A photo of a sonic boom.

[terbang]

Well, there you go. I had always assumed that the condensation cloud marked the region of high pressure of the shock wave itself. I had reached this conclusion because I hadn't thought too deeply about it, but I always drain a fair bit of water out of the receiver of my air compressor. So I assumed that compressing air increased the relative saturation and so caused condensation.
Now you tell me that decompressing the air does this and I can't disagree because, firstly, I had always thought that the shock wave should form further forward on the aircraft than the cloud does and secondly, I have often seen condensation trails on the top surface of the flap tips of landing 737's (obviously a low pressure area) in conditions of high relative humidity.
Somewhere around here, I have a psychrometric chart which probably explains this, but since I can't find it at the moment, can you tell me why a pressure shift in either direction appears to increase relative saturation?

Ranty Dave is quite correct about the visible moisture with the formation of a shockwave. The trails that you see off the flaps on a 737 are of a much lower speed nature though created by the same principle. A lowering of the pressure of a parcel of air cools it (Adiabatic process) as conversely, increasing its pressure heats it. Put your finger over the end of a bicycle pump and give it a good pump and watch your eyes water. As a parcel of air cools the relative humidity (RH) increases and when the RH reaches 100%, you will get visible moisture (cloud). Thats why we tend to get lots of clouds on cold days and clear skies on the warm days. Ever notice that it often clouds over in the evenings as the day cools.
Using a bit of instructors licence rather than scientific, to produce lift an aeroplane wing creates a low pressure zone over its curved upper surface and like any fluid, higher pressure areas tend to move toward, to 'fill in', low pressure areas. Thats why the high pressure air in yer tyres is always trying to get out rather then in. Rather successfully in my case.
Higher pressure air (though only marginally above ambient air pressure) on the wing's under surface attempts to move toward the lower pressure on the upper surface. Because the wing is there in the way, it imposes a force on the wing giving us a lifting force. Viola, we fly! There are also those that say the wing is sucked into the air by lower pressure above it. This is not the explanation that aviators like to use though. So nuff said on that. The higher pressure air under the wing, in it's relentless and natural quest to fill in the low pressure areas on the top, tends to also have a spanwise component (or along the wing) toward the tip, attempting to rotate around the wing tip to 'fill in'. Essentially the wing moves on due to the forward movement of the aircraft so this leaves a rotating vortex off the wingtip (or tip of the flap if they are down). Like a tiny tornado, the wingtip vortex has a low pressure core. Applying the old adiabatic process and if the atmosphere has already got a fairly high RH, our little tornado could easily be further cooled to reach 100% RH. Quite spectacular visible moisture!

[007XX]

Awesome pics and great reading about the technical explanations of it...Even though I do not have the technical knowledge to keep up with this discussion and participate with gobsmacking clever comments, I still find it very interesting...Cool stuff guys!

[Bass]

Nonetheless, the pressure in the reciever is lower than that in the compressor. If the air is close to saturated then any drop in pressure will cause condensation, there's just less mass there to saturate. Also, warm as the reciever is it's still way cooler than the incoming air, the compressor has just squished it into a tenth of it's previous volume. I can't be bothered looking it up but it'll be fuggen hot. .

This doesn't gel for me.
The absolute humidity is fixed by the condition of the incoming air.
The relative humidity determines whether or not condensation occurs.
We appear to be agreed that lowering the pressure increases the relative humidity - does it therefore follow that raising the pressure decreases RH?
Certainly the heat of compression raises the temperature and so decreases RH, so if the increased pressure also decreases it then we have air in the receiver that is at a lower RH than the inlet air both for pressure and temperature reasons. How can it condense? Yet it does - and this is not explained by the temperature hump it has gone through - especially in the case of the screw compressor because the oil acts as a continuous intercooler and so the air entering the receiver is at almost the same temperature as the oil. This is of course way cooler than for a piston compressor.
Sorry, but I think we are missing something here.

Seems likely, reports seem to be from low altitude at transition to supersonic. I heard of one such incident as an F18 flew past a carrier at under 40ft, the whiteout was instant and apparently extended for hundreds of metres. Temperature was sub-zero, don't know if that had any bearing on the outcome.

Yep, supersaturation usually occurs when saturated air is cooled, so low temperature is a factor.

[terbang]

I always get confused between Boyle's law and Charles's law... looks... http://en.wikipedia.org/wiki/Gas_laws

Mr Bernoulli had something to say here as well.

[Bass]

Ranty Dave is quite correct about the visible moisture with the formation of a shockwave. Applying the old adiabatic process and if the atmosphere has already got a fairly high RH, our little tornado could easily be further cooled to reach 100% RH. Quite spectacular visible moisture!

So far, so good. This is pretty standard Bernoulli stuff. Now Mr Bernoulli was a pretty bright cookie, but in the end, he simply integrated Euler's equations and there are some aspects of flight which Bernoulli does not explain fully, momentum and drag in particular. More modern standing vortex theory does rather better but has no place in a motorcycle forum.

As for vapour trails, they require supersaturation. This is witnessed by their lifetime. If the air is not supersaturated, then the moisture would evaporate again and the trails would disappear as the tip vortex dies away.
The life of the vortex is measured in minutes but the trails can last in the order of hours.

[Bass]

Mr Bernoulli had something to say here as well.

Good find - thanks for that.
It does however point out that Bernoulli's equation is at heart simply an expression of conservation of energy and that you have to be careful with the boundary conditions.

[terbang]

So far, so good. This is pretty standard Bernoulli stuff. Now Mr Bernoulli was a pretty bright cookie, but in the end, he simply integrated Euler's equations and there are some aspects of flight which Bernoulli does not explain fully, momentum and drag in particular. More modern standing vortex theory does rather better but has no place in a motorcycle forum.

As for vapour trails, they require supersaturation. This is witnessed by their lifetime. If the air is not supersaturated, then the moisture would evaporate again and the trails would disappear as the tip vortex dies away.
The life of the vortex is measured in minutes but the trails can last in the order of hours.

Vapour trails and wing tip vortex are not the same. Vapour trails, the ones you see way up in the atmosphere are produced by an aircrafts engines. These would be more correctly called Ice crystal trails as they consist mainly of Ice crystals. I have climbed through many 'vapour trails' in the troposphere and, like cirrus clouds, you can hear a 'rustling sound' over the cockpit as you pass through them. Climbing further into the stratosphere (in a Gulfstream not a Boeing) I notice that our own 'vapour trails', viewed from our own external vigilance cameras, don't exist anymore.
The, sometimes visible, tip vortex is created as a 'byproduct' of lift. Mainly at low speed. The wing tip vortex is actually considerably larger than the core that you often see, as much as half of the aircrafts wing span. Wing tip vortex has extremely high local velocities that are root of the wake turbulence danger that we are faced with every day. Wing tip vorticies also add to the aircrafts drag profile and much effort is put into reducing it wirh all sorts of devices like winglets, fences and so on. Nice picky here of a light aircraft using smoke to highlight the vorticies.



Not only do winglets serve to decrease drag, thus increasing range, but they also look damn sexey.

[avgas]

Sorry, but I think we are missing something here.

Have you considered the concept of the surface temperature venturi effect.
While you are getting a pressure change on the inlet of your compressor you are also getting a change in air speed. Faster air = more air = more air hitting the sides of the bottleneck....which in turn is cold.
Lots of air hitting cold surface.....condensation.
Similar to air ram systems on a cold night.
Sorry cant be more technical than that, i only did mechanical stuff for a few years. Their wasn't enough flashing lights for me

[Ocean1]

This doesn't gel for me.
The absolute humidity is fixed by the condition of the incoming air.
The relative humidity determines whether or not condensation occurs.
We appear to be agreed that lowering the pressure increases the relative humidity - does it therefore follow that raising the pressure decreases RH?
Certainly the heat of compression raises the temperature and so decreases RH, so if the increased pressure also decreases it then we have air in the receiver that is at a lower RH than the inlet air both for pressure and temperature reasons. How can it condense? Yet it does - and this is not explained by the temperature hump it has gone through - especially in the case of the screw compressor because the oil acts as a continuous intercooler and so the air entering the receiver is at almost the same temperature as the oil. This is of course way cooler than for a piston compressor.
Sorry, but I think we are missing something here.

Yup, incoming air at ambient pressure will contain a quantity of water vapor, that doesn’t change. Saturation occurs when the ratio of partial pressure of water vapor exceeds the saturation vapor pressure of the other gasses in the mix,(Nitrogen, Oxygen etc). That’s more or less the definition of relative humidity. The relative humidity increases with both an increase in temperature and a drop in pressure. Both these things happen inside a compressor system, repeatedly, in spades. Enough to cause precipitation even when the ambient humidity is reasonably low, (on a reasonably dry day). Pressure changes continue to happen throughout the reticulation system too. That’s why it’s good practice to install a drain leg below the air feed to any equipment, traditionally with an increased diameter to force a further pressure drop.

I think the observed effect is exagerated by the speed at which precipitation occurs, much faster than evaporation, so the accumulated effect of all of the pressure changes throughout a compressed air system is to make water. Even on a well set up system you can see evidence of water (or ice) around any high volume exhausts, (ever used a pneumatic breaker?).

[Bass]

Vapour trails and wing tip vortex are not the same. Vapour trails, the ones you see way up in the atmosphere are produced by an aircrafts engines.

DOH!! Where is my head at today?
You are of course quite right and it is particulates from the engine exhausts that provide coalescence nuclei. However, supersaturation can be condensed at lower altitudes (much rarer phenomenon) by turbulence such as the tip vortex.

These would be more correctly called Ice crystal trails as they consist mainly of Ice crystals. I have climbed through many 'vapour trails' in the troposphere and, like cirrus clouds, you can hear a 'rustling sound' over the cockpit as you pass through them. Climbing further into the stratosphere (in a Gulfstream not a Boeing) I notice that our own 'vapour trails', viewed from our own external vigilance cameras, don't exist anymore.

I can't explain that - can you?
There must be a limit to the altitude at which the required conditions occur. I suspect it stems from from those conditions being generated by rising air which is then cooled past its dew point without condensation.
Is the tropopause a natural limit to the distance that air can rise due to thermal currents?

The, sometimes visible, tip vortex is created as a 'byproduct' of lift. Mainly at low speed. The wing tip vortex is actually considerably larger than the core that you often see, as much as half of the aircrafts wing span. Wing tip vortex has extremely high local velocities that are root of the wake turbulence danger that we are faced with every day. Wing tip vorticies also add to the aircrafts drag profile and much effort is put into reducing it wirh all sorts of devices like winglets, fences and so on. Nice picky here of a light aircraft using smoke to highlight the vorticies.


Not only do winglets serve to increase drag, thus increasing range, but they also look damn sexey.

Ummm decrease?
Lots of work in recent years on adapting the way that a wing sheds the tip vortex. If the vortex can be kept outside the wing span, then the drag is hugely reduced - very hard to do though.

[terbang]

I can't explain that - can you?
There must be a limit to the altitude at which the required conditions occur. I suspect it stems from from those conditions being generated by rising air which is then cooled past its dew point without condensation.
Is the tropopause a natural limit to the distance that air can rise due to thermal currents?

Not too sure either, but it will have something to do with the moisture content up there I guess though I have observed shockwaves on various parts of the airframe up there as well. The normal weather that we know is supposed to stop at the tropopause which is the boundary (essentially a temperature inversion layer) between the troposphere, which is where we live and the stratosphere (where we only visit). Though I have encountered very low (outside the normal theory) temperatures and also strong winds at 45000'. I'll have to dig up me notes and have a wee think.

Ummm decrease?
Lots of work in recent years on adapting the way that a wing sheds the tip vortex. If the vortex can be kept outside the wing span, then the drag is hugely reduced - very hard to do though.

Argh yes decrease.

[Korumba]

A few more picks..

[Bass]

Yup, incoming air at ambient pressure will contain a quantity of water vapor, that doesn’t change. Saturation occurs when the ratio of partial pressure of water vapor exceeds the saturation vapor pressure of the other gasses in the mix,(Nitrogen, Oxygen etc).

Not sure of your meaning here.

That’s more or less the definition of relative humidity.

I have always defined relative humidity as the absolute humidity now, divided by the absolute humidity at saturation at the same temperature and pressure. In other words, as a percentage, how close it is to saturation or simpler yet, how humid it is, relative to its saturation value

The relative humidity increases with both an increase in temperature and a drop in pressure.

Now that, I just can't go along with. The RH goes down as temperature rises and does so rapidly because the saturation humidity goes up. Hot air can hold more water. It's the reason that driers work.
Similarly, if the RH rises as pressure falls, perhaps it drops as pressure rises. Both these effects are counter to your argument of what goes on in a compressor.

Both these things happen inside a compressor system, repeatedly, in spades. Enough to cause precipitation even when the ambient humidity is reasonably low, (on a reasonably dry day).

Well we agree that it gets wet - I guess that's a start

Pressure changes continue to happen throughout the reticulation system too. That’s why it’s good practice to install a drain leg below the air feed to any equipment, traditionally with an increased diameter to force a further pressure drop.

Yes, but that's the pressure down, RH up effect and that is the second thing we agree on so far.

I think the observed effect is exagerated by the speed at which precipitation occurs, much faster than evaporation, so the accumulated effect of all of the pressure changes throughout a compressed air system is to make water. Even on a well set up system you can see evidence of water (or ice) around any high volume exhausts, (ever used a pneumatic breaker?).

Yep but same as above plus a bit of Messeurs Joule and Thompson
God save us - if we can't get this straight, imagine the thread if we took say, hydrogen into account. You know what I mean here? Ambient temperatures are above the critical point for hydrogen and so the Joule Thompson effect is reversed i.e. it heats as it EXPANDS and vice versa. Crack the valve on a hydrogen cylinder and it can catch fire.

[Bass]

The normal weather that we know is supposed to stop at the tropopause

That would explain it.
Supersaturation occurs in cooling and rising air - if the air cannot rise past the tropopause then neither can supersaturation
That's not to say that the air is perfectly dry - it will still contain moisture and so shock waves will be visible - only that vapour trails will not

[Ocean1]

God save us - if we can't get this straight, imagine the thread if we took say, hydrogen into account. You know what I mean here? Ambient temperatures are above the critical point for hydrogen and so the Joule Thompson effect is reversed i.e. it heats as it EXPANDS and vice versa. Crack the valve on a hydrogen cylinder and it can catch fire.

Dude you're confusing me, a task I like to reserve for myself, when the sun's below the yardarm, (and that's just plain bewildering). Time to look instead of shooting from the hip... http://www.domnickhunter.com/tech_Ce...getIndex=false

Looks like the higher pressure squishes the water out but the effect's temporarily offset by the increase in temperature. Suppose that explains why all the rain happens in the reciever.