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Jet Fighter vapour effect - a.k.a. the "Prandtl-Glauert singularity"

felix

felix

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hi sci-fi dorks <3

see this?

jethalo2ub7.jpg


there is a name for that effect. does anyone know what the name of it is? a 'halo' something or other...?

links?

cheers! :D
felix
 
Google much?

Not so hard

to find answers

Btw, the last link indicates:
Dr. Frysinger wrote: said:
The explanation
A conically shaped high pressure surface is formed by objects traveling faster than the speed of sound in a medium, e.g., a jet flying through the air.

This cone is caused by the sound source (jet) traveling faster than the sound waves that are produced by it can travel.

The result is an "envelope" of overlapping circles ("sound wave crests"), each with its center lying ahead of the last sound crest. That envelope is conically shaped with its point at the actual sound source. Lying just inside this conically shaped sound pressure wave (above ambient pressure) is a similar, conically shaped surface of below-ambient pressure air ("sound wave trough").

This is a necessary result of the wave nature of sound waves. Any decent physics text will have a sketch of this. If the humidity level is high enough (e.g., just above sea surface on a warm Pacific Ocean afternoon), the humidity in the air may condense in that trough of low pressure and form a cloud, only to be reabsorbed by the air when pressure returns to normal.

Such clouds tend to form on the noses, nacelles, an leading egdes or tips of wings, canards, etc. These are not unlike contrails except that the water vapor is present in the air before the jet passes, as opposed to being the result of burning jet fuel.

The sonic "boom"
Pictures of these show the result of moisture condensing in the conically shaped wave trough just behind the shock wave. The sonic "boom" reported with these events is NOT caused at the time the jet's speed rises past the speed of sound. It is in fact the continuous sound that the shock wave "crest" represents. A person farther down the path of the jet will hear the same "boom" but later than a person nearer the jet's direction of approach. The speed of the sound crest is equal to the speed of the jet in the direction of travel and equal to the speed of sound perpendicular to the cone's surface. The movie clip on my website at www.cofc.edu/~frysingj/fs14ss.mpg shows the jet approaching and at that moment it has not yet been heard by the observer. Many of the stills being passed around are single frames from similar video clips; the camera was rolling before the "boom" was heard.

A personal account
I was fortunate during my last tour of duty to be standing atop a building at the end of the main runway at NOB Norfolk, a large naval base during a show by the Navy Blue Angels. My height above ground was equivalent to being on the fourth floor, but I was on top of it and "in the elements". The announcer let us know that a jet was coming in at a supersonic speed and would pass at that speed down the length of the runway and "very close to the deck". I saw it coming and aimed my camera slightly downward to its altitude and then tripped the shutter on my tripod-mounted camera as it went by. Immediately, I was blown over backwards and my camera and its tripod landed on top of me. The sequence--all in a fraction of a second--was click, boom, thud. Having been alerted I didn't need to "wait for the boom" to tell when to take the picture. And since the shock wave is NOT a momentary phenomenon, I could let my eye judge when he would be "dead ahead". All this happened so fast that it seemed for a moment to be simultaneous, but that was an illusion. Yes, I got the picture, looking slightly down into the cockpit and with a clear view of the pilot. Humidity levels that day were too low for a contrail-type cloud to form as they did in the movie clip cited above.

The technical explanation
The geometry works out such that the sine of the apex half-angle (half the angle at the point of the cone) is equal to the ratio of the speed of sound in air of that temperature, pressure, and humidity to the jet's speed. That speed ratio is called the Mach Number. So, sin(theta) = V/v or sin(theta) = 1/(Mach Number) where v is the speed of the jet, V is the speed of sound at that location, and theta is half the angle made at the point of the cone.

Notice that if the speed of the jet equals the speed of sound, the "cone" is actually a flat surface perpendicular to the direction of travel. If the jet's speed is less than the speed of sound, the equation is not valid; there IS no superposition of crests taking place (although there is a large pressure build up, similar to a bow wave on a ship).
Click to expand...

and I believe the technical term is 'an airplane in a tutu', also called a 'felix'.
 
grrrr... i did google it very much, but it looks like i didn't know what to google for. plus there's the fact that the internet doesn't even know how to spell the word 'vapour'. :|

anyway

Prandtl-Glauert singularity

this is another extremely cool example:

Crossroads_baker_explosion.jpg


A 21 kiloton underwater nuclear weapons effects test, known as Operation CROSSROADS (Event Baker), conducted at Bikini Atoll (1946).

(cheers %) )
 
anyone know where the best place to happen to see a fighter jet crossing the speed of sound would be?

i really wanna see this in person
 
felix said:
grrrr... i did google it very much, but it looks like i didn't know what to google for. plus there's the fact that the internet doesn't even know how to spell the word 'vapour'. :|

anyway

Prandtl-Glauert singularity

this is another extremely cool example:

Crossroads_baker_explosion.jpg


A 21 kiloton underwater nuclear weapons effects test, known as Operation CROSSROADS (Event Baker), conducted at Bikini Atoll (1946).

(cheers %) )
Click to expand...

Can you imagine what that did to the marine life in the area? Marine mammals, like whales? The pressure wave from that must travel hundreds of miles underwater, fucking up every living thing along the way.

Testing nuclear weapons is crazy enough - doing it underwater, in life-rich tropical environments to boot. . . who the fuck thought that was appropriate?

Sorry, just had a moment there. I wonder what the hairless primates would think if a race from outside our neck of the galaxy decided to use Earth for some good 'ole weapons testing. Yeah, they could have used a lifeless planet. . . but they wanted to see a more colorful destructive result. :X

A friend of mine once compared humanity to a toddler with an automatic weapon - it might not intend to do extraordinary harm, but one way or another that's going to be the result of poor judgment combined with heavy weaponry.

Peace,

Fausty
 
^ you'd think they would've given those boats some warning too. :|

BM - maybe you should get a job on an aircraft carrier? =D
 
Believe it or not, Bikini Atoll is--today--a thriving oasis of sea life, and littered with wrecks from Operation Crossroads that are a tourist attraction for well-heeled scuba divers. Ironically, the lagoon is only thriving because no one's fished it in over 60 years; the islands themselves are still not safely inhabitable, though this isn't because of the two Crossroads tests.

Here is footage of the test in question, Baker, which had a yield equal to the Hiroshima/Nagasaki blasts. In one of the later scenes, you can actually see the shock wave traveling across the water:

http://www.youtube.com/watch?v=oxayEJWqjbQ

The real atrocity with Operaion Crossroads, IMO, were the people who were put on the wrecks immediately after the tests, when they were still seething with radiation. Needless to say, many of them didn't live long, happy lives.
 
Bikini Atoll is absolutely gorgeous!! We didn't ruin it for the fish, apparently.
 
Belisarius said:
Believe it or not, Bikini Atoll is--today--a thriving oasis of sea life, and littered with wrecks from Operation Crossroads that are a tourist attraction for well-heeled scuba divers. Ironically, the lagoon is only thriving because no one's fished it in over 60 years; the islands themselves are still not safely inhabitable, though this isn't because of the two Crossroads tests.

Here is footage of the test in question, Baker, which had a yield equal to the Hiroshima/Nagasaki blasts. In one of the later scenes, you can actually see the shock wave traveling across the water:

http://www.youtube.com/watch?v=oxayEJWqjbQ

The real atrocity with Operaion Crossroads, IMO, were the people who were put on the wrecks immediately after the tests, when they were still seething with radiation. Needless to say, many of them didn't live long, happy lives.
Click to expand...

That's a true irony, and I'd also heard that it was a really beautiful place now that the local ecosystem has recovered. The only way we can protect someplace from human overfishing is to drop a nuclear bomb on it. 8)

Again, the horrific part of this (to me, anyway) isn't the local reef fish. It's the impact that a pressure wave like that would have on marine mammals - not just in the Atoll itself, but hundreds of miles away. Lest we forget, they navigate and communicate using sonar - sound waves. In many species, a large organ on or above their forehead - filled with a fatty fluid - acts as an ultra-sensitive detector for echolocation (that's the stuff that they used to kill Sperm whales for in order to make oil for candles and lamps, btw).

I can only imagine the effect of a massive pressure blast underwater on those sensory organs - probably something equivalent to having a hundred-million candlepower light shone in your eyes with your eyelids stitched open. Even the much lower-power "active sonar" tools used by the U.S. Navy in recent times has been shown to cause severe behavioral and social abnormalities in dolphins and whales dozens or hundreds of miles from the testing zones. Many of the "mysterious beaching" events of dolphins and whales in recent decades are most likely caused by such underwater sound overloads - researchers speculate that the creatures are trying so hard to flee the pain from the unrelenting, overwhelming sound that they literally beach themselves out of the water to get away - dying in the process.

Now imagine a nuclear bomb - sound (particular lower-frequency ranges) travels very, very far underwater. Some humpback whale songs have been calculated to circle nearly half the globe whilst still being identifiable.

Oh, and on the "brain index" scale (cerebral mass relative to body mass), some species of marine mammal come out on top of the "most intelligent creature" (self-titled, of course) on the planet: homo sapiens. Are they smart? Truly, we don't know - and we don't know if our definition of "smart" is even relevant.

No, I'm not some "love the whales" hippie. I am fascinated by the alternate neurological architecture of the marine mammals - given that the human brain is suffused with our visually-oriented perception apparatus (per Crick's The Astonishing Hypothesis), we can only guess at this point what the world "looks like" for a creature whose brain is deeply structured towards a three-dimensional (genuine, not imputed like we do) conception of the world in which they live. My guess: they are really, really good at vector calculus! %)

So, yeah, setting off nuclear bombs in their habitat - and not even considering that it's likely to wreak massive havoc and immeasurable pain on them in the process - seems like a particularly asshole-ish thing to do. We have no idea how it effected the marine mammals - back then we were still more engaged with killing them than studying them.

So long, and thanks for all the fish. . . :\

Peace,

Fausty

ps: yeah, I've also seen videos of the hapless sailors (and soldiers) sent in to poke around test sites right after the bombs went off. . . wtf? Surprisingly, one doesn't see many admirals or generals in those video images, eh?
 
Ham-milton said:
Bikini Atoll is absolutely gorgeous!! We didn't ruin it for the fish, apparently.
Click to expand...

Saw this article in New Scientist following up on the same topic as our discussion. It reads:

----------------------------------
Nuked coral reef bounces back
* 18:30 14 April 2008
* NewScientist.com news service
* Catherine Brahic

What does a coral reef look like 50 years after being nuked? Not so bad, it seems. Coconuts growing on Bikini Atoll haven't fared so well, however.

Three islands of Bikini Atoll were vapourised by the Bravo hydrogen bomb in 1954, which shook islands 200 kilometres away. Instead of finding a bare underwater moonscape, ecologists who have dived it have given the 2-kilometre-wide crater a clean bill of health.

"It was fascinating – I’ve never seen corals growing like trees outside of the Marshall Islands," says Zoe Richards of the ARC Centre of Excellence for Coral Reef Studies in Australia.

Richards and colleagues report a thriving ecosystem of 183 species of coral, some of which were 8 metres high. They estimate that the diversity of species represents about 65% of what was present before the atomic tests.

The ecologists think the nearby Rongelap Atoll is seeding the Bikini Atoll, and the lack of human disturbance is helping its recovery. Although the ambient radiation is low, people have remained at bay.

Atomic idyll
"Apart from occasional forays of illegal shark, tuna and Napoleon Wrasse fishing, the reef is almost completely undisturbed to this day," says Maria Beger of the University of Queensland in Australia. "There are very few local inhabitants and the divers who visit dive on shipwrecks, like the USS Saratoga, and not on the reef."

Beger took a Geiger counter with her on dives and says that the background levels were similar to that at any Australian city. The same could not be said of coconuts growing on the islands.

"When I put the Geiger counter near a coconut, which accumulates radioactive material from the soil, it went berserk," says Beger.

Journal reference: Marine Pollution Bulletin (DOI: 10.1016/j.marpolbul.2007.11.018)
----------------------------------

Peace,

Fausty
 
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