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.
Ultrasonic waves can be detected by various methods as listed below:
(a)Using radiometer: Ultrasonic waves can be detected using Radiometer. In this method ultrasonic beam is made to fall on a thin mica fan suspended by a thin wire carrying a small mirror from one end of a light rod. Due to pressure exerted by ultrasonic waves the fan gets detected along with the mirror. The deflection can be noted by a lamp and scale arrangement. A beam of light is made incident on the mirror and reflected beam falls back on the origin of scale attached to lamp. When mirror shows deflection by angle 0, then reflected beam on the scale defects by angle 20. Since 20 can be noted from scale, hence deflection of mirror can be found. The deflection is directly proportional to the intensity of ultrasonic waves. Hence we can calculate the intensity of ultrasonic waves with this method. Continue reading “Detection of ultrasonic waves”
A magnetostriction generator can produce ultrasonic waves of comparatively low frequency. For generating high frequency ultrasonic waves, piezoelectric generator is used.
Piezoelectric Generator. It is found that when pressure or compression is applied on two opposite faces of a quartz crystal, then charges are produced on a set of opposite faces which are perpendicular to the faces at which pressure is applied. The magnitude of charge developed is proportional to the amount of pressure applied. Charge produced on one face is positive and on other face is negative. Further more, if instead of compression, the faces of crystal are subjected to some tension, then nature of charges developed also gets reversed. “The process of appearance of charges on transverse faces of certain crystals when subjected to external stress is called piezoelectric effect.” Continue reading “Production of ultrasonic waves using piezoelectric generator”
When magnetic field is applied across the length of a ferromagnetic rod such as Nickel, then change in the length of rod is observed. This is called Magnetostriction. The charge in length is propotional to the strength of megnetic field. The increase in length is very small in practice. Amongst all ferromagnetic substances, increase in length is maximum for Nickel. Continue reading “Production of ultrasonic waves by magnetostriction method”
Ultrasonics can be produced by means of (i) Galton’s Whistle (ii) Magnetostriction Generator (ii) Piezoelectric Generator or Oscillator.
Galton’s Whistle. It contains of essentially a short cylindrical pipe blown in the form of an annular nozzle. The distance of nozzle from the edge of pipe can be varied by turning a micrometer screw. By suitable adjustment of this distance and the pressure of air blast, the pipe is set into resonant viberation at a frequency depending on length and diameter of pipe. The Galton’s Whistle method can be used to produce ultrasonic waves of low frequency upto 100kHz.
I will discuss the other methods in next articles.
Definition of ultrasonic waves:
Ultrasonic waves are acoustic waves whose frequency is more than 20kHz .They travel with the speed of sound. Hence their wave length is smaller than 333200cms-1/ 20000Hz = 1.66 cm
(ג = v/υ) . These waves possess a number of properties of sound waves and exhibit some new phenomena also.
Continue reading “4 types of ultrasonic waves”