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Difference between blast, shock, pressure and sound waves?

by autodidude
Tags: blast, difference, pressure, shock, sound, waves
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autodidude
#1
Nov8-12, 09:10 PM
P: 333
What are the differences (I'm really trying to understand the differences in the context of an explosion). When you hear it, are you hearing all 4 waves? Which waves cause the damage and why - why not the others?

Also, when NASA launches the space shuttle, they release water to absorb the 'acoustical energy', these are soundwaves?

EDIT: I read somewhere that a sound wave turns into a pressure wave if exceeds some dB - what happens to it here?
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Hetware
#2
Nov8-12, 09:35 PM
P: 125
One thing that distinguishes a shock wave from a normal sound wave is that it travels faster than what we typically call sound. This is because its intensity is such that it significantly heats the gas media adiabatically. The speed of "sound" is an increasing function of temperature. The wavefront is pushing into undisturbed air, so the excess velocity is not due to the immediate effect of the pressure at the front. What happens is that any other pressure waves behind the front in the region of significantly excess pressure propagate faster than the front, and therefore catch up with the front and help it push that much harder.
Hetware
#3
Nov8-12, 11:09 PM
P: 125
See also "Partin Brisance".

boneh3ad
#4
Nov9-12, 12:25 AM
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Difference between blast, shock, pressure and sound waves?

They are all essentially the same phenomena with different amplitudes... in a sense.

A pressure wave is just a general term for a pressure disturbance moving through a medium. This happens to be exactly what a sound wave is. These disturbances move at the speed of sound in the medium in which they are traveling. There is no formal distinction between the two, as any amplitude of pressure wave could be heard as sound provided the listening device was sensitive enough.

A blast wave would be a pressure wave created by an explosive blast. Depending on the nature of the explosion, it may or may not be a shock wave.

A shock wave is a specific type of pressure disturbance moving through a fluid (or stationary in the fluid depending on the situation. Consider a pressure wave (sound wave) moving through a fluid. For small amplitudes, the wave passes by and the medium more or less returns to its initial state. However, if the amplitude is large enough, it will drag a little bit of the air along with it. That means any sound waves propagating behind it will tend to catch up with the original wave and drag the fluid behind them still faster. That process stacks up and eventually you can have a number of pressure waves coalesce into a shock wave.

Shocks waves are near discontinuous changes in pressure, velocity, temperature and density in the fluid (changes occur over 6 or so mean free paths between molecules). To the stationary observer, they can appear to be moving faster than the speed of sound, though they will always be traveling below the speed of sound in the medium out of which they are propagating, even if they are traveling faster the speed of sound in the medium into which they are propagating.

When an explosion occurs such that a shock is generated, a few things happen when the shock passes over you. First, there is a sharp rise in pressure, temperature and density, so that pressure spike itself can do a lot of damage. However, at least as important is the flow behind the shock. When the shock passes, it is dragging an awful lot of fluid with it. In fact, the flow being dragged behind the shock can even be supersonic if the shock is strong enough! The potentially high velocities combined with high density behind the shock means that a lot of the real damage correlates with the incredibly high dynamic pressure behind a passing shock wave.

As a quick example, if you were to explode a bomb at ground level such that the shock was moving at Mach 1.8, the simple shock wave that passes by would cause the pressure to spike to about 3.6 times the ambient pressure. Additionally, say the ambient pressure was 0.3 atm and the ambient temperature was 220 K. You would have over 0.5 atm of dynamic pressure behind that passing shock as well due to an induced velocity of just shy of 310 m/s. That is where a lot of the damage comes from.

However, consider a second case. A bomb is usually detonated in the air such that the shock is coming down at the ground at an angle. That results in not only the original shock, but a reflected shock, which effectively compresses the flow twice. The pressure would increase almost 10 times over ambient! The induced velocity doesn't change much, but because of the increased pressure and density, the dynamic pressure would now be roughly twice as high as the single shock case. That is why bombs are usually detonated in the air and not at ground level.

That probably went a lot farther in-depth than you wanted. Oops.
autodidude
#5
Nov12-12, 08:00 PM
P: 333
Thanks very much guys!

@Hetware: Are you referring to the Oklahoma City lectures on Youtube? I'm gonna watch that now

@boneh3ad: No way - the more details the better!

By the way, do you generally get to learn this in first year physics (if not, what subjects would it usually be taught under?). I have a 'Physics for Scientists and Engineers' text and it doesn't really go into this sort of stuff
boneh3ad
#6
Nov12-12, 09:15 PM
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Shock waves and pressure waves and the like were originally typically part of physics curricula, but they were phased out years ago to save time for things like quantum mechanics and relativity. These sorts of things are primarily fluid mechanic phenomena and fall under various engineering disciplines most of the time these days, most notably aerospace and mechanical engineering. It would be given quite a bit of treatment in any physics department doing acoustics research as well.
Vadar2012
#7
Nov12-12, 09:29 PM
P: 208
Yeah I did them for my mechanical and aerospace engineering degree. My Undergrad and Masters theses were pretty much simulating shock waves at around mach 15-20. That's funny bonehead, your answers are usually vague or extremely detailed... that answer in particular is awesome. I was going to write a couple of sentences.
boneh3ad
#8
Nov12-12, 09:33 PM
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Quote Quote by Vadar2012 View Post
That's funny bonehead, your answers are usually vague or extremely detailed... that answer in particular is awesome. I was going to write a couple of sentences.
It all depends on my mood and how swamped I am with real work. Generally the more swamped I am, the more detailed my answer since (A) I am already in work mode and (B) I'll take any excuse possible to procrastinate. Take right now, for example, when I am supposed to be writing up some stuff for a review session for my class tomorrow.


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