Shock Wave Meaning

In Physics, a shock wave is also known as shock waves. It is a strong pressure wave in an elastic medium such as air, water, or any solid material ejected from explosions or lightning, or other phenomena that create variations in pressure.


It is a type of disturbance that propagates at a speed greater than the speed of sound in the medium. 


As a rule, like ordinary waves, shock waves carry energy and can propagate through a medium. 


Above all, we characterize a shock wave by a sudden, nearly discontinuous, change in pressure, temperature, and density of the medium. 


On this page, we will understand the shock wave in detail.


What is Shock Wave?

When an aeroplane travels at a speed less than that of sound, in the first place, the air stays ahead of it.


In the meantime,  the air begins to flow out of the way, before the plane approaches it. The pressure waves that aeroplane creates past the air, eventually end up being smooth and gradual.


However, when an aeroplane reaches the speed of sound and catches up to its own pressure waves, the air ahead of it receives no signal of the plane’s approach. 


The aeroplane flows through the air, creating a strong pressure wave known as a shock wave. 

In this case, when air flows through the shock wave, its pressure, density, and temperature increase sharply and quickly.


Shock Wave Example 

A medium carries various characteristics with itself, such as stress, density, and temperature. 

When a supersonic aircraft, lightning, or explosions expel a strong pressure wave or a shock wave in an elastic medium, an intense change in pressure occurs inside the medium.


Here, the strong wave pressure from a supersonic aircraft is a cone comprising spherical wavefronts. 


Also, the term “supersonic,”  itself says that it expels the waves having much more speed than the speed of sound in the air. 


The below image describes the spherical wave fronts:

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Shock and Wave

Do you know how a shock wave differs from a sound wave? Well!  Shock waves differ from sound waves in the following ways:


In a shock wave, compression takes place in a region of abrupt and violent change in stress, density, and temperature of a medium it propagates/travels. 


As a result, shock waves propagate in a manner varying from that of ordinary acoustic waves. 


Particularly, shock waves travel faster than sound, and their speed increases consequently with the increase in the amplitude.


However, the intensity of a shock wave also decreases more quickly than that of a sound wave.


Further, some of the energy of the shock wave is dissipated by heating the medium through which it travels. 


The air explosion that creates a strong shock wave has a property known as amplitude. This property decreases conversely by the square of the distance until the wave becomes low to obey the laws of acoustic waves. 


Shock waves can reshape the electrical, mechanical, and thermal properties of materials.

Thus, we can use shock waves to study the equation of states, such as the relation of pressure, temperature, and volume of any material.


Characteristics of Shock Waves

Comparing supersonic flows, we may achieve an increase in an expansion through an expansion fan. For instance, the known expansion fan is a Prandtl-Meyer expansion fan. 


Coupled with, expansion wave may approach, collide and lastly recombine with the shock wave, creating a process of destructive interference. 

The sonic boom associates with the passage of a supersonic aircraft, which is a type of sound wave produced as a result of constructive interference.


When a shock wave passes through matter, energy preserves, however, entropy increases. This change in the matter's properties, as a rule, manifests itself as a decrease in energy. However, we can extract work from this energy, also, as a drag force on supersonic objects.


Therefore, shock waves are strongly irreversible processes.


Additionally, we have a few types of shock waves, let’s understand these:


Types of Shock Waves

  • Concussive Wave

A concussive wave is a blast of energy given off by an exploding object. 

Further, the explosion compresses the air immediately surrounding the site of detonation. As a result, the compressed air scatters in all directions away from the source - growing weaker the further, the wave travels. 

Incidentally, a person close to the source of detonation can get affected by the blast wave.

  • Oblique Shockwave 

For analyzing shock waves, we attach them to the body. After that, we find that the shock waves deviate at an arbitrary angle from the flow direction.

Meanwhile, we call this shockwave the oblique shock wave.

As a matter of fact, these shock waves require a component vector analysis of the flow.

However, doing so permits the flow treatment in a direction orthogonal to the oblique shock as a normal shock.

  • Mach Wave

A mach wave is a pressure wave in fluid dynamics. It travels with the speed of sound by a slight change of pressure added to a compressible flow.

FAQs (Frequently Asked Questions)

Q1: What is a Moving Shock Wave?

Ans: A  moving shock wave, as a rule,  comprises a shock wave propagating into a stationary medium.


In this case, the gas ahead of the shock is static (in the laboratory frame) and the gas behind the shock can be supersonic in the laboratory frame. 


The shock propagates with a wavefront that is orthogonal to the direction of flow. 


Here, the speed of the shock is a function of the original pressure ratio between the two bodies of gas.


Moving shocks productions are the results of the interaction of two bodies of gas at varying pressure, with a shock wave travelling at a lower pressure gas and an expansion wave travelling into the higher pressure gas.


For instance, Balloon bursting, Shock tube, the shock wave from the explosion are all examples of a moving shock wave.

Q2: Describe Shock Waves in Astrophysics.

Ans: Astrophysical environments use several shock waves. Familiar examples are supernovae shock waves or blast waves propagating via an interstellar medium.


Moreover, the bow shock is caused by the Earth's magnetic field, when colliding with the solar wind.


Also, shock waves are the result of collisions between galaxies. 


Quasi-steady reverse shock/termination shock is an interesting type of shock in astrophysics that terminates the ultra-relativistic wind from young pulsars.

Q3: Explain Shock Waves in a Pipeline Flow.

Ans: A pipeline flow shock appears aftermath deceleration of supersonic flow in a pipe.


Examples:

In supersonic propulsion: ramjet, scramjet, unstart.

Inflow control: needle valve, choked venturi.


In this case, the gas to the front of the shock is supersonic, and the back of the shock system can either be supersonic/oblique or subsonic/normal. Above all, the shock is the result of the deceleration of the gas by a converging duct, or as a result of the growth of the boundary layer on the wall of a parallel duct.