Nuceli Effects on Tip Vortex Cavitation Scaling

Abstract

A cavitation susceptibility meter has been used by the US Navy for 16 years to measure the tensile strength of water in the NSWCCD water tunnels, Lake Pend Oreille in Idaho, Exuma Sound in the Bahama Islands, the Pacific Ocean along the US West Coast, and the North Atlantic Ocean. This meter measures the variation of cavitation nuclei with water tension. Using the bubble stability equation, these tensions are related to equivalent size bubbles to produce a spectrum of bubble concentration versus bubble size. This paper compares these nuclei sizes in the lake and ocean waters. Results of bubble dynamics calculations show a relationship for estimating the nuclei effect on scaling that is separate from the viscous effects on the pressure fields. The influence of these nuclei variations on cavitation inception scaling are discussed for a hypothetical propeller tip vortex cavity. The scaling of two model inception conditions are studied for nuclei variations in the model environment as well as variations in the full-scale environment. The sizes of the nuclei in the two environments differ, but the resulting effects on inception are predicted to be minor, and this trend may be globally true for nuclei variations within natural water bodies. The variations illustrated may be minor compared to the nuclei effect for scaling water tunnel model tests to full-scale.