Cavitation and what it does

If you have never heard of the term’ cavitation’ then you may think that it is something to do with visiting the dentist for fillings in your teeth.  But you would be wrong.  The term Cavitation first appeared in the late 19th century when Lord Rayleigh carried out studies and investigated the breakdown of a round void with placed within a liquid environment.

So what actually happens in the process of cavitation?  Very simply, vapour cavities or holes are formed within a liquid.  These can be described as bubbles and they are often created when a fluid has rapid changes or pressure applied to it.  The cavities or bubbles form when the pressure is fairly low.  If the pressure is then increased, these bubbles will collapse creating a strong shockwave.

But where would you find this process of cavitation taking place in the real world?  Often this can take place in certain engineering scenarios.  When these voids break down and implode next to a metal surface they can create stress and damage.  If this action repeats itself then metal fatigue can occur, often also referred to as ‘cavitation’.  The types of sites where you may find this in action are within pump impellers or any place that has liquid contained within twists and curves.  There are two kinds of cavitation, transient and non-transient (sometimes referred to as inertial).

The differences between the two are this:

Transient cavitation happens when a bubble (void) breaks down quickly and sends out a shock wave.  This can happen naturally within nature as well as in engineering environments.

Non-transient cavitation can be seen when a bubble contained within a liquid is made to change in size and/or shape due to energy being inputted, thus creating intentional shock waves.  This process can be used in industry to do such things as ultrasonic cleaning and is also found within most propellers and pumps.  Because these shock waves can be pretty strong they can cause damage to moving parts if not kept under control, so non-transient cavitation that is not created on purpose is inconvenient and definitely something that needs to be kept in check.

The only time when this is not damaging is when the bubbles break down at a distance away from the machinery.  This is called super cavitation and is not as problematic.

So as can be seen, cavitation is a very useful process when controlled but can cause plenty of damage if not kept in check or identified.



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