Last month, on a flight from London to Washington DC, the Boeing 747 I was flying on had to wait a little while for take-off on the runway at Heathrow. It was a very wet day and I noticed that the engines appeared to be sucking up water from the tarmac in front of them. Strangely, the water rose vertically in a very narrow stream less than 10 centimetres wide from a point on the tarmac directly in front of each engine. Then, when the vertical columns of water reached a point about a metre in front of the centre of each engine, they changed direction to head horizontally into the middle of each turbine. They looked, in effect, like large walking sticks made of water pouring upwards into each engine. I can accept that jet engines suck in huge amounts of water on wet days but presumed they did it more generally from the air around them rather than somehow sucking it up vertically from the tarmac in such a specific way. What is going on?
The peculiar moisture cloud you saw is in the core of an intense vortex, much like those in dust devils or tornadoes. Mathematically, a vortex cannot terminate in free space, but must either form a loop, or attach to a surface 鈥� hence the vortex from the engine bends down and connects to the ground where it is held stably. These engine-intake vortices have occasionally resulted in accidents where people have been sucked into the engine. Watch a .
鈥淭he peculiar moisture cloud is in the core of an intense vortex, much like those found in dust devils or tornadoes鈥�
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Ralph Lorenz, Columbia, Maryland, US
The air is being drawn evenly into the engine when you consider only the portion of atmosphere directly in front of the turbine. However, the presence of the tarmac boundary means the bigger picture is rather different. Such a configuration can be thought of as a flat layer with the air being drawn towards a low-pressure region in the centre.
The conservation of angular momentum means that any rotation in the air mass intensifies as you get closer to the centre, just as it does with low-pressure weather systems. Regarding the case in hand, the speed of rotation near the centre becameso intense and the pressure so low that water was lifted from the ground and along the vortex right into the intake. A tornado above a body of water can act in a similar way. The photograph (above) shows the same effect, but here the extreme low pressure and high atmospheric humidity made the water condense out to form a visible mist.
A more familiar example for most of us would be to consider a body of water, such as a water trough, with a horizontal drainpipe removing the water from a short distance below the surface. If the drain pressure is low enough, the angular velocity of the water as it approaches the drain becomes such that a familiar air-filled vortex forms on the surface and extends down to a point just ahead of the drain before turning horizontally into it, just like the engine intake vortex, but with the fluids and airspace reversed.
Laurence Dickie, Brighton, East Sussex, UK