Talk:Shock wave/Archive/1

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[edit] Questions about Sonic Booms

When the Shuttle takes of, why don't people on the ground hear a sonic boom? Is it the sound wave is radiated away from the ground toward space? If so, I would think some of the sound waves would be reflected back towards earth and something would be heard. Or is a boom heard but the Shuttle engines are too loud and the boom is drowned out?

When a vehicle goes supersonic, do the people in the vehicle hear the boom? Or is it now that they are traveling faster then the sped of sound the sound wave never catches up to them so nothing is heard?

When a vehicle goes subsonic, does it create a sonic boom? If so, is the wave radiated backward or forward?

When the shuttle returns to earth, is the reason the sound is different, “the crack sound”, because it’s not really a sonic boom it’s a thunder clap? Is this because the air is so thin an air pressure wave is not created?

—The preceding unsigned comment was added by 70.90.163.246 (talk) 17:06, 11 December 2006 (UTC).

[edit] examples

An "everyday example" would be a balloon popping or something, not the A-Team.  :-) - Omegatron 17:51, May 13, 2004 (UTC)

Actually, I found that example clear, understandable, and relevant; a balloon shock wave has little actual effect, as opposed to the clearly visible shock wave in the A-Team. Meelar 17:53, 13 May 2004 (UTC)

This irked me too, it just seemed like a lame example. Replaced "is from the TV series The A-Team (or any other action series or movie). When handgrenades are thrown at the bad guys they are supposedly blown away, flying through the air, by the blast waves of the grenades." with an example almost everyone is familliar with: a sonic boom.--Deglr6328 01:49, 17 Jul 2004 (UTC)

"There are two basic types of shock waves: blast waves and driven waves."

Which one is an aircraft? - Omegatron 13:22, Jul 22, 2004 (UTC)

shocks around bodies moving through fluid tend (like aircraft) would probably be categorized as driven waves,

although that distinction (blast/driven) tends to refer to unsteady, one-dimensional configurations.

[edit] mach wave

i'm surprised the most common type of shockwave, the mach wave (sound wave), is not mentioned in the article. it is an infintely weak shock. 141.211.174.250 23:00, 8 Nov 2004 (UTC)

[edit] friction

"When meteors enter the earth's atmosphere, this phenomenon causes them to heat up and disintegrate; this is sometimes erroneously attributed to friction." I'm not sure I really understand the difference. If air molecule hit the survace and each other they heat up. If they hit each other in shock wave they heat up. Wouldn't both of these actions called friction? --Gbleem 00:32, 19 Nov 2004 (UTC)

(someone correct me if I go astray). Gbleem: Strictly speaking, you are correct, except the difference is that fluid friction is usually only understood to mean lateral transfer of mean momentum by collisions between atoms, whereas shock waves and other drastic processes tend to also have friction associated with other kinds of molecular motion besides translation (in particular, rotation, which is where bulk viscosity comes from). In a sense, shocks are dissipative, "friction"-driven processes. These processes are mostly identifiable within the shock thickness itself, which is only a few molecular mean free paths thick (tens of microns). The reason saying "friction heats up a re-entering meteor" is misleading is that frictional heating is mostly due to the shearing action between adjacent layers of fluid moving along a body (like in a boundary layer) and is caused by velocity gradients in shear layers of far greater thickness than molecular mean free path. However, a blunt body like a meteor has a shock wave standing off it. By the time any fluid reaches the meteor, it has already been slowed down tremendously by the shock wave and is already very hot. Viscous dissipation at the meteor's surface will be small compared to the heating already done by the shock. Further, shock waves, though physically speaking are accompanied by viscous processes within them, are caused by drastic changes in pressure and can actually be accurately described (macroscopically) by the Euler equations which govern inviscid fluid flow. It's just that to these equations (and to most observers) the shock appears to be an infinitely thin discontinuity.

[edit] soliton

I don't think so. Usual definition of soliton is a smooth self-reinforcing waveform; a shock wave is a discontinuity.

[edit] Bad article

I must say, I found this article really quite misleading and inaccurate.

The equations of fluid mechanics are non-linear, with wave-like solutions. In an ideal fluid, compression waves tend to collate and form discontinuities that are known as "shock waves", while expansion waves tend to disperse. Thus, even the first sentence of the article needs to be more specific in its definition; a shock wave is specifically a compression wave, as a non-linear expansion wave will not form a shock.

It can also be when the actual molecular or particle speed is moving faster than the wave propagation speed

This doesn't really describe a shock wave, more a possible cause of a shock wave. Even then, I'm not sure it makes sense.

In compressible fluids such as air, disturbances such as the pressure changes caused by a solid object moving through the medium will propagate through the fluid as pressure waves traveling at the speed of sound.

In general, the speed of a shock is a function of its strength and the compression waves caused by a subsonically moving disturbance will collate and form shocks just like any other compression waves. The speed of sound is the speed of infinitessimally small waves, but there is nothing magical about it. There is no strong distinction between linear and non-linear waves; all fluid waves are non-linear in their behaviour, but for very small ones, the non-linearity is often insignificant in the length- and time-scales of interest.

When the cause of the disturbance is moving slowly relative to the speed of sound, the pressure wave takes the form of conventional sound waves.

And yet, in the case of explosive shock waves, the disturbance is stationary. These waves move at a speed depending on their size. Again, there isn't anything different about sound waves, they are simply the limiting form of a shock or expansion wave as its strength approaches zero.

The pressure waves enable the fluid to redistribute itself to accommodate the disturbance, and the fluid behaves similarly to an incompressible fluid

This in general is not true, the fluid doesn't "behave like" an incompressible fluid. Low mach number flows can and do behave compressibly. Take acoustics, for example; Imagine a loadspeaker generating sound at 1KHz, with its diaphragm moving 1mm. The maximum disturbance speed is therefore 6.3m/s, which is way below the sound speed in air of 340m/s. Yet, it is still the compressible, wave-like behaviour of the fluid that is of interest. Note that 6.3m/s will also be a typical fluid velocity of the flow. If the flow is steady and low mach number, then the steady-state solution will look incompressible, but transient behaviour will still be wave-like. It's important to remember this when considering things like explosive flows, where the disturbance is stationary, and the flow can be relatively low mach number, but it's the fast transient that dominates the flow picture.

However, when a disturbance moves faster than the pressure waves it causes, fluid near the disturbance cannot react to it or "get out of the way" before it arrives. The properties of the fluid (density, pressure, temperature, velocity, etc.) thus change almost instantaneously as they adjust to the disturbance, creating thin disturbance waves called shock waves and shock heating.

Again, I really don't like this! A supersonic disturbance creates supersonic waves. If these are compression waves, then they collate to form discontinuous shocks, if they are expansion waves then they spread out (e.g. a Prandtl-Meyer expansion fan).

Shock waves are not sound waves...Over time a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy

I find this strong distinction between shock waves and sound waves very misleading. Also it should be mentioned that the shock strength tends to be reduced mostly by spreading.

I think this page could use some heavy editing!

Steve Thomas 16:48, 21 September 2005 (UTC)

I've just replaced all of the useless and incorrect information with a functional stub. Happy Editing! AKAF 14:34, 11 November 2005 (UTC)

[edit] Accessibility

As it is now this page is neigh on incomprehensible to people with little knowledge of phyisics. The language is way too technical. A basic intro is missing.

[edit] Reinstated material

Since user Robinh insists on reinstating material, and I have no particular interest in a reversion war, this section is a discussion of what material belongs in this article and what doesn't.

However, when a disturbance moves faster than the pressure waves it causes, fluid near the disturbance cannot react to it or "get out of the way" before it arrives. The properties of the fluid (density, pressure, temperature, velocity, etc.) thus change almost instantaneously as they adjust to the disturbance, creating thin disturbance waves called shock waves and shock heating.

My problem with this paragraph is that there is no such thing as a disturbance wave, and that there is certainly no "thin disturbance wave called shock heating". Additionally, since a shock wave is a particular kind of pressure wave, it can't be caused by a disturbance moving faster than the pressure wave it causes. I know what you're trying to say, but as formulated it is either impossible to understand or wrong. Take your pick.

Shock waves are not sound waves; a shock wave takes the form of a very thin membrane (sheet of energy) on the order of micro-meters in thickness. The pressure excursion within the shock wave is so extreme that it causes the speed of sound within the wave to change. Shock waves in air are heard as a loud "crack" or "snap" noise. Over time a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy. The sound wave is heard as the familiar "thud" or "thump" of a sonic boom, commonly created by the supersonic flight of aircraft.

My problem with this paragraph is that shock waves are not membranes, and are not sheets of energy in any literal sense, and I think that this explanation is just confusing. One speaks of pressure change, not "excursion", and although some models use a speed of sound change within the shock wave to model it, I am aware of no measurements of this phenomenon (if you have a reference, maybe I'll agree with this). The speed of sound changes behind the wave.

There are two types of shock waves: normal shocks and oblique shocks. A normal shock extends perpendicular to the flow of fluid, and the flow goes from supersonic upstream of the shock wave to subsonic downstream. An oblique shock is formed at an angle to the flow, and although the component of flow perpendicular to the oblique shock goes from supersonic to subsonic in crossing the wave, the tangent component of flow is not affected, so the net flow may remain supersonic downstream of an oblique shock wave.

My problem with this explaination is mainly that normal and oblique waves are not different. A normal shock is merely a special case where the oblique angle is 90 degrees. The explanation here is impossible to understand, and I would suggest either expanding it significantly or deleting it.

A cage around the engine reflects any shock waves. A spike behind the engine converts them into trust.

To generate lift a supersonic airplane has to produce at least two shock waves: One over-pressure downwards wave, and one under-pressure upwards wave. Whitcomb area rule states, we can reuse air displacement without generating additional shock waves. In this case the fuselage reuses some displacement of the wings.

(Images linked here)

These images make no sense here. They are not referenced or explained in any way. They appear to be copied from supersonic, where they are at least somewhat at home. It's my opinion that the quality of the first picture is so poor as to be useless, especially since it's just explaining Busemann's shockless internal flow concept. The area rule picture is taken from Whitcomb_area_rule and should be referenced as such rather than duplicating data.

Analogous phenomena are known outside fluid mechanics. For example, particles accelerated beyond the speed of light in a refractive medium (where the speed of light is less than that in a vacuum, such as water) create shock effects, a phenomenon known as Cerenkov radiation.

There are two basic types of shock waves: blast waves and driven waves. A blast wave is produced by explosive phenomena. Blast waves can travel out from their source at supersonic speeds. A driven wave is produced by a source that constantly ejects matter (for example, the solar wind). A driven wave can reach a static state where it bounds the wind.

When meteors enter the earth's atmosphere, this phenomenon causes them to heat up and disintegrate; this is sometimes erroneously attributed to friction.

Another example of a shock wave is the boundary of a magnetosphere. At the shock wave, particles from the solar wind will abruptly slow to subsonic speeds.

I suppose that the Cerenkov radiation example is relevant, although it's drawing a long bow. The description of the different kinds of shock waves is basically wrong, and I don't see that it's close enough to being a good description to be kept. The meteor example is wrong, you're thinking of the combination of stress by differential heating and vaporisation of gaseous internal components. The third example is ok.

I've linked this on your talk page, and I hope we can come to some resolution AKAF 13:37, 8 December 2005 (UTC)

[edit] M<1 behind an oblique shock?

Physically I'd expect this was possible (just view a normal shock with small sideways translation) and it's apparently easy to find such solutions on the nasa 'wedge' applet. Here (at a NACA/NASA webpage)

[1]

Could you tell me what data you've put in?, I don't see M2<1 for an oblique shock, but if you could tell me what upstream Mach number and turning angle you used?AKAF 06:55, 26 April 2006 (UTC)

For the gamma =1.4 M1=2 base case, a wedge angle of >22.65 gives a shock angle of 61.5 and M2=1.002. At 22.9 the flow is subsonic (M2=0.949) and the shock is still oblique (63.5); at 23.15 the shock stands normal and M2 is about 0.5. So it's a small range, but it doesn't look implausible :-)

Also note the words on the NASA web-page, after the maths but before the applet:-

'For the Mach number change across an oblique shock there are two possible solutions; one supersonic and one subsonic. In nature, the supersonic ("weak shock") solution occurs most often. However, under some conditions the "strong shock", subsonic solution is possible'.

and, at [2] essentially develops the point made earlier (only the normal component of flow goes from supersonic to subsonic) Linuxlad 07:58, 26 April 2006 (UTC)

I don't necessarily see the applicability of the strong shock approximation to converging pipe flow (I've noted it in the detached shock section-thanks). In this case wouldn't the effect of any wall convergence be equal to the farfield effect of a detached shock, which is the weak shock solution? I think (correct me if I'm wrong) that in the case where the strong shock is applicable to the general duct flow, that the duct must choke and push a normal shock upstream. I'm not sure whether the Mach number < 1 area can extend to be a significant area of the pipe? I'm not saying it's wrong (and I'll leave the comment there), but I'm not managing to get my head around it.AKAF 11:02, 26 April 2006 (UTC)

I'd not appreciated you were talking about nozzle flow only at this point. (I would need to get out a good book or two to pursue this further.) Linuxlad

[edit] Supersonic?

In my opinion shock waves can be created by supersonic (but not only) objects, but the created waves themselves travel with speed of sound. Take evergreen example of a jet plane. The plane creates shock waves, but at the time the shock wave arrives to your ears, the plane is gone far far away and you need some amount of experience to get a sight of it. Marcel

That's not a shockwave that's just ordinary soundwaves.

To get a shockwave something has to be travelling faster than sound. In that case some parts of the air around the aircraft are travelling at exactly the speed of sound *towards* the aircraft, so that the soundwaves leaving the aircraft pile up on each other, sort of like a tailback on a road, and a shockwave forms, the pressure goes up and up and up there, and then spreads out sideways. Because of this amplification effect, a shockwave is very intense, more like an explosion when you hear it (not coincidentally, since explosions create shockwaves.)WolfKeeper 13:18, 11 May 2006 (UTC)

I really like this clarification, so I copied it to the main page. Hope that's OK. AKAF 14:12, 11 May 2006 (UTC)

This is totally wrong! You have to understand that get shock wave you do not need to move above the speed of sound. In fact, any movement creates shock wave. The faster the movement the larger the shock. In every explanation of the speed of sound is originated by a very small movement. Check any compressible flow book that you like, you can use my here www.potto.org or you can use shapiro or Saad or any thermo book. No matter what book you choose the results should be the same. Now take for example piston moving with Mach number 0.1 what will be the velocity of shock ahead and according to you it will be below the speed of sound. Of course you are wrong! if you where right than

you never hear car approaching you. The shock will move in Mach=1.062 for =1.4. Think differently, if you were right than zero movement (almost zero creates sound wave) yet larger velocity creates smaller velocity. You were right it will violate the second law of thermo. --potto 04:10, 2 February 2007 (UTC)

[edit] Detonation wave

Some one has just added quite a good edit to Detonation setting out the classical ZND theory quite clearly. The first 2 paras of section 6 here are not really consistent with this. On the ZND model, detonation _is_ usually modelled as a shock-initiated combustion/heat release. Modelling the chemistry as being within the shock (ie within the zone where longitudinal heat and momentum transfer matter), (though it was assumed by the ancients, and is not invalid}, does not lead to the easiest way of physically understanding what's going on, especially with the CJ condition. Is there anything to be gained by not rewriting??? Linuxlad 09:47, 17 June 2006 (UTC)

There are, in fact, significant differences between shock induced combustion and detonation. Many one dimensional detonation models attempt to model some particular aspects of the detonation (ie wave speed, total heat release, total combustion efficiency), and are perfectly adequate for their purpose. However these models do not describe the difference between a shock wave and a detonation wave.

I think that it's important to have a fundamental description of a detonation wave in this article, since so many people are unclear about what a shock wave is. A detonation wave has some pretty fundamental differences to a shock wave, even when shock induced combustion (SIC) is present:

• The angle of an oblique detonation wave is different to that of a shock with SIC
• The total pressure loss over a detonation wave is different to that of a shock with SIC
• The combustion efficiency is different over a detonation wave than for a shock with SIC
• The wave speed is different for a detonation wave than for a shock with SIC

And all of these points occur due to the fact that the combustion is taking place in the shock layer. Amongst other things, this explains why the conditions downstream of the detonation wave affect it, when the shock is independant of the downstream conditions. Optimally we'd just send readers to detonation to learn about the difference, but I don't think that's practical (or that the difference is very well explained there). If you have a rewrite which clarifies these points, I think it belongs, but a description of how to calculate the bulk properties in the wake of a detonation probably doesn't belong here.AKAF 08:50, 19 June 2006 (UTC)

It's not of the essence that the energy release in a detonation is within the lead shock. The classical ZND model did not assume that, and it was both predictive and influential (eg in the modelling of steam explosions). I think you're being too influenced by the modelling of chemical explosions with their Arrhenius reaction rates and complex 3-d cell structures.Bob aka Linuxlad 10:29, 19 June 2006 (UTC)

Not within the lead shock, of course. A detonation needs at least N mean free paths, where N is the number of reactions to exothermic release, whereas the shock itself is on the order of the mean free path. However "within the shock" is a good shorthand for explaining the difference between detonation and shock induced combustion. Can you explain why the angle of an oblique detonation wave is different to that of a shock with SIC, without an explanation that the reaction occurs within the shock layer? If so, then feel free. I find the difference very much of the essence when the primary observable structure in the flow (shock/detonation wave) takes a different form (angle in the oblique combustion case). In fact ZND and CJD theories rely on the energy release being "within" the shock since they only describe the case for a normal shock where the effective thickness of the shock layer (the area of the flow behind the shock which can affect the shock) is large. In fact you only need to look at the failures of the ZND theory to predict the detonation speeds in condensed explosives to see that the rules governing explosives are different to those of shock waves. Which is kind of the point. AKAF 12:02, 19 June 2006 (UTC)

Well in the naive days of my youth, I thought the shock was where the longitudinal viscosity term (real or von Neumann-enhanced) was significant, and the zone of reactive flow was the region where I needed to supplement Euler's equation (NO viscosity term needed) with an equation of state with a reaction progress parameter. Computationally very different - But both 'close-coupled' (at least in 1-D), since the flow was still subsonic. I don't call all this 'within the shock', but it looks like you do. (Words from a Dylan song come to mind :-))

Incidentally, I see the stellar nucleosynthesis guys still like using ZND. Bob aka Linuxlad 14:09, 19 June 2006 (UTC)

Yep, its still in wide use, the PDE people being another one. I think that there's a lot of wooly terminology around. The real gas researchers for re-entry capsules talk about the "shock layer", which is where the energy of the shock is acting on gas which still hasn't had time to relax into its equilibrium state in the stagnation region. In this case the flow is divided into three (or more regions): freestream, shock layer, and regions behind the shock; where actually only the regions in the shock layer need good finite-rate chemistry, and are coupled with the shock. This is why I tend to think of "shock layer" when thinking about detonation waves :-).

I certainly wouldn't have a problem with changing what's in the article to reflect that the coupled region is not actually "inside" the shock. To be honest, I couldn't think of another way of doing it which would be relatively concise, and so I chose the sloppy method as a starting point. Perhaps a paragraph about the formation of a shock layer which can affect the shock? I think that there's a value to explaining the difference between a shock and a detonation wave (which is why I also added the bit about shock-induced combustion), since earlier versions of this article couldn't distinguish between shock waves and detonation waves. I think that there was a bit in there about the detonation wave radiating through the air from an explosion being a kind of shock wave.

The last reason is that detonations tend to have a lot of the nastiness of cell combustion, with wavelets combining at the detonation wave (Just from my head, isn't spin (sp?) detonation also a nasty creature in that regard?). This isn't what you see in a shock wave, and so I think it's worth distinguishing.

Why don't you have a bash at it, and we'll see where it goes? I'll refrain for a few days to see where it takes you. AKAF 16:18, 19 June 2006 (UTC)

[edit] Overhaul needed

The article seems to be written primarily from an aviation perspective, but many shock waves exist that are not due to fast objects moving through air. Examples should include blast waves, termination shocks, standing shock waves in flows (such as entrance to a conventional ramjet that is traveling supersonically), rate limiting of Petchek-style magnetic reconnection (in an MHD context), Cerenkov radiation and other wake effects like shocked wakes from power boats and the Earth's Alfven bow shock in the solar wind, and (as others have mentioned above) detonation waves such as knocking in an internal-combustion engine. zowie 19:49, 14 July 2006 (UTC)

[edit] Still some facts wrong here

Shock waves are characterized by a sudden change in the characteristics of the medium (such as pressure, temperature, or speed) as a positive step function

Flow speed decreases through a shock wave, pressure and temperature increase. This is important because a shock wave has to obey the usual conservation laws (mass, momentum and energy). The mass flow through a shock has to be continuous, the momentum decreases and this is balanced by the increase in pressure (change in momentum = force acting on fluid), the kinetic energy decreases and this is balanced by an increase in thermal energy. This conversion of kinetic energy (work) to thermal energy (heat) is what leads to the entropy rise across a shock wave and is why engineers are generally desperate to avoid strong shocks (unless they are trying to dump energy, as in the case of spacecraft reentry).

Shock waves are not sound waves; a shock wave takes the form of a very sharp change in the gas properties on the order of a few mean free paths (roughly micro-meters at atmospheric conditions) in thickness. Shock waves in air are heard as a loud "crack" or "snap" noise. Over time a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy. The sound wave is heard as the familiar "thud" or "thump" of a sonic boom, commonly created by the supersonic flight of aircraft.

I still don't understand the desperation to distinguish sound waves from shock waves. While the nonlinearity is small for sound disturbances, it is nevertheless there. There is no transition from a non-linear wave into a linear one. All fluid waves are nonlinear, even if the effect is small.

To get a shockwave something has to be travelling faster than the local speed of sound. In that case some parts of the air around the aircraft are travelling at exactly the speed of sound with the aircraft, so that the soundwaves leaving the aircraft pile up on each other, sort of like a tailback on a road, and a shockwave forms, the pressure goes up and up and up there, and then spreads out sideways. Because of this amplification effect, a shockwave is very intense, more like an explosion when you hear it (not coincidentally, since explosions create shockwaves).

This is simply untrue. An explosive shockwave can be created without the fluid velocity anywhere ever exceeding the speed of sound. The only thing the travels supersonically is the disturbance, the fluid barely moves.

In this case, the gas ahead of the shock is stationary (in the laboratory frame), and the gas behind the shock is supersonic in the laboratory frame. The shock propagates normal to the oncoming flow. The speed of the shock is a function of the original pressure ratio between the two bodies of gas.

Again, this is not generally true. The downstream gas will only be supersonic for relatively large shocks where the difference between the upstream and downstream mach numbers in the shock frame is greater than 1.

[edit] Shock Wave vs. Shockwave

The entire first part of the entry uses "shock wave". Then in the second half of the entry the word "shockwave" appears. Personally I think the second form doesn't belong here, but I don't care about that as much as I'd rather see one or the other used used throughout.

Agree, I have replaced shockwave by shock wave. myth 03:28, 15 September 2006 (UTC)

[edit] Fundamental properties

What are the fundamntal diffs between ordinary logitudinal compression/rearefaction waves in , say, air or water, and real shock waves?--Light current 07:39, 21 October 2006 (UTC)

The classical difference is that standard waves follow a sine wave in pressure (or height, whatever), and that shock waves follow a step function. AKAF 15:52, 23 October 2006 (UTC)

Also shock waves move faster than the speed of sound in the medium. This can be a bit confusing if there are several frames of reference, since it means that the speed difference between the fluid directly before the shock, and the shock is greater than the local speed of sound in the fluid directly before the shock. This means one property of a shock is that it is independant of the properties of the gas behind the shock. AKAF 15:57, 23 October 2006 (UTC)

If shock waves move faster than the speed of sound in the medium, this must be due to non linearities in Youngs modulus of the medium. is that correct?--Light current 16:29, 23 October 2006 (UTC)