Cavitation Testing in Water Tunnels

Abstract

There are direct and indirect measurements reported in the literature which indicate boundary pressures in incipient-bubbling cavitation zones in water ranging as high as a foot or more of water above vapor pressure to below absolute zero (liquid tension). Tests have been conducted to study some of the factors believed to affect the cavitation susceptibility of water in a water tunnel and to determine if the use of a measured pressure instead of vapor pressure in computing the cavitation index would result in more consistent results in tests of incipient-bubbling cavitation on slender bodies. Factors studied included total gas content of the water, the free carbon dioxide, the nitrogen-oxygen ratio, changes in surface tension, colloidal solid nuclei added to the water, temperature, and velocity. The total gas content and temperature were the only known factors which changed the measured cavitation pressure significantly at a given velocity, although one or more unknown factors were effective since tests were seldom reproducible. Incipient cavitation tests of the tunnel test section, and of a 3- and 6-caliber ogive head form, indicated that the incipient cavitation index was more constant when based on a measured pressure than when based on the conventional vapor pressure.