Wind Force

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wattspr

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Is there any scale that will relate the wind speed to a force in say newtons or kilogrammes per square metre of an object.

There must be a scale somewhere but I have been unable to find it.

Many Thanks

Philip Watts

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Not detracting from Richard&tracy reply, I have been after this info for what seems like forever. I have spoken to numerous designers, like Naval architects, sailmakers etc etc & no-one either knows or will let on. I simply cannot believe that there is no "ready reckoner" type info available for this. I am damn sure that sail designers, boat, ship & rigging designers dont do it by sticking a finger in the air & saying "thats about 5kn/ft sq" or somesuch.
Look forward to any further replies on this.
Brgds
Mike

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Just reading Bob Fisher's "Armchair Sailor" and there's an extract from an Uffa Fox book quoting mph vs pressure. I'll try and remeber to post the info later

<hr width=100% size=1>Larry Botheras

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I seem to remember reading somewhere (and at my age memory aint what it used to be!) that the actual force exerted by wind varies not only by speed, but by density and humidity and temprature. So that the force exerted by say a 10knot breeze per square metre varies appreciably. Vague memories of school physics lesons about density and mass seem to be relevant here.

Any of our more scientifically trained minds care to comment?


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The Vent Axia extract seems to come direct from BS CP 3 chapter 5. That's not to say it's wrong - quite the reverse. Unfortunately, there is little data on slot effects (eg between Jib & Main). The slot effect can considerably alter the pressures & loads.

I agree that sailmakers & designers don't stick a finger in the air & guess. The more up to date ones will use computational fluid dynamics (CFD) software. I've never had any sucess in getting a functional free version of CFD software out of anyone (not for want of trying either).

You could try making your own wind tunnel and doing practical tests. That's what the Wright Brothers did 102 years ago when they found all published lift data for aerofoils was wrong.. Can't accuse them of being high tech!

Regards

Richard.


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A tabulation could only apply to one very specific situation. eg, it could apply to a mast of circular cross section.

A simplified formula for force due to air drag is:

Force = 0.5 rho CD A V^2

The parameters are:

F Force Newtons assuming object is normal to the direction of airflow

C suffix D Drag coefficient - varies according to the 3D shape of the object, from about 0.05 for an aerofoil to about 1.3 for a plate. This assumes turbulent flow

rho air density - this will be about 1.25 Kg/m^3 and will vary with humidity and temperature, but insignificantly in comparison with the estimated value for CD

A cross sectional area in m^2

V air speed in m/S


Hope this helps, but if you have a practical application, I would be interested to look at it.


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I'm not sure what you are looking for but if you try "Wind Loading" on Google you'll get lots of hits relating to wind loading in the building industry.

My first hit gave me <A target="_blank" HREF=http://www.anemos.co.uk/download.htm>http://www.anemos.co.uk/download.htm</A>. Their free software may, or may not, be of help /forums/images/icons/smile.gif

joe

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One formula states :
Wind Pressure (lbs/sq ft) = 0.004 x velocity squared in knots
8 knts = 0.25 lbs/sq ft
12 = 0.6
16 = 1.0
20 = 1.6
25 = 2.5
30 = 3.6
35 = 4.9
Then covert from 'olde' currency into S.I. units.

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