Every time you drive a boat, turn on a pump, or even turn on your faucet, tiny destructive underwater explosions occur. These explosions are vapor bubbles generated rapidly by boiling water, in a process called cavitation.
The basics of cavitation
The basics of cavitation are based on Bernoulli’s principle. It states that the pressure of a fluid decreases as the velocity increases and vice versa. In combination with this idea, we must remember that the boiling point of water changes depending on the pressure it is subjected to. That’s why pressure cookers make cooking more efficient. Once we understand this concept, it is easier to see why cavitation occurs.
Whether you know it or not, you’ve probably seen cavitation before. These bubbles floating on the surface behind moving boats are actually traces of cavitation. When a propeller spins rapidly on a boat, the motion increases the localized velocity of the surrounding water. This creates a low pressure area on the aft side of the propeller.
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This low pressure causes the water to boil at room temperature of the fluid, so small bubbles of vapor are created due to the boiling. These steam bubbles then implode due to the surrounding water pressure and then form again due to the transferred energy from the implosion. This principle can be seen in the following GIF of a ball entering water.
For marine engineers, cavitation is hopelessly avoided. Explosions, especially underwater steam explosions, fascinate observers, but for a propeller they are incredibly destructive. When cavitation occurs on or near a boat’s propeller, or even inside pipes and pumps, the process breaks down on the surface layer of the metal.
It seems counter-intuitive to imagine small implosions in the water damaging and destroying the metal, but with a mental understanding of the relative energy of the implosion and the relative damage it causes, everything begins to fall into place. When cavity bubbles implode near the metal, it causes high frequency vibrations on the surface of the metal, which crumbles on the surface. Eventually, after constant cavitation, entire layers and sections of the metal separate, leading to catastrophic failure.
However, modern engineers mitigate cavitation through innovative propeller design. In fact, the following video on the USS Pennsylvania, an American submarine, is a perfect example:
Where does cavitation occur?
Cavitation occurs wherever water experiences a rapid change in velocity, causing significant long-term damage. This shortens the life of impeller pumps, which makes their design tricky. If the inlet fluid pressure is lower than the vapor pressure when water is drawn into the pumps, the extra energy from the impeller can trigger cavitation. This cavitation then continues into the fluid around the impeller, significantly damaging the internal mechanics.
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Cavitation occurs in pipes and valves. If we open a faucet or open a valve too quickly, cavitation can occur in the mediator fluid transition zone. This will damage the pipe or valve over time, creating the possibility of a possible catastrophic failure.
Cavitation is just one example of the extremely powerful and destructive effects of the physics around us. For those who don’t know, it’s a big, unseen danger to engineers around the world.