Cavitation and ultrasonics
Cavitation is formation of vapor bubbles of a following liquid in a region where the pressure of liquid falls below its vapor pressure. Cavitation is usually of two types namely inertial (or transient) cavitation and non inertial cavitation. Inertial cavitation is the process where a bubble in a liquid rapidly collapses and produces a shock wave. Such cavitation often occurs in control valves, pumps, propellers and in the vascular tissues of plants. Non internal cavitation is the process in which a bubble in a fluid is forced to oscillate in shape or size due to some form of energy input such as acoustic field using ultrasonic waves. Such kind of non inertial cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps and propellers etc.
Since shock waves formed by cavitation are strong enough to damage the moving parts, cavitation is an undesirable phenomenon in many applications of industry. It is specifically avoided in the design of machines such as turbines and propellers and eliminating cavitation is a major field in the study of fluid dynamics.