究科, 教授 (60174918)
TAKAHASHI Satoshi OKAYAMA UNIVERSITY, Faculty of Engineering, Assistant Professor, 工学部, 助教授 (20236277)
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| Project Period (FY) |
2001 – 2003
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| Keywords | Cavitaion / Hydraulic oil / Point of flow separation / Light emission / Electrification / Heat generation / Microscope / Absolute negative pressure |
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| Research Abstract |
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.
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