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Buchanan Quarrying and Anchovy Inc. (a
wholly-owned subsidiary of Buchanan
Phone and Wire) is proud to announce the
opening of our new installation at Dover,
the world's first free public clifftop
skydiving park. BQA has donated this
facility to the children of Dover, to help
restore a
little pleasure to their lives after
the tragedy at the Buchanan Chalk
Refinery (a company totally unconnected
to BQA).
Cut into the cliff face, about 50 feet above
sea level, you will notice the large,
rectangular opening, about 150 feet wide
and 80 feet high. As this tunnel
dissappears horizontally into the cliff, it
narrows to about 50ft by 30ft, before
curving gently upwards to emerge on top
of the cliff, a few hundred feet inland from
the edge.
You will also notice the sturdy stainless-
steel mesh covering the top opening of
the tunnel.
As the sea breeze picks up, children and
the young-at-heart wait anxiously on the
clifftop, to the seaward side of the mesh.
You'll notice leaves and the occassional
piece of litter skittering skywards as the
drift over the mesh with its focussed
updraft of air.
By the time the sea breeze has picked up
to a brisk 20 knots, the air coming up out
of the tunnel is moving at about 120 miles
per hour (a little less toward the edges,
due to drag and turbulence where the
updraft meets the adjoining air). When
they feel the moment is right, the waiting
children run towards the centre of the
mesh, pitch themselves headlong, and find
themselves flying, lofted by the updraft as
in a skydiving trainer.
No-one can float more than a few feet
above the mesh: the rising current has to
battle with the air it meets, and soon loses
momentum, ensuring that nobody can get
dangerously high. Practiced skydivers can
float for minutes at a time, turning and
sliding as they would in freefall, until a lull
in the wind lowers them gently to the
mesh. Even relative-work is possible
when the wind is steady enough and the
mesh is not too crowded.
People drifting - or intentionally tracking -
towards the edge of the mesh will find
themselves gently lowered to the ground
as the vertical windspeed drops near the
periphery of the column.
Enjoy, enjoy, enjoy. After your visit, why
not drop in to the conveniently located
craft shop, where the totally unrelated
Buchanan Chalk Refineries, Inc., will sell
you an attractively-priced pack of
commemorative chalks.
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Annotation:
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What does a small, salty fish have to do with quarrying (or skydiving, for that matter)? |
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Attempts to sweep the tragedy at the Buchanan Chalk Refinery under the rug will be contested by the Halfbakery Free Press. |
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Currently the link between BCR and BQA is the subject of an active investigation. |
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You'd have to discuss that with the
directors of our holding company,
Buchanan Rug and Brush International. |
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"It's a taxation issue."
Isn't it always? |
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This idea may put the wind up people....... |
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Nicely illustrated, [2-fries]. |
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I initially read this incorrectly, and thought it would be a luge for daredevils, which deposited them in the English Channel. When I realised my mistake, I walked off in disgust. Now that I've calmed down, I've decided it's still a good idea. Just not *as* good. Bun anyway (+) |
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Chalk is notoriously unsuitable for lugeing
down, since it is too soft to provide a
smooth surface (like ice), yet not soft
enough to schloosh out of the way (like
mud). Instead, it simply results in very
high friction all the way down. Just ask
Barry "half thickness" Mandible, captain of
the Dover Luge Team. |
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I haven't laughed out loud whilst on the HB for a long time. Thanks Max. |
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Give me a reason why not. |
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If it doesn't, lay down some tarpaulin and make it a luge. Please? |
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Possibly. I'll have to see what Legal says
about it. |
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Disclaimers. You can get round *anything* with disclaimers. |
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Yes, but we cannot be held responsible for
that. |
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/Give me a reason why not./ |
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The air speed at the narrow end will be, at best, similar to that of the onset breeze. The airspeed at the mouth of the tunnel will be so much slower in proportion to the area ratios. |
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Hold up a kitchen funnel to a convenient breeze. Are you faced with a high speed jet of gaseous fury coming out the small end? No. What if you made the small end smaller? No improvement, it just restricts the flow further. What if you made if bigger? When it gets to the same size as the big end, i.e. a parallel tunnel, that's about as good an exit speed as you're going to get for an internal flow. |
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Air is compressible and has a non-zero viscosity. These factors conspire against you. |
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You could have a wind-powerered fan. A big wind-powered fan is geared up to rotate a small fan. Could this go in the same tunnel or would they need to be in seperate airflows? |
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I agree with [Texticle]. Nice idea, but narrowing the pipe won't magically increase the velocity of the air. I'm afraid the widely publicised launch of this will be a bit of an embarassment, somewhat justifying the complaints of the environmentalists who didn't like the huge hole cut into the cliff, or the destruction of several 'practice' cliffs by Buchanan Drilling and Landscaping Inc. (now ceased trading) engineers. Then, the salty sea air will corrode the cheap steel mesh (if only BQA had forked out for stainless steel mesh...) and local children playing near the abandoned site will fall through the rusty mesh and either plummet to their death or die from tetanus. |
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//Physics// Texticle, I took the trouble
to
model this in COMSOL, and it works.
You'll also note that, at certain points in
a
city on a windy day, there are wind-
speeds
considerably in excess of the ambient
wind speed. |
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Vertical wind-tunnels (ie, the
conventional fan-driven ones) rely on
the inverse ratio of velocity and cross-
section; the air path (normally two,
forming a sideways figure 8) widens
outside the flying area to reduce the
velocity and allow debris to fall out of
the airstream. |
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Yes, air is compressible; are you
suggesting that the air coming out of
the thin end is at 5 atmospheres of
pressure? No, it's negligibly over
pressure but has a higher velocity
(conservation of mass and all that). |
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Yes, air has non-zero viscosity, and
hence there are losses, as with any fluid
jet; try partially blocking a stream of
very viscous water, and the flow
nevertheless accelerates through the
narrower channel. |
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Finally, I put it to you that you have not
actually done the funnel experiment
that you describe. Try it. A standard
funnel is not an efficient shape for this,
and you won't get much velocity
increase, but you will get some if you
have a suitable measuring device. A
larger funnel (proportionally lower wall-
related drag) and smoother taper will
give you something closer to the
inverse-area velocity gain. |
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Seriously, either do the experiment or
run some finite-element modelling
before you tell me this won't work. |
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I look forward to BQA solving the world's energy crisis. Bernoulli be damned. |
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Your comments on FEA are somewhat unexpected given the very valid points you made re: The Pointless Calculator. |
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Alas, BQA is not in a position to
generate energy as efficiently as a wind
turbine (though a few wind funnels
might enable turbine farms to be more
compact). The best we can do is to
funnel the wind, increasing its speed,
reducing its cross section, and losing a
proportion of its kinetic energy in the
process. Our lawyers have asked to
adhere to the laws of physics at all
times. |
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Regarding finite element analysis, I
don't get it (I mean I don't get your
comment). But FEA seems to work OK;
COMSOL is easy to use, but seems to
hold up as well as the more
impenetrable FEA software. |
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I took the trouble to model this |
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Wind around buildings, water in a hose pipe, these examples you cite are completely different animals to the system as described. |
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Wind can speed up as it flows _around_ buildings, similar to how it speeds up across the top surface of an aeroplane’s wing. This is external flow, and as such is markedly different to the creature with which we a dealing. |
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Of course water will speed up through a narrowed opening in a hose pipe, as it is being _forced_ through said pipe. This situation is akin to considering a big fan blowing into the mouth of you tunnel. Of course the air would speed up through the narrowing area, because it is being forced through the chalken nozzle. This gives me some clues as to where you may have gone wrong with your FEA model. |
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I suspect that you are considering only air blowing through your tunnel, constrained in COMSOL such that the initial velocity is that of the open air breeze. |
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Expand the control volume, both in COMSOL and in your own visualisation. Start with air blowing _at_ the mouth of the tunnel, with the mouth of the tunnel being insignificant in area compared to the area _around_ the tunnel. |
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You should find that no matter what shape the tunnel, be it smooth, gradual reduction or reminiscent of the most abrupt and angular kitchen funnel, the ambient air will elect to flow around the tunnel, rather than through it. A bit will flow through, of course, but in the absence of external work being applied (e.g. a fan) the speed of the flow will not be amplified as you may or may not still think. |
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EDIT: /Regarding finite element analysis, I don't get it (I mean I don't get your comment)./ |
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Sorry, it was in fact [Custardguts] who made the comment re: FEA in an annotation about the Pointless Calculator. Remember, FEA does not give 'the answer', it merely solves what you ask it to solve. |
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...or perhaps the question is "What's the limit to speed increase through an optimum-profile, frictionless funnel?" - you wouldn't expect a funnel which tapered from 100m diameter down to 1mm diameter to give a 10^10 increase in airspeed. |
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The air also seems to be heating up
(pictures anorak wearing figures,
blowing air through funnels, as they sit
hunched over copies of Junior World
Physics, their walls decorated with
fading polaroids of copper
calorimeters), but (+) for giving me idea
of converting cliffs of Dover into a giant
wall of Swiss cheese. |
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//a giant wall of Swiss cheese// - or a massive church organ. |
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