Study on the cavitation mechanism in hydraulic oil flow using a needle projection

Abstract

Cavitation occurring at a sharp projection in a hydraulic oil flow was observed in as much detail as possible, using, variously, a microscope, a high-speed video camera, laser beams, an electric charge detector and a photomultiplier. At the tip of the needle employed as a projection, a tiny cavity as small as several tens of micrometres in length suddenly emerged and would not go away. As cavitation became more vigorous, flashes occurred intermittently around the needle and positive electrical charges were generated which synchronized with the flashing. The electrode inserted downstream from the needle detected negative charge from the oil, which was also synchronous with the flashing. Observing the needle tip with the microscope made it possible to determine exactly the moment of cavitation inception, which turned out to depend on the oil temperature as well as the downstream pressure. All these findings tend to reinforce the ‘rip-off’ hypothesis previously proposed by the present authors for the cavitation mechanism.

Study on cavitation starting at the point of separation on a smooth wall in hydraulic oil flow

“…When the luminescence was watched on the original video lm, what looked like a gleam to the human eye turned out to be in reality an intermittent ash (http://keilab.mech.okayama-u.ac.jp/ research/cavitation/cavitatione.html). This certainly is the same phenomenon as previously observed in the cavitation on a needle projection [2], and suggests an occurrence of electric discharge in a near-vacuum cavity. Fig.…”

“…Therefore equations (3) provide a theoretical basis for believing that within a very small area just upstream of the separation point, a large tensile stress is generated in the direction normal to the wall. It can also be seen from equations (3) that the magnitude of the tensile stress s y increases as the pressure p decreases and the viscosity · increases, which seems to explain why cavitation inception depends on the pressure and temperature, as was experimentally con rmed in a ow over a needle projection [2].…”

“…In previous studies on cavitation using a rectangular constriction [1] and a needle projection [2], light emission and electric charge were recognized in vigorous cavitation. M oreover, it was discovered with the cavitation on a needle that the steel needle and the oil were charged positively and negatively respectively, and their transient variations were synchronized with light emission [2].…”

“…M oreover, it was discovered with the cavitation on a needle that the steel needle and the oil were charged positively and negatively respectively, and their transient variations were synchronized with light emission [2].…”

“…Although the nucleus theory was widely accepted and seemingly unassailable, in the early 1990s the present authors, motivated by the wish to settle these doubts, started to investigate from scratch the mechanism of cavitation. By means of non-traditional microscopic observations of oil ows at the points of separation of a rectangular constriction and a needle projection, the following phenomena were observed and recorded [1,2]:…”