Broken column
Question: Why does smoke from a cigarette rise in a smooth, vertical column
before it suddenly seems to scatter in a multitude of directions?
Answer: Smoke rises from a cigarette because it is hot and lighter than air.
The stream is smooth and shaped like a column because it is laminar, and all the
smoke particles travel in parallel.
The flow is laminar because the cigaretteâs burning area is small, and the
energy output that is driving the stream upward is only about one watt. Larger
sources of smoke such as smokestacks or bonfires clearly do not produce laminar
streams.
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The scattering of the smoke marks the transition of the flow from laminar to
turbulent, and is caused by the growth of small, invisible disturbances to the
smoke stream that eventually create scattering. Turbulent flow is characterised
by random fluctuations of speed and direction.
Careful measurements using lasers and special imaging techniques have
revealed that the smoke stream is only a small part of a much larger invisible
plume that rises from the cigarette, with the hottest and fastest moving part of
the plume actually a few millimetres in front of the smoke stream.
As the plume rises, it takes the surrounding air with it and cools the air
down which, surprisingly, makes it move faster.
In very still air, I have observed stable plumes that are as long as 30
centimetres, which become wavy just prior to scattering.
Peter Lipowicz
Senior principal scientist
Philip Morris USA
Richmond, Virginia
Answer: The effect is caused by a transition from a laminar to a turbulent
air flow. The column of rising smoke consists of fine particles suspended in air
which is warmer than its surroundings and therefore less denseâso it tends
to rise.
Because the surrounding air is stationary, a boundary layer exists between
the moving air and the still air surrounding it. Air on the inner wall of this
boundary layer is moving upwards at the same speed as the smoke column, whereas
air on its outer wall is stationary. Between these two extremes, air rises at a
range of intermediate speeds.
At the bottom of the column, the boundary layer is thin and the flow within
it is smooth. Each molecule of air is confined to a thin sheet or lamina of air.
The laminae slide past one another quite easily, like a telescopic aerial being
extended. Movement of air between the laminae is minimal, because the viscosity
of the air tends to dampen any small ripples.
At a critical point, however, this situation cannot be maintained. The air
accelerates as it rises, and eventually the damping effect of viscosity isnât
enough to maintain laminar flow, and the small irregularities grow into eddies
that break up the laminae to form a turbulent, irregular flow pattern.
David Roche
Helensburgh, New South Wales
Answer: The smooth vertical column is laminar flow; the scattered column is
turbulent. The stability of fluid flows depends on the dimensionless Reynolds
number (Re) which equals LU/n
where L and U are respectively a characteristic
length and speed of the flow and n is the fluid kinematic viscosity (about
0.00001 for air).
Depending on the stillness of the air in the room, the flow is unstable if Re
exceeds a critical value of about a few thousand. Estimating that cigarette
smoke rises at about 1 metre per second and that the critical Reynolds number is
about 5000 gives a âsmooth vertical columnâ length of 5 centimetres, a very
plausible result.
Alistair Fitt
Faculty of Mathematical Studies
University of Southampton
This weekâs questions
Drinkersâ legs: If you add a little whisky or another spirit to a glass and
then take a sip, the inside wall of the glass becomes coated with a thin film of
the liquid.
As the film drains back into the liquid in the bottom of the glass it tends
to form into rivulets which are roughly equally spaced from each other. The
spirit is then said to âhave legsâ. Presumably this effect is caused by the high
alcohol content in the liquid because it does not occur with drinks of lower
alcohol content. But why do these âlegsâ form?
John Yeadon
By e-mail, no address supplied
Ice chiller: Last night I visited an Indian restaurant with three friends. We
ordered, among other things, a 1-litre glass bottle of sparking water. On
arrival it looked perfectly normal, but within about 8 seconds of being opened
most of the water had turned to ice.
We watched in amazement as the rapid solidifying started at the top and
eerily worked its way right down the bottle. Needless to say we were
dumbfounded. What is the explanation for this bizarre occurrence?
Chris Jeffs
London
Cracking on: Some days ago I was standing underneath the flight path taken by
aircraft landing at Dublin Airport.
I noticed that after a plane had passed overhead and landed there was an
audible crackling noise that could be heard moving directly above me, in the
same direction as the aircraft.
The sound was not unlike high-pitched thunder, and didnât arrive until up to
one minute after the plane passed overhead. In some cases it was accompanied by
ribbons of vapour which floated downwards and then dissipated. This was more
noticeable with larger aircraft.
What is the mechanism responsible for this phenomenon and why did I not hear
it immediately after the plane passed overhead? The planes were flying at only
around 150 metres and the weather was calm.
Fergal Tierney
Garristown, Ireland
Eyes up: Many insects such as bees, wasps and flies have large, multifaceted
eyes. These cover a wide field of view without having to swivel as human eyes
do.
However, unless these insects point their heads downwardsâwhich would
make flying somewhat unpredictable to say the leastâ it would seem
inevitable that some part of their eyes would always be looking directly at the
Sun.
In view of the dangers of looking directly at the Sun, which were publicised
at the time of the total eclipse, why donât these insects go blind? Do they have
some protective mechanism in their eyes, or is it that they just donât live long
enough to suffer any significant damage?
John Hann
Etchingham, East Sussex
