Research has been conducted to evaluate a technique for measuring the
cavitation strength of water. The technique is based on counting cavitation
events as a function of cavitation number on a standard body, thereby
producing cavitation characteristic curves. It is assumed that water
cavitates because of nuclei carried in the water and that the measured
characteristic curves must therefore be related to the nuclei which are
In this research it was hypothesized that the nuclei, whatever their
real nature, could be represented by a distribution of equivalent gas
bubbles of neutral density. The standard bodies were designed according
to potential flow theory so that the bubble trajectories, along with their
cavitation rates, could be calculated. By calculating cavitation characteristic curves
for various bubble numbers and size distributions and comparing
the calculated curves with measured curves for the same body, it was possible
to infer the specifications for the equivalent bubble nuclei that were present
in any test. The equivalent nuclei can be described using N, the
number of cavitatable bubbles per unit volume of water (a number far smaller
than the total number of nuclei per unit volume); rmax, the likeiy radius
of the largest bubbles in the distribution; and a, a parameter of the exponential
size distribution assumed for the bubbles.
As a consequenoe, given (N,rmax,a) and an experimental realization
whose flow field can be calculated by potential flow theory, it should be
possible to predict incipient cavitation. The experiment devised for
verifying this statement proved to be faulty, and experimental verification
has not yet been accomplished.
An alternative for future study has been proposed which calls for
abandoning the hypothesis of equivalent bubble nuclei and determining the
relative cavitation strength of water by direct comparison of a measured
cavitation characteristic curve obtained on a standard body in a specific
test configuration with a catalogue of such curves kept on file for a
family of standard bodies.
Silberman, Edward; Schiebe, Frank; Mrosla, Edward.
The Use of Standard Bodies to Measure the Cavitation Strength of Water.
St. Anthony Falls Hydraulic Laboratory.
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