An Assessment of the Influence of Environmental Factors on Cavitation Instabilities

Abstract

Cavitation induced flow instabilities are of interest in numerous applications. Experimental and numerical investigations of this phenomenon are taking place at several institutions around the world. Although there is qualitative agreement among the numerous recent papers on the subject, there is a lack of agreement with regard to important details, such as the spectral content of unsteady lift oscillations. This paper summarizes observations of a cavitating NACA0015 foil in three different tunnels that revealed remarkably different cavity shedding appearances and behaviors. Some of the differences were attributed to system instabilities. However, in addition to a different cavitation behavior attributed to system instabilities, it was found that differences in gas content could significantly alter the lift spectrum of a cavitating foil. For a certain range of the composite parameter σ∕2α near 4, the dominant frequency appears to double when the gas content is reduced by a half. It is also argued that surface effects can have a significant influence on fully wetted time during cavity shedding. Normally, surface effects are assumed to play an important role in the initial inception of a fully wetted hydrofoil with gas content being the primary factor governing developed cavitation behavior. However, the repetitive nature of the process implies that each shedding cycle is an individual inception process. Hence, the unexpected role of surface effects in partially cavitating hydrofoils. The conclusions reached have important ramifications concerning numerical code verification that is a topic of major concern.