Observation of cavitation in hydraulic oil flow separated from a smooth surface on which an incipient cavity is unable to stay

Cavitation has been observed in a hydraulic oil flow separated from a smooth cylindrical wall, using various methods such as a microscope mounted with either a digital still camera or a high-speed video one, laser beam transmission, photo-multiplier and an electric charge detector. At the incipient phase of cavitation, a cavity suddenly emerged at the separation point with its upstream tip attached on the wall. Differently from cavitation previously observed in such flows as separated from an edge or a projection, however, the present incipient cavity neither stayed nor made a stable bubble on the wall, which was smooth this time. While drifting downstream, the newly born cavity was substantially enlarged, underwent severe deformation and ultimately vanished from the tail in as short a time as one-tenth of milli-second. When being enlarged, the cavity emitted flashing light similar to the one formerly observed in the cavitation on the needle projection. Transient electric charge synchronized with the light emission was also detected from the oil in the downstream. Moreover, the streamline extending from the separation point was spontaneously visualized, indicating heat generation at the separation point where significant shear stress acts. All these results seem to support the "rip-off" hypothesis previously advocated by the present authors.