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Public clifftop skydiving

Grind cliffs and pour wind through them.
  (+10)(+10)
(+10)
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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.

MaxwellBuchanan, Mar 11 2008

[link]






       What does a small, salty fish have to do with quarrying (or skydiving, for that matter)?
neutrinos_shadow, Mar 11 2008
  

       It's a taxation issue.
MaxwellBuchanan, Mar 11 2008
  

       Attempts to sweep the tragedy at the Buchanan Chalk Refinery under the rug will be contested by the Halfbakery Free Press.   

       Currently the link between BCR and BQA is the subject of an active investigation.
normzone, Mar 11 2008
  

       You'd have to discuss that with the directors of our holding company, Buchanan Rug and Brush International.
MaxwellBuchanan, Mar 11 2008
  

       "It's a taxation issue."
Isn't it always?
phoenix, Mar 11 2008
  

       .+
..
..
(..)
  

       This idea may put the wind up people.......
8th of 7, Mar 12 2008
  

       Nicely illustrated, [2-fries].
MaxwellBuchanan, Mar 12 2008
  

       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 (+)
jtp, Mar 12 2008
  

       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.
MaxwellBuchanan, Mar 12 2008
  

       I haven't laughed out loud whilst on the HB for a long time. Thanks Max.
jtp, Mar 12 2008
  

       It'll never work.
Texticle, Mar 12 2008
  

       Give me a reason why not.
MaxwellBuchanan, Mar 12 2008
  

       If it doesn't, lay down some tarpaulin and make it a luge. Please?
jtp, Mar 12 2008
  

       Possibly. I'll have to see what Legal says about it.
MaxwellBuchanan, Mar 12 2008
  

       Disclaimers. You can get round *anything* with disclaimers.
jtp, Mar 12 2008
  

       Yes, but we cannot be held responsible for that.
MaxwellBuchanan, Mar 12 2008
  

       I'll take my chances.
jtp, Mar 12 2008
  

       /Give me a reason why not./   

       Physics.   

       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.   

       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.   

       Air is compressible and has a non-zero viscosity. These factors conspire against you.
Texticle, Mar 12 2008
  

       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?
marklar, Mar 13 2008
  

       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.
hippo, Mar 13 2008
  

       //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.   

       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.   

       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).   

       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.   

       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.   

       Seriously, either do the experiment or run some finite-element modelling before you tell me this won't work.
MaxwellBuchanan, Mar 13 2008
  

       I look forward to BQA solving the world's energy crisis. Bernoulli be damned.   

       Your comments on FEA are somewhat unexpected given the very valid points you made re: The Pointless Calculator.
Texticle, Mar 13 2008
  

       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.   

       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.
MaxwellBuchanan, Mar 13 2008
  

       (marked-for-tagline)   

       I took the trouble to model this
normzone, Mar 13 2008
  

       Wind around buildings, water in a hose pipe, these examples you cite are completely different animals to the system as described.   

       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.   

       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.   

       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.   

       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.   

       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.   

       EDIT: /Regarding finite element analysis, I don't get it (I mean I don't get your comment)./   

       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.
Texticle, Mar 13 2008
  

       ...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.
hippo, Mar 14 2008
  

       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.
xenzag, Mar 14 2008
  

       //a giant wall of Swiss cheese// - or a massive church organ.
hippo, Mar 14 2008
  
      
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