Shock Waves: Cause of Sonic Boom

Hello Steemians, how you doing? Hope All is Well. Today, I want to share my knowledge about "Shock Waves" though this post.

Before the development of means of transportation, human beings used natural locomotive organ i.e. foot for locomotion. Then, Human beings used horses, donkeys for quick and comfortable transportation. Human beings always wanted faster means of transportation. Car, ships, rail, airplane etc. were made in this course. But, one of the major milestone was to conquer the speed of sound. This in fact was due to increased turbulence due to speed.

The aircraft moving with speed greater than the speed of sound creates a wave called shock wave. Due to this, thunder like noise can be heard. It is called sonic boom. It can be fatal to human and animals.

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Consider speed of source of sound be 'v' and local speed of sound be 'a'. There is important non dimensional aerodynamic parameter to relate these speeds, it is called Mach Number(Ma) and it is given by;

Ma = v/a

if Ma < 1; Flight is subsonic

if Ma >1; Flight is supersonic

Condition 1: When source is not moving(Ma=0)

When the source is not moving i.e v=0; the concentric spherical wave fronts are formed.

Condition 2: When source is moving with speed less than speed of sound(Ma<1)

When the source is moving with speed less than speed of sound i.e. v < a; The wave fronts in front of source are concentrated and hence, the frequency increases. This effect is called Doppler effect. Example: The frequency of horn of car increases while approaching towards you.

Condition 3: When source is moving with speed equal speed of sound(Ma=1)

When source is moving with speed equal speed of sound i.e v =a; there is strong concentration of pressure wave in the point shown( sound is pressure wave after all). There is high pressure in the point. So, the structure should be such that it should safely bear that pressure.

Condition 4: When source is moving with speed greater speed of sound(Ma>1)

When source is moving with speed greater speed of sound i.e v > a; the one dot in above condition changes to cone. The crest of wave meet at this cone. Faster the speed of aircraft, sharper the point of cone. This very wave is called shock wave.

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The angle of mach cone called mach angle can be found out using simple trigonometry.

In the figure,

When source travels from point A to point D, the wave travels from A to point E, let time taken be t. Mach angle is the angle made by mach line with the direction of source and denoted by µ.

The mach angle is crucial as the supersonic flow and disturbances are limited to mach angle.

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Sources

1. Introduction to flight

2. Supersonic Flow and Shock Waves

3. ocw.mit.edu

4. Glenn Research Center, NASA