A Method for the Study of Unsteady Cavitation - Observations on Collapsing Sheet Cavities

Doctoral thesis, 2000

When subjected to sufficiently low pressures, water will eventually come apart. Gas-filled cavities appear in the water: it is said to cavitate. When the cavities collapse, they often do so quite vigorously, causing noise, vibrations and erosion. A well-known example of cavitation is that found on heavily loaded ship propellers. The wake flow at the propeller is seldom rotationally symmetric, so the resulting flow met by each propeller blade changes periodically as the propeller rotates. These variations of the inflow make the resulting cavitation significantly more destructive. This thesis deals primarily with the design and testing of a new type of flow equipment which, although it does not fully reproduce the flow around a propeller blade, still has attractive features compared with its precursors. The propeller is replaced by a single fixed wing, while the asymmetric wake is simulated by a train of gusts generated by a pitching wing located upstream of the fixed wing. During the course of testing, several mechanisms that play significant parts in the formation of erosive cavitation were studied. A mathematical model was formulated for the flow in the jets that sometimes re-enter sheet cavities from their downstream edge. In qualitative agreement with observations, this model was found to describe mechanisms that enable the re-entrant jet to pinch off part of the sheet cavity, which is known to trigger violent cloud cavitation. Sheet cavities that lose their hold on the leading edge of the wing, which means they could collapse quite symmetrically, can do so with great force. This type of collapse was easier to produce with the new equipment than with earlier set-ups. Observations led to the conjecture that the pressure pulse emanating from the collapsing sheet cavity could synchronise a concerted collapse of accompanying bubbly fragments, thereby enhancing the erosivity of them. A mechanism that may connect the collapse intensity of a cavity and the optical brightness of the consequent rebounding cloud, was suggested. It is based on the phenomenon that the spatial frequency of Rayleigh-Taylor instability fingers at the cavity interface becomes greater in more violent collapses; this produces a denser cloud with finer bubbles, which looks whiter.