Hydrodynamic cavitation, hydraulic and ocean engineering

As Acosta and Parkin emphasize in a past review of this 

For instance, appreciable tensile strength had occasionally been noted in water tunnels (ref. 16). (The tensile strength of a liquid is defined here as the minimum tensile stress in the liquid at which it ruptures or cavitates (ref. 57).) The inception of cavitation in these water tunnels has occurred at higher stress levels than ordinarily expected. Higher flow velocities or lower tunnel pressures than normal have been needed to produce cavitation about a test body. The tensile strength acts as if an additional static head were present in the system. In this case, appreciable tensile strength is undesirable in order to make for a uniformity of test results, and duplicate prototype conditions (ref. 16). 

The detailed work of Bernd (ref. 15-17) and other investigators has also shown that the tensile strength of water is set by the gas nuclei (i.e., microbubbles) present in the water. (Accordingly, the earlier-mentioned definition of the tensile strength of a liquid can be restated as the minimum tensile stress at which the gas nuclei in the liquid start to explode . This property is also often referred to as the cavitation susceptibility (ref. 57).) Using specially constructed sonar transducers, the behavior of gas nuclei was followed by Bernd by measuring tensile strength. Surface 

in practical flow situations the water is not pure gas bubbles and small impurities are embedded within the liquid. Small gas bubbles can stay in suspension for a long time, because the relative motion in an upward direction due to gravity is opposed by transport in the downwards direction by turbulent diffusion (ref. 57). These microbubbles are initially trapped in the liquid mostly by jet entrainment, cavitation, and/or strong turbulence at a gas/liquid (usually air/water) interface (ref. 58). 

During the last four decades, measurements of weak nuclei (i.e., gas nuclei) in liquids have become especially important in view of their influence on cavitation inception (e.g., ref. 57-59). Understandably, the gas nuclei concentration is closely coupled with the free gas content of the liquid (ref. 58, 60). (A distinction is made commonly in the engineering literature between free gas content and dissolved gas content. The free gas content is that portion of gas which has the normal physical properties of bulk gas. In practical situations, the free gas concentration within the liquid is usually several orders of magnitude lower than the dissolved gas concentration (ref. 58).) Many investigators have developed instruments to detect this free gas content (ref. 58,60) and the freestream gas nuclei concentration associated with it (e.g., ref. 58-60). 

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