What is Cavitation? (with AvE)

If you subject a fluid to a sudden change in pressure, some interesting things can happen. You can cause tremendous damage to moving parts, but you can also harness that destructive power for many beneficial uses from mantis shrimp killing their prey to ultrasonic cleaning. So many things rely on this fluid phenomenon.

You may have heard of this principle which basically says that all energy in a fluid can take one of three forms kinetic potential or internal energy. And the total amount of energy is the same along a streamline. So if you change one, for example, if you increase the kinetic energy of a fluid by speeding it up, you have to accommodate. In this example, the fluids pressure goes down. Being able to lower the pressure of the fluid or create a vacuum just by constricting the flow area makes a venturi a very useful tool that can be found in all kinds of devices, from engines to trombones to scuba diving regulators.

It didn’t or at least I have to assume it didn’t, because 10 months later I got this in the mail instead of giving me the hard truth that my sketch was poorly considered and I wasted his weekend, he gave me something even better, a care package, including a clear acrylic liquid flow meter that was designed by someone who knew what they were doing. And if you look closely at this flow meter, you might recognize the shape as a venturi, which is perfect because I need a venturi to show you this fluid phenomenon.

Here’s myself, I am my garden hose running into the garage kind of pressure goosebump feeding a manifold that connects to a pressure tank, a pressure gauge, and this flow meter, I modified the meter. So it acts like a venturi by gluing the weight to the center post so it can’t slide up and down. And I have a differential pressure gauge to measure the pressure drop across the country. The drop in pressure is the whole purpose of this demonstration. To understand why, we need to look at the phase diagram of water. We know that water changes state based on temperature. It’s a solid one. It’s cold, a.k.a. ice, a liquid at room temperature and a gas when it’s hot, a.k.a. steam. But the face of any substance also depends on the ambient pressure. You can see that even at room temperature, water can turn to steam at very low pressures. This is true for a lot of substances. If I force this water through a small enough opening in the Venturi, according to Banally, I’m decreasing the pressure and converting it to kinetic energy. And if I get the flow going extremely fast, I can decrease the pressure below the vapor pressure of the water, converting it to Steve. Steam on its own isn’t that big of a problem, but the issue comes on the other side of the Venturi, when the pressure goes back up above the vapor pressure and the steam collapses back into water on a larger scale, this collapse can lead to thermal shock. Check out my video on Steam Hammer to learn more. But on a smaller scale, collapsing steam bubbles are called cavitation. And even though the scale is smaller, the damage cavitation can cause can be just as destructive.

This is because collapsing steam causes water to accelerate and decelerate violently. Water is compressible, so it slams into itself, creating a shockwave. It’s like a thousand tiny water hammers in many cases where cavitation is occurring. You can even hear the shockwave which sound like gravel moving through a pipe. If I build up enough pressure in this tank and open the valve to the vent, you can clearly see and hear the cavitation occurring. I can’t measure the pressure at the constriction of inventory, which will be a very strong vacuum, but this gauge measures the total loss and pressure caused by the turbulence and cavitation just for reference and because it looks cool. Needless to say, in many cases, cavitation could be bad news. It can erode pipes, impeller and other moving parts leading to accelerated wear or even catastrophic failure. So engineers generally avoid designs that might suggest the liquids to sudden changes in pressure. Pipes get smooth bends rather than abrupt changes in size or direction. Both propellers and pump and peddlers are carefully designed to match with the speed and power of the motor to which they’re attached. And dam spillways are designed to avoid any protrusions into the high velocity flow. However, although it’s generally avoided in all kinds of industries, cavitation can be a force for good. Ultrasonic cleaners use cavitation to agitate a solvent and break the strong bonds between a contaminant and apart. Some industries use cavitation to mix compounds that are difficult to combine like paints.

Finally, some shrimp can move so quickly they create a cavitation bubble to kill their prey. As for this flow meter, it seems to be holding up fairly well. So far, the acrylic seems to be able to absorb the shock waves better than Meadowood, so it’s probably best that our collaboration worked out the way it did.

Leave a comment

Your email address will not be published. Required fields are marked *