Name for speed at which tropospheric bubbles occur?

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Drauts
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Name for speed at which tropospheric bubbles occur?

Postby Drauts » Thu Jul 02, 2015 10:11 pm UTC

I would have added 'cavitation' to the title, had I had the room to do so.
Basically, I'd like to know if there's a "mach" style name for the speed at which an aeroplane leaves a wake of empty space.

If there's no name, maybe we can make one up.

If anyone wants to discuss the practicalities/lack thereof of the concept, feel free to make another thread; it might get a lot of posts

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Re: Name for speed at which tropospheric bubbles occur?

Postby Neil_Boekend » Fri Jul 03, 2015 7:12 am UTC

I'm not sure what you're trying to ask. The wake of a plane travels with the plane, so it travels at exactly the same speed as the plane, for example 800 km/h.
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Re: Name for speed at which tropospheric bubbles occur?

Postby Sandor » Fri Jul 03, 2015 1:00 pm UTC

If there were a name for the speed at which vacuum cavitation happens in air (if that's what you are referring to, and if it happens at all), I would expect it to be mentioned on the Wikipedia page for hypersonic speed, but it's not.

The idea reminded me of this passage from The City and The Stars (aka Against the Fall of Night)
Arthur C Clarke wrote:The ship was now only a dark stain against the sky, and of a sudden Rorden lost it altogether. He never saw it going, but presently there echoed down from the heavens the most awe-inspiring of all the sounds that Man had ever made - —the long-drawn thunder of air falling, mile after mile, into a tunnel drilled suddenly across the sky.

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measure
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Re: Name for speed at which tropospheric bubbles occur?

Postby measure » Fri Jul 03, 2015 2:43 pm UTC

I think cavitation is only important/relevent in water (and other liquids) because they are incompressible, and thus the bubbles cause significant shocks when they implode.

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Re: Name for speed at which tropospheric bubbles occur?

Postby Qaanol » Fri Jul 03, 2015 5:27 pm UTC

Should be around the speed of air molecules, so on the order of 500 m/s.
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Re: Name for speed at which tropospheric bubbles occur?

Postby cyanyoshi » Fri Jul 03, 2015 7:30 pm UTC

Qaanol wrote:Should be around the speed of air molecules, so on the order of 500 m/s.


There is still no upper speed limit for the gas particles. For an ideal gas, I suppose that there should always be some nonzero pressure behind a moving object because there are always particles moving fast enough to catch up (assuming you don't have zero ambient pressure).

Of course, gases are not ideal in the real world, and weird things do happen to boundary layers in certain conditions (e.g. the no-slip condition breaks down for rarefied flow). Unfortunately, leaving behind a vacuum in an airplane's wake does not seem to be a thing that happens unless you have a very loose definition of "vacuum".

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Re: Name for speed at which tropospheric bubbles occur?

Postby Zamfir » Fri Jul 03, 2015 7:37 pm UTC

measure wrote:I think cavitation is only important/relevent in water (and other liquids) because they are incompressible, and thus the bubbles cause significant shocks when they implode.

Cavitations happens when the local pressure is reduced so much, that the liquid boils to vapour bubbles. So it's restricted by its very nature to liquids. You can't cavitate air, just as you can't boil air. It's already pre-boiled ;-)

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Re: Name for speed at which tropospheric bubbles occur?

Postby sevenperforce » Tue Jul 07, 2015 1:34 pm UTC

Since cavitation in liquid takes place when the bow shock exceeds a certain factor and causes phase transition, an analogy to cavitation could be phase transition from gas to plasma, leaving behind a soft vacuum encased in a conical plasma tail.

Of course, if you're interested in the behavior of air at high speeds, what better place to look than the Wikipedia article on atmospheric re-entry?
An approximate rule-of-thumb used by heat shield designers for estimating peak shock layer temperature is to assume the air temperature in kelvins to be equal to the entry speed in meters per second — a mathematical coincidence. For example, a spacecraft entering the atmosphere at 7.8 km/s would experience a peak shock layer temperature of 7,800 K. This is unexpected, since the kinetic energy increases with the square of the velocity and can only occur because the specific heat of the gas increases greatly with temperature (unlike the nearly constant specific heat assumed for solids under ordinary conditions).
I know that a re-entering spacecraft leaves a plasma trail. But a plasma trail alone is not good enough; you need a plasma trail which will essentially "stretch out" behind and completely encapsulate the vehicle, ostensibly for the purposes of eliminating form drag.

The stability of plasma, however, is an incredibly complex subject. So...good luck!

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Re: Name for speed at which tropospheric bubbles occur?

Postby p1t1o » Tue Jul 07, 2015 2:46 pm UTC

sevenperforce wrote:Since cavitation in liquid takes place when the bow shock exceeds a certain factor and causes phase transition, an analogy to cavitation could be phase transition from gas to plasma, leaving behind a soft vacuum encased in a conical plasma tail.

Of course, if you're interested in the behavior of air at high speeds, what better place to look than the Wikipedia article on atmospheric re-entry?
An approximate rule-of-thumb used by heat shield designers for estimating peak shock layer temperature is to assume the air temperature in kelvins to be equal to the entry speed in meters per second — a mathematical coincidence. For example, a spacecraft entering the atmosphere at 7.8 km/s would experience a peak shock layer temperature of 7,800 K. This is unexpected, since the kinetic energy increases with the square of the velocity and can only occur because the specific heat of the gas increases greatly with temperature (unlike the nearly constant specific heat assumed for solids under ordinary conditions).
I know that a re-entering spacecraft leaves a plasma trail. But a plasma trail alone is not good enough; you need a plasma trail which will essentially "stretch out" behind and completely encapsulate the vehicle, ostensibly for the purposes of eliminating form drag.

The stability of plasma, however, is an incredibly complex subject. So...good luck!


For this analogy to hold, the space shuttle would have to be encapsulated by air with a bubble of plasma "trapped" in the shuttle's wake. I feel like the opposite is true - the shuttle becomes encapsulated in a plasma "sheathe" with normal (if hot) air in its wake. Unless Im way off.

I'd say a more apt analogy is simply the formation of shockwaves, though it doesn't have the same effects as liquid/gas cavitation. It has the sharp changes in density/pressure that a gas/plasma transition may not demonstrate. I feel like the rules that apply in the hypersonic regime are too different for an exact recreation of subsonic cavitation.

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Re: Name for speed at which tropospheric bubbles occur?

Postby sevenperforce » Tue Jul 07, 2015 3:49 pm UTC

p1t1o wrote:For this analogy to hold, the space shuttle would have to be encapsulated by air with a bubble of plasma "trapped" in the shuttle's wake. I feel like the opposite is true - the shuttle becomes encapsulated in a plasma "sheathe" with normal (if hot) air in its wake. Unless Im way off.

But...

Image

The sheath of plasma contains hot air because the inside of the plasma bow shock has time to collapse back into ordinary air. I think that if the space shuttle was moving fast enough, it would pass through its own length before the plasma deionized; thus, the entire "bubble" encapsulating the shuttle would be pure plasma. The density of the bubble would drop to nearly zero within the bow shock, which would reduce drag considerably:

plasma.png


In order for this to work, you'd have to be moving a lot faster, and you might need some sort of magnetic field within the ship to help maintain the stability of the plasma.

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Re: Name for speed at which tropospheric bubbles occur?

Postby p1t1o » Wed Jul 08, 2015 8:11 am UTC

I think that the formation of the bow shock might be responsible for the majority of drag in this context, which would still be present.

But as to the OPs question - what would you call the speed necessary for this to occur?

Some suggestion:

Plasmabolic speed
Aerolytic speed
Ludicrous speed
Ludacris speed
Critical velocy Mp (More power)
UltraMegaSonic
Fast

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sevenperforce
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Re: Name for speed at which tropospheric bubbles occur?

Postby sevenperforce » Thu Jul 09, 2015 1:12 pm UTC

p1t1o wrote:I think that the formation of the bow shock might be responsible for the majority of drag in this context, which would still be present.

IIRC, hypersonic flows have a squared-term problem with the boundary layer responsible for form drag. As the speed increases, the bow shock becomes more and more acute, decreasing the distance between the bow shock and the body. Once the distance becomes low enough, the boundary layer between the the bow shock and the body can no longer be approximated as an inviscid flow and the form drag will increase with the square of the Mach number.

Thus, it might actually be very helpful to use a specially-designed shape and some sort of electromagnetic field to produce a stable plasma sheath around the entire body. Of course, this would probably only be possible at ultrasonic speeds, which rather limits application.

To answer the OP -- I would think the name you're looking for would be something like "persistent ionization speed" or "plasma persistence speed".

Oxygen and nitrogen atoms ionize at around 9000K, which (entirely by coincidence) means you need a speed of at least 9 km/s for the formation of a thermal plasma layer. However, an oxygen-nitrogen plasma at 9000K will be radiating heat away in the form of blackbody radiation rather rapidly, which will immediately reduce its temperature. To create a plasma bubble using thermal factors along, you need a high enough plasma temperature that the plasma will remain ionized for a significant period of time, long enough for the ship to pass "through" the sheath that is being formed.

Using the Stefan-Boltzmann Law can give us an idea of how quickly the plasma will lose energy. At 9,000K, the power radiance of a plasma is 3.7e8 W/m2; at 10,000K, it is 5.7e8 W/m2; at 20,000K, it is 9.1e9 W/m2. Unfortunately, the heat capacity of plasma is itself temperature-dependent, which complicates things considerably.

If we suppose that the plasma layer is initially on the order of 1 cm thick, and use 100 kJ/kmol*K from this paper as our median specific heat capacity, and we further assume that air plasma is about as dense as air at sea level and thus contains on the order of 2,000 moles of atoms per cubic meter, then we can get a VERY rough idea of how long it will take for the plasma to drop below 9000K again. At 10 km/s, it will take about 0.0035 seconds, giving you a "bubble" about 35 meters in length. At 12 km/s, your bubble is 96 meters long. At 15 km/s, your bubble is 167 meters long.

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Re: Name for speed at which tropospheric bubbles occur?

Postby p1t1o » Thu Jul 09, 2015 2:40 pm UTC

sevenperforce wrote:IIRC, hypersonic flows have a squared-term problem with the boundary layer responsible for form drag. As the speed increases, the bow shock becomes more and more acute, decreasing the distance between the bow shock and the body. Once the distance becomes low enough, the boundary layer between the the bow shock and the body can no longer be approximated as an inviscid flow and the form drag will increase with the square of the Mach number.


I would think this would strongly depend on the shape of the body, becoming more true the less "blunt" the body is.

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Re: Name for speed at which tropospheric bubbles occur?

Postby sevenperforce » Thu Jul 09, 2015 3:32 pm UTC

p1t1o wrote:
sevenperforce wrote:IIRC, hypersonic flows have a squared-term problem with the boundary layer responsible for form drag. As the speed increases, the bow shock becomes more and more acute, decreasing the distance between the bow shock and the body. Once the distance becomes low enough, the boundary layer between the the bow shock and the body can no longer be approximated as an inviscid flow and the form drag will increase with the square of the Mach number.


I would think this would strongly depend on the shape of the body, becoming more true the less "blunt" the body is.

Only because a blunter nose will result in a greater amount of the body being contained directly behind the bow shock formation region. Re-entry vehicles take advantage of this by orienting themselves so the entire vehicle is substantially "behind" the frontal surface that is exposed to bow shock formation. Of course, the whole design is to maximize controllable drag, whereas this "plasma bubble" idea would be designed to minimize total drag.

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david.windsor
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Re: Name for speed at which tropospheric bubbles occur?

Postby david.windsor » Fri Aug 28, 2015 9:15 pm UTC

p1t1o wrote:I think that the formation of the bow shock might be responsible for the majority of drag in this context, which would still be present.

But as to the OPs question - what would you call the speed necessary for this to occur?

Some suggestion:

Plasmabolic speed
Aerolytic speed
Ludicrous speed
Ludacris speed
Critical velocy Mp (More power)
UltraMegaSonic
Fast

Really Fast
Really Really Fast
Stupidly Fast
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Re: Name for speed at which tropospheric bubbles occur?

Postby jewish_scientist » Tue Sep 01, 2015 2:37 pm UTC

The speed at which an object transfers enough kinetic energy to the air that the air ionizes into plasma I would call the ionization speed.

The speed at which an object transfers enough kinetic energy to the air that the air ionizes into plasma and the plasma exists behind the object I would call the plas-trail speed. I picked this name because if an object was traveling at relativistic speeds, the plasma it would leave behind would resemble a contrail, but would be awesomer. Unfortunately, anything that is near the realm of practical would only leave a plas-trail of a couple feet at most.
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