Every time you drive a boat, turn on a pump, or possibly even start your faucet, tiny and destructive underwater explosions occur. These explosions are rapidly-generated steam bubbles caused 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 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. This is why pressure cookers make cooking more efficient. Once we grasp this concept, it's easier to see why cavitation happens.
Whether you know it or not, you have probably seen cavitation. Those bubbles that float to the surface behind moving boats are actually traces of cavitation. When a propeller spins fast on a boat, the motion 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 causes 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 desperately avoided. Explosions—especially underwater steam explosions—are mesmerizing to observers, but for a propeller, they're incredibly destructive. When cavitation happens on or near the propeller of a boat—or even inside pipes and pumps—the process chips away at the surface layer of the metal.
It feels counterintuitive to imagine small implosions in water damaging and destroying metal, but with a mental grasp of the relative energy of the implosion, and the relative damage it causes—everything begins to fall in place. When cavitated bubbles implode near metal, it causes high-frequency vibrations on the surface of the metal, which chip away at the surface. Eventually, after consistent 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 about the USS Pennsylvania—an American submarine—is a perfect example:
Where does cavitation occur?
Cavitation happens wherever water undergoes a rapid change in velocity, causing significant long-term damage. This shortens the lifespan of impeller pumps, which makes their design a tricky proposition. If the fluid pressure at the inlet is lower than the vapor pressure as water is sucked up into the pumps, 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 occurs in pipes and valves. If we switch on a faucet or open a valve too fast, cavitation might happen in the transition area of the mediating fluid. This will damage the pipe or valve over time, creating the possibility of eventual catastrophic failure.
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 engineers around the world.