Shockwave?

[E-Diddy!]

Boy we sure draw the academic crowd in, don't we?

[Gabriel]

I don't want to enter personal discussions with any of you. I'll just state some verifiable facts:

A 747 traveling below M 0.6 just lacks the capablity to accelerate the airflow past M=1 around the nose.

Shock waves, bow or whatever, just don't hapen below the speed of sound. The speed of sound needs to be exceeded, even if locally, to have a shock or bow wave. Why? Because the shock waves hapen when you SLOW DOWN a SUPERSONIC flow (yep, the shockwave doesn't happen where the air exceeds the sound barrier but where air that has already ecxeeded islows down, even if it is still supersoninc after sowing down).

The pressure field some 15ft around the nose of a 747 traveling at M=0.6 or less is nearly undistrubed from the atmospheric condition. Yet, if this halo was really happening around the nose and moving together with the airplane, It's the best explanation I can think of. An no, it's NOT a shock wave.

Anything that vibrates is moving. Unles you are willing to accept that if I repeatedly punch you in the face with one hand, the hand is not moving provided that it returns to the original position after each punch.

[Dmmoore]

Dear ATFS_CRASH
How many standing shock waves have you seen?
I have seen several on 707's, 747's, 727's, DC-8-63, DC-10's & MD-11. All of the ones I observed were during test flights where the aircraft was operated at or above VMO. None of the ones I have witnessed behaved as those in the video's you have linked.
Standing shock waves do not dance, they drift slowly in response to airspeed / air flow changes.
You have opinions, you are entitled to them.
I have opinions, I am entitled to them.
We are not entitled to insult the intelligence of other forum members.
I have >40 years of experience as an aviation professional including 20 years as an engineer. My opinion's reflect my years of observation and consultation with my peers.

[SYDCBRWOD]

It stays in place and oscillates in frequency enough to vibrate the air around it?

Oscillation = movement. No movement from the vocal chords = no sounds produced.

[ATFS_Crash]

I saw a video of what the poster thought was a standing shock wave however what I saw was a flaw (blister) in the window through which the video was taken.

I disagree. While your theory is fairly intelligent I disagree with your opinion and think your observation skills are somewhat lacking in this case. I think if you use better observation, logic and even greater intelligence that it will debunk your flaw (blister) theory.

Notice how the wave is not in sync with the camera vibration, also notice that the wave does not change in proportion to camera angle that debunks your flaw (blister) theory.

Standing Shock-Wave on Subsonic Commercial Aircraft
http://www.youtube.com/watch?v=TI_usGqlbjI

In the second video notice how the size and shape of the wave changes in a manner that a flaw (blister) or smudge would not.

Standing Shock-Wave on Subsonic Commercial Aircraft
http://www.youtube.com/watch?v=duCHFaWm-ms

While in the case of these videos by Aardvark it does seem to be likely transonic shock waves as the arguments posted by Aardvark.
http://www.pprune.org/tech-log/25921...-aircraft.html

I will stand by my guns that the photograph posted by G-DALE has a shockwave doughnut of vapor at the rim of the bow wave; as air pressure decompresses from the bow wave.

Guess again. For it to be a "Shock-wave" it has to be a "Sonic Shock-wave". Other wise it's just a wave.
You have just defined a sonic shock-wave.

Guess again yourself. There is more than one definition and interpretation; you only to choose to acknowledge the definitions and interpretations that meets your own myopic view.

The vapor donut indicates there was a shock ( a rapid change in pressure and temperature). The doughnuts seems to indicate a shock wave even though the waves are nowhere near approaching the speed of sound. A shockwave does not have to be a (super)sonic shockwave.

All aircraft disturb the air through which they move. The faster they move, the greater the disturbance. The disturbance is what causes "Wake Turbulence". Below critical Mach numbers the disturbances are unorganized and do not form measurable pressure patterns until after the aircraft passes. The disturbed air remains close to that part of the airframe causing the disturbance. In this case the nose appears to be creating a pressure wave in front of the aircraft.

Rubbish.

NASA Airliner Wing Vortice Tests
http://www.youtube.com/watch?v=E1ESmvyAmOs

There are shock waves at subsonic, often it is measurable and is organized and sometimes even precedes the aircraft. You’re obviously talking doubletalk because your own statement contradicts itself.

The pressure wave in front of the aircraft is the bow wave, the “doughnut” in the photograph seems to be at the point where the bow wave would start to decompress. The doughnut also could be a resulting shock wave from intersecting waves. In either case in my interpretation the doughnut is caused by a decompression from waves.

Contrary to what you say. Watercraft much like aircraft; pressure builds up in front of an aircraft even if it’s only traveling subsonicly, forming a bow wave. You might want to study aerodynamics and fluid dynamics.

No it can't. By definition, a "Shock-wave" causes a sudden, abrupt, change in pressure and temperature. You can't have one without the other. The "Shock" portion only occurs at higher Mach numbers where they are measurable.

Wrong. You’re in denial, the photograph seems to suggest that the shockwave is measurable even subsonic. If you would’ve ever studied Schlieren photography thoroughly you would know that shock waves can happen and are measurable subsonically. Of course they become more obvious and easier to measure at higher speeds; if you were more observant you might notice them at lower speeds.

Temperature and pressure waves can be seen coming from a candle using Schlieren photography. Even the naked eye can see the heat and temperature waves of a mirage.

The vapor doughnut indicates that there was an abrupt change in temperature and pressure. Thusly the doughnut indicates a shock.

Additionally, pressure waves always fade out (feather) into nothingness as the wave moves away from the energy source. The one depicted in the photo has a ring around the outer diameter of the ring forward of the nose.

What photograph are you looking at? Try to stay on topic. The photograph depicted in this thread shows an aircraft with a doughnut behind the nose, in the same area at the bow wave would start to decompress. The artifact seems to be a vapor effect from the bow wave decompression.

No it cannot. Subsonic waves are just waves.

As you seem to be contradicted by the photograph. The photograph seems to indicate a subsonic bow wave decompression as indicated by the doughnut.

Yes but that's not what I see. I don't see any kind of a wave. Why? Because what is shown can't happen in nature.

So you don’t believe what you see? So you must think it’s Photoshop or an optic effect?

The wave itself is mostly transparent however the footprints of the wave is evident by the decompression vapor that forms a doughnut.

The vapor indicates a shock of pressure/temperature.

You seem to be rambling and contradictory circles.

Take a look at a bow wave on a boat. It takes a knot or two before any organized wave is generated. In the displacement mode, once the wave is generated, the faster the boat moves, the sharper the angle between the boats bow and the wave. A bow wave never moves at 90 degree angles to the bow.

Why how I’ve never done that before (sarcasm).

Were those waves intersect can often cause a “shock” that can result in standing waves or in vortices or other phenomena.

A shock-wave forms almost perpendicular to the airframe at Mach 1.

What kind of drugs are you on? Perpendicular as in how? The horizontal or vertical part of airframe? Either way perpendicular does not make sense.

http://library.thinkquest.org/12228/Page5.html

In order for shockwave to be perpendicular to the airframe the speed of the shock wave would have to be infinite or the airframe would have to be stationary.

At Mach numbers above Mach 1 the angle becomes more acute.

Wow, I never would’ve known that (sarcasm).

All of the evidence I have seen shows the shock wave is swept back even at supersonic speeds; of course the angle becomes more acute as an aircraft velocity becomes higher.

Even as a child I seemed to have better understanding and intuition of shock waves than you do. I was in an elementary school tour of aviation technology. I was debating and contradicting the tour guide; shortly thereafter I was whisked away by security personnel that interrogated me, that wanted to know where I got my information. When I explained that I had experience in Schlieren photography and I was familiar enough with the it as a child that I could fairly estimate the design speed of an aircraft by looking at the degree of sweep back. I was blasting through their cover stories that was understating the speed of some of the aircraft. I was one of the few children to understand and be privileged enough to consult with Harold Edgerton

In it's liquid state, compressibility is very slight.

Using regular Earth physics liquids are only slightly compressible. However an extreme physics; liquids become a very compressible. IE Astronomy: If I remember correctly the pressures within a black hole are great enough to compress the earth down to something like the size of a basketball. Of course under those conditions the lines might blur and might become confusing between gas, liquid, solid and plasma.

It’s moot anyway because relatively compressible or not; waves can form subsonically in liquids and gases.

[E-Diddy!]

I don't know what a "sonic wave" is. But shock waves and sound waves are two absolutely, and I mean ABSOLUTELY, different phenoms. The sound wave is an periodic oscilation of pressure (imagine a cosin function), a shock wave is one singular abrupt change in pressure (imagine a step).

A sonic wave is the same thing as a sound wave which is the same thing as a sonic "Shockwave." The only difference being that normal sound from say, your voice is many very short period low amplitude waves, where the pressure change is very slight because you hear all the waves, A sonic shockwave on the other hand is only one very abrupt change in pressure where all of the sound waves created by the object have been forced into one wave front. You hear the shockwave as a sonic boom.

http://www.kettering.edu/~drussell/D...r/doppler.html

Are we done yet?

And now please explain me again how do your vocal string (or any other thing) can vibrate without moving?????

It stays in place and oscillates in frequency enough to vibrate the air around it?

[JordanD]

Oh god, that guy wouldn't back down either.

[Dmmoore]

Do I recall somebody posting a video of an alleged shockwave going over the wing of an MD-80 the exact second their camera was trained on it?

I saw a video of what the poster thought was a standing shock wave however what I saw was a flaw (blister) in the window through which the video was taken.

[Spad13]

Do I recall somebody posting a video of an alleged shockwave going over the wing of an MD-80 the exact second their camera was trained on it?

[Gabriel]

Isn't a sound wave, the one you make when you talk, a sonic wave? Being of the basic understanding that sonic = sound? Isn't the sonic boom a product of the way your ear interprets a sonic shockwave as the compressed air vibrates your tympanic membrane?

And your vocal strings don't move at all, they vibrate.

I don't know what a "sonic wave" is. But shock waves and sound waves are two absolutely, and I mean ABSOLUTELY, different phenoms. The sound wave is an periodic oscilation of pressure (imagine a cosin function), a shock wave is one singular abrupt change in pressure (imagine a step).

And now please explain me again how do your vocal string (or any other thing) can vibrate without moving?????

[Gabriel]

I think you have used the speeds for the 747-400.
The 747-200 speeds are:
VMO=370 KIAS / M 0.92

Thanks for the correction. Anyway you can see that little changes with either set of speeds.

[Gabriel]

Water vapor is highly compressible.
Water in its solid form is considered incompressible.
In it's liquid state, compressibility is very slight.
http://data.piercecollege.edu/weather/water.html
[/font]
For practical purposes water is incompressible except in cases where the compressibility factor needs to be considered.[/left]

Amen for that last sentence!

If the solid water (ice) could be always considered incompressible, the sound in the ice would travel faster than the speed of light.

An interesting example of the consequences of the compresibility of the water is this: In metrology there is a strong tendency to replace phisical representations (masters) of the units by phisical definitions so the measurement can be reproduced anywere. One master that survived is the kilogram, which is by definition the mass of a certain individual object. It was once considered to use "the mass of one 1000cm3 of H2O", however the density of the H2O (and hence the mass of a given volume) is a function of its temperatura and, to a lesse but not negligible degree, pressure, so one should add "at a given temperature and pressure". But pressure is force per unit of area, and force is mass times acceleration, so you would have mass inside the definition of mass!!

Anyway, the only thing I wanted to show is that the reason why a bow wave happens in the air when you exceed the speed of sound and in a boat "in the water" a very slow speeds is absolutely not related with the air being comopressible while the water not. You ove something blunt in the water at a speed above the speed of sound in the water (that is, VERY fast) and you have exactly the same shockwave than in the air. Move something at any speed in the interface of any two fluids (including two compressible gases of different density) and you have surface waves.

[E-Diddy!]

And of course there are waves in the air at subsonic speeds. Just talk and you form waves in the air (pressure waves, sound) even if you are not moving (ok, your vocal strings are moving, at a very subsonic speed of course).

Isn't a sound wave, the one you make when you talk, a sonic wave? Being of the basic understanding that sonic = sound? Isn't the sonic boom a product of the way your ear interprets a sonic shockwave as the compressed air vibrates your tympanic membrane?

And your vocal strings don't move at all, they vibrate.

[Dmmoore]

Now let's put the things in perspective:

The 747-200 has a Vmo/Mmo of 355 KIAS / M 0.88. Meaning that you are not to exceed any of those speeds.

I think you have used the speeds for the 747-400.
The 747-200 speeds are:
VMO=370 KIAS / M 0.92

[Dmmoore]

And the water IS compressible. Sonic shockwaves happen in water if something travels fast enough in it. Fast enough is transonic speeds, which are much higher than transonic speeds in the air because the speed of sound in the water is much faster than the speed of sound in the air, because yes, the watter is much less compressible. But it has nothing to do with the waves of a canoe at 3 kts.

Water vapor is highly compressible.
Water in its solid form is considered incompressible.
In it's liquid state, compressibility is very slight.
http://data.piercecollege.edu/weather/water.html

Water's Compressibility
Water at temperatures common in nature has a compressibility factor of around 0.0000034, meaning that a hydrostatic pressure of 6.89 kilopascals (1lb/ sq. in) would reduce unit volume by about 0.0000034 of the original volume.
This compressibility of water is so slight we could never actually see it with our own unaided eyes. We might think water is not compressible. However, if that were the case then the oceans would be about 30 meters higher than they are now, and therefore cover an extra 5 million square kilometers of Earth!

For practical purposes water is incompressible except in cases where the compressibility factor needs to be considered.