Question seen in New Scientist

[Caudillo]

Was lucky enough to chance upon only a very lightly soiled copy of the New Scientist magazine on the train the other day and saw something in the questions and answers section in the back of it which got me wondering.

From what I recall, somebody had written in saying he'd been a passenger in a 74 and whilst stationary on a very wet runway with engines running, he'd noticed one of the engines cause water to collect in a patch on the ground about a metre forward of the intake lip, rise vertically upward and turn ninety degrees and head into the turbine. I can't say I've noticed this myself, or that this is a verbatim account of the question - but my sim partner swore he'd seen it happen himself in the past.

Any ideas?

[Kestrel_909]

It happens, check out these shots. Maybe a bit surprising it happened if at idle power, sorry I can't really offer a proper explanation.

http://www.airliners.net/open.file/0326528/L/

http://www.airliners.net/open.file/0518744/L/

http://www.airliners.net/open.file/0283585/L/

[haughtney1]

High pressure (outside the engine) to an area of low pressure (inside the intake and engine)....the pressure drop is due to in part the air being accelerated and hence lowering it pressure.

Here endeth my understanding of Mr Bernoulli's theory:ok:

[Spitoon]

Very common with low slung engines a la B737.

[lomapaseo]

High pressure (outside the engine) to an area of low pressure (inside the intake and engine)....the pressure drop is due to in part the air being accelerated and hence lowering it pressure.
Here endeth my understanding of Mr Bernoulli's theory
Good description of the suck part of the suck, squeeze, bang and blow theory, now if only you could move on to the part about why do we also have a vortex :)
I think it has to do with velocity vectors and not just gross pressure gradients. So if you have two velocity vectors creating a shear component at the same time that you also have a gross pressure gradient, viola, you have a vortex. Now just add some visible water vapor and we have a photographers' dream :}

[Slow Roll]

Yes, all that suction combined with fast rotating compressors makes rising columns of water entirely plausible- I've also seen it with sand and dust and was glad it wasn't my aircraft. Jet engines suck, as this well thrashed link (http://www.youtube.com/watch?v=L_gpPbpONK4)proves all too well

[Gonzo]

I've seen this a few times with my own eyes, and it's always been when a C-17A has been doing its party piece at airshows; using reverse thrust to reverse back down the runway. I see that the three pictures that Kestrel909 has linked to show the same.

Is this significant? That this happens whilst the a/c is actually travelling backwards? I guess this would increase the pressure differential between 'high' and now 'even lower', but would the 20kts or so make that much difference?

[Mark 1]

You get a phenomenon called a ground vortex due to the sink-flow into the intake.
I've known one pull up a lump of loose tarmac into an engine on a test stand.

[ICT_SLB]

We had a similar vortex appear above the inlet when we ran the RJ-700's engines in the climate test hanger at Eglin AFB. Probably partly due to the temperature stratification in the -40 cold. I got it on video but no way to upload the analog right now. The snowfall as they brought the chamber temp down was worthy of a Hollywood spectacular.

[Max Angle]

You see it a lot as aicraft taxi over puddles of water, makes you realise how dangerous the things are even at idle. Ramp staff take note!.

[MrBernoulli]

Tornado bomber/fighter is very good at this ....... and sucking on stones too!

[enicalyth]

I've seen it too, many times. Often like a cobra dancing to an invisible snake charmer. But the cobra mist disappears very quickly.

If I may pick up on Mr Haughtney's observation.

The inlet inducer or diffuser is part of the initial internal slowing down process. Agreed anything outside the engine is "sucked" towards the inducer but once inside the duct the flow is slowed down and pressure rises. It is this compression process which if it overcomes all losses and the work extracted by the turbine will create the nett jet thrust.

The turbine is there to turn the compressor and effectively pushes the aircraft back, hence all those lovely thrust balance sketches.

Compression is a rise in temperature and pressure. Expansion is a loss of pressure and temperature. Clearly any expansion anywhere in the front end of the jet engine creates an adverse pressure gradient. Not the same as a surge which is choking and the choking is seen as an adverse pressure gradient causing the flame to come out at the front if it can! And a bang whether it can or can't, leading to oscillating and more bangs.

When Whittle propounded his views people not unnaturally said "But because the turbine turns the compressor won't it consume so much as to make the whole engine not to run at all?" Whittle would add "But I am adding heat to make up for these subtractions and losses - I am adding heat by burning paraffin in air". Heat is work and provided that burning fuel more than makes up for mechanical losses and the work extracted by the turbine to turn the compressor then the engine runs and we get nett forward thrust.

What Whittle, Ohain and others then wrestled with is not to have the compressor choke and stall, in which some compressor stages actual "turbine" and rob thrust to the extent of creating an adverse pressure gradient pushing back to front.

Whittle used a centrifugal compressor and an axial turbine. Ohain went for an axial compressor and a centrifugal turbine. Hence nothing to do with war winners and losers Whittle has the more rightful claim to precedence because his design through either insight, luck or both is the more thought through. If Ohain beat Whittle to the punch it was in spite of his design rather than because of it. Both Ohain and Whittle suffered bureaucratic obstinacy, Ohain perhaps less so because he won the heart and mind of a manufacturer.

The immediate foreunner of the Avon (I forget its name) was another engine that handled compression badly, never actually running properly at all and the remarkable thing is how quickly Rollers cottoned on and designed that amazing single shaft engine - the Avon. But as usual I am digressing. The cobra vapour effect on the ground....

How I think it happens is like this. Sat on the ground air speeds up outside the inlet but once it has got inside it must slow down otherwise it negates the purpose of the compressor. Pressure must not be allowed to fall until we get to the turbine stages. Any such loss of pressure is a loss of thrust. So very soon inside the inlet the cobra mist effect must disappear 'cos the air is rapidly slowing down and warming up by good design that Whittle appears to have understood, Ohain didn't and Rollers had to learn the hard way.

Right up until the last stage of the compressor the whole process of the jet engine is to slow down and compress the air which gets hotter and by getting hotter is available to do work. Some of the work is absorbed by the turbine driving the compressor hopefully leaving enough to supply forward thrust.

(When we set fire to Whittle's paraffin to overcome turbine and mechanical losses we try to ensure that the temperature is always high enough, raising the local speed of sound high enough that critical stages do not choke too soon. As soon as the air reaches local Mach One in the engine it is remarkably disinclined to shift faster).

That the water vapour is picked up from a stationary pool and made to move into the engine is suggestive of a pressure gradient outside the inlet (otherwise air would never enter) but once it is inside it is then compressed and slowed down, not speeded up further until expanded through the turbine and nozzle. Hence if you look at that cobra mist it appears to be attached to a point a very short distance inside the inlet.

Nevertheless there is a considerable mass flow front to rear. Anyone "sucked" into an engine will rapidly find himself being compressed but it will be all right in the end. He gets expanded in the turbine and exit nozzle. So that's all right then. Just a few smuts and bruises and a looooooooong story to tell in the bar.

PS Ohain "went" to the USA and his mastery after an unpromising start showed him to be at least Whittle's equal and perhaps sounder business judge.