|Frame from a video on the referenced article. Note the|
cloudy region in the bottom of the liquid near the blades.
This is caused by minute bubbles created by cavitation at
the tips of the blades. The brown layer on top of the water
Cavitation occurs when the pressure in a liquid drops below the equilibrium vapor pressure. It is a common occurrence in industrial settings, such as around the tips of rotating blades. Pressure drops occur in a variety of settings ranging from the flow of rivers around and over objects to flow in nozzles, to the wiggling of the tails and fins of swimming animals, and cavitation is a possibility in any of these settings. A bubble formed by cavitation is unstable. When such bubbles collapse, often asymmetrically, a tiny jet is formed. When these jets impinge on either the opposite sides of the bubble walls or on an adjacent surface, very high pressures, thousands of times atmospheric pressure, can occur. These jets erode the adjacent surfaces, causing structural damage. Cavitation around the tips of dolphin or tuna tails may limit the speed with which these animals can swim! Cavitation is also a major problem at spillways from dams, and played a role in the near-failure of the bypass tubes during a major flood crisis at Glen Canyon Dam in 1983.