Every time you drive a boat, turn on a pump, possibly even start your faucet, tiny and destructive underwater explosions occur. These explosions are rapidly-generated steam bubbles caused from boiling water in a process called 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 visa-versa. In combination with this idea, we have to remember that the boiling point of water changes based on the pressure it is under – why pressure cookers make cooking more efficient. When we grasp these concepts, we can begin to understand why cavitation occurs.
Whether you realize it or not, you have probably seen cavitation for yourself. Those bubbles that float to the surface behind moving boats are actually results of cavitation. For boats specifically, as a propeller spins fast, it increases the localized speed of the water around it. This creates a low-pressure zone on the trailing side of the propeller. This low pressure cause the water to boil at ambient temperatures of the fluid, so small steam bubbles are created due to boiling. These steam bubbles then implode due to the surrounding pressure of the water, then form again due to the energy transferred from the implosion. This principle can be seen in the following GIF of a bullet entering the water.
For marine engineers, cavitation is to be direly avoided. These underwater steam explosions may be fascinating to an observer, but to a propeller, they are incredibly destructive. When cavitation occurs on or near the propeller of a boat, or inside pipes or pumps for that matter, it chips away at the surface layer of the metal.
It may be hard to understand how exactly small implosions in water can destroy metal, but you need to understand both the relative energy of the implosion and the relative damage it causes. When a cavitated bubble implodes near metal, it causes high-frequency vibrations on the surface of the metal which begin chipping the surface. Eventually, after consistent cavitation occurs, layers of the metal are slowly chipped away, possibly leading to catastrophic failure.
Cavitation can be mitigated in propeller design. One example of such can be seen in the following video about the USS Pennsylvania.
Anywhere that water undergoes a rapid change in velocity, cavitation can occur and cause significant damage over time. The effect creates problems as to the longevity of impeller pump designs. As water is sucked up into pumps, if the fluid pressure at the inlet is lower than the vapor pressure, then added energy from the impeller can start cavitation. This cavitation then continues in the fluid around the impeller, significantly damaging the internal mechanics.
Cavitation again occurs in pipes and valves. If you were to switch on a faucet or open a valve too fast, cavitation can occur in the transition area of the fluid involved. Like mentioned before, this cavitation will damage the pipe or valve over time, creating the possibility of catastrophic failure down the line.
Cavitation is only one example of the extremely powerful and destructive effects of physics around us. To those unaware, it is a great unseen danger to engineering around the world.