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Roof aerodynamics


Rich Griff
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I suppose this could be air brakes/spoilers related but please bear with me...

 

There is a really worrying weather system on it way being described as a weather bomb ?

 

Probably another period of phenomenal weather storm force 13 + similar to a couple of years ago off Scotland.

 

South facing conservatory plastic sheet roof, a lean to at a shallow angle, 5 degrees approx. The sheets rattle and have been seem to " bow" in very strong winds.

 

I have thought about fitting an "air fence" similar to glider etc. air brake come spoiler.

 

The turbulent area/pocket behind the air fence, does that area have a low pressure "bubble" ?

 

I am thinking of placing some heavy car wheels with tyres to add weight and "spoil" air flow on the shallow angle plastic roof.

 

A sensible think to do ?

 

Thoughts please...

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7 minutes ago, Ron Gray said:

If you have the room and can get to the ends think about flat straps that are used for caravan awnings when a storm is coming. I used them several times for my motorhome awning and they worked really well (they go over the top of the roof).

That reminds me of the ropes and weights used in the Northern Isles to keep their roofs on and they know about wind up there. I wonder if I can find a picture.

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             I live on an exposed West Wales Coast and keeping buildings put is a bit of a challenge. A common roof fail is for the downwind side to come off. Some years ago the back roof section of a slated farm building was just "sucked" off in an explosive type event, no slates finished up in the building they were all on the ground behind the building.

     Heavy car wheels, blocks do help on lighter structures. Ratchet straps are a great invention both for going over things and holding from within to anything fixed or heavy like a tractor. A stich in time of a screw, nail on anything starting to come loose is good.

SAM_0817.JPG

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Hi jd8, that site looks familiar, in Pembrokeshire.

 

We holidayied in Solva and toured around. You seen the beware horses signed that shows a unicorn at night, how we laughed.

 

Anyways at that site we saw a hundred or so mini wind turbines, terrific amount of noise.

 

The airbrakes vortex, a low pressure area ?

 

Interesting about the down wind side of a roof being sucked off , a very low pressure area in that side ?

 

Vortex due to apex of the roof ?

 

Windward side forced down by the wind enough to prevent lifting ?

 

Interesting and worrying...

 

A bottle of wine before bed me thinks ?

 

Years ago we had a conversation about a proposed building development at a corner site where wind was funneled between tall houses.

 

A very strong wind in a gale due to funneling.

I asked at what speed would the houses roof blow off.

The architect said "100 mph"

 

Mmmmm we thought.

 

Earlier that year the morning after, after a storm, a trimerang was found upside down on the road a good 100 metres from where it was moored.

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  Hi Rich, That is the Strumble Head light you may well have visited. Photo is a bit of an optical illusion as I am about a mile away up the hill.

 I recon a roof can act much like a simple jedelsky wing [ flat sheet of balsa with a kink about one third chord ] so you get that low pressure on the back side.

For buildings there are two choices I recon. Seal up tight/fix firm so when gust comes through it can not go inside and assist the lifting. For more open barns it seems better if both ends are open to allow wind to pass through and pressure inside and out to equalize quickly.

 

 Biggest gust recorded at Strumble Head some years ago 136 mph!

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When I was about 15 I had a shed/workshop and a small tornado swept through our garden after causing a swathe of local damage.  The (floorless) shed was lifted into the air complete with contents, including a heavy table that I used as a workbench and deposited inverted on the lawn with the table upside down on top of the wreckage.

 

A good demonstration of the effect of negative pressure on a structure - I recall reading somewhere that in the Caribbean, it was traditional advice, rather than boarding up, to leave doors and windows of simple structures open in a hurricane so that pressures were equalised.

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5 hours ago, Rich Griff said:

Thanks guys, 91mph winds forecast for this area....

 

A spoiler/air brake on a glider wing, the turbulent "rolling" air behind it, would that be a low/lower pressure vortex ?

That would be ,lower pressure. So a fence in front of your conservatory would probably lift your roof even more !

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Despite it's modern usage, it really is "airbrake CUM spoiler"!, But the the idea of low prrssure is intriguing.  A long long time ago a friend of mine who was a sign writer was explaining it's the low press that pulls signs off the walls.  The wind blows over them, they bow and Bernouil (sp?) takes over!   Equally, the same friend told me that if you place a brick on the top surface of the wing of a Cessna 150 it's enough to stop it blowing away in a gale.  I know thats wrong but the signs I believe.  Odd chap my friend, good bloke tho.

 

 

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Rubber bands for the roof, now there's an idea, better than bolts just pulling thru, sorry, the roof pulling thru the bolts eh.

 

Which is stronger, the bolt fastened to the structure or the plastic sheeting, even with a "large washer" come reinforcement plate, to spread the load.

 

its been said the pressure difference between the jumbo jets wing lower and upper surface is equivalent to that of a baby sucking a straw !

 

How heavy is a jumbo jet I wonder.

 

thank God it has big wings.

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We have 65 mph winds forecast for tomorrow and we're quite high up and fairly exposed.  There's a hollow ash tree just down the lane that's been on its last legs since we moved here in 1972 - every time the wind blows I'm expecting that it'll be across the lane but it's survived so far.  Just hope our unique 6 sailed village windmill is OK.  It was completely overhauled a few years back.  It's been there since the late 18th century.

 

 

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1 hour ago, Colin Carpenter said:

My wife’s Uncle Elvid delivered the coal to Strumble Head light. Very hard work and he was no youngster. Off topic I know ! ???

  Hi Colin, I had heard of your wife's uncle Elvid and his job of shifting coal to keep the lighthouse men warm. A ton of coal in cwt bags would be dumped at the entrance and he with a bag on his back carry it down the steps to the foot bridge that linked the lighthouse island to the mainland and then up the steps on the other side. I have never counted them but there are a hell of a lot!  Funny thing was the light was powered by kerosene at that time. The tanks were filled by gravity from a high point on the mainland through a pipe that was also a hand rail for the steps.

   For those upon the sea the Strumble head Light is known as Big Four, as it is the most powerful lighthouse in the world and has a four flash sequence. Not a big bulb [ about 250w in old money ] but nine tons of glass lenses  floating around on a bed of mercury, you can move it with one finger.  Cheers, John.

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2 weeks ago a Sunday , too windy to fly so we decided to mow the patch for the first time this year .The mower lives in a shed 5x5 x10 ft, posted into the clay, the door is top hinged and takes two people to lift it open, safety feature so you cannot mow alone !  , We mowed , went to put the mower away and the door had vanished , It had gone up over the roof,  ripped all the hinges out and was lying on top .

It was windy but not that windy we thought , but air flow can be very "turbulent "dont trust it !

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Where is Michael Fish when you need him, He will  put us all at ease with ensuring us it's not a hurricane and to sleep easy!!?

what's that 1987 and again down here 1989 worse , many roofs especially shallow pitch ones with no tiles left.

 

If it's going to be  that strong a wind stay indoors, it's the flying debris that gets you, had a friend at the time had a branch come through his windscreen and it was a tragic end to a young life and a good chap.

 

 

 

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