Here we have a lovely example of supercavitation. Regular cavitation occurs when the pressure of a fluid drops below vapour pressure in the presence of a weakness (termed nucleus) in the water. This forms a vapour cavity at the location of the nucleus. When the cavity collapses, it is a highly energetic, chaotic, and violent process. However, when the cavity becomes so large that it engulfs the whole body (this is termed supercavitation), the flow is much more quiet. Here is a photograph of supercavitation about a sphere. The cavitation number (difference between the pressure and the vapour pressure as a ratio of the dynamic pressure) in this case is 0.3, and the Reynolds number is 1.5 million (for the lay person, that means the fluid is water at room temperature, going past a 150mm sphere at 10.5 meters per second).

Supercavitation about a sphere.
Supercavitation about a sphere.

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