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Wind Weight

Printed From: Yachts and Yachting Online
Category: General
Forum Name: Weather
Forum Discription: What is it up to?
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=1201
Printed Date: 24 Jul 21 at 3:30am
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com


Topic: Wind Weight
Posted By: Skiffman
Subject: Wind Weight
Date Posted: 17 Nov 05 at 1:24pm

In the 4th November issue of YnY, Andy Rice was talking about wind weights and americas cup. A reader has sent an email in this month explaining that it is not possible, I wondered what other people thought.

Myself I think that there is such a thing as wind weight. Because last year the 29er Worlds were in Silvaplana which is 1850m above sea level were the air is less dense and i remember the first day reading 25knots average my anemometer when only felt about 18knots. Get back to the UK doing some winter training at weymouth (-13 degrees windchill) with anemometer reading 18knots when it felt like 25knots. Or am i just stupid and the wind is always the same?

Also the reason why the anemometer will read the same wind speed for 2 different air densitys is because the air/paritcles in the air is going that speed but the particles are more spread out in the less dense air so there are less particles hitting your sail even though there hitting it at the same speed therefore it feels like less wind?????

what do other people think?

I dont really know but find it quite interesting



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49er GBR5

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Team Fletcher and Sign campaign site



Replies:
Posted By: Pierre
Date Posted: 17 Nov 05 at 3:14pm

Good question and I just found this by googling...

Wind does not have weight, but the air that wind moves
does have mass and thus weight.  The wind does exert force
and that force is what moves the air molecules.  What you
feel as wind is the force of the air molecules moving
against you.


David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory

Looks about right to me.  The operative word is MASS.  Thinner air has less mass.



Posted By: Stefan Lloyd
Date Posted: 17 Nov 05 at 3:56pm

Originally posted by Skiffman

Also the reason why the anemometer will read the same wind speed for 2 different air densitys is because the air/paritcles in the air is going that speed but the particles are more spread out in the less dense air so there are less particles hitting your sail even though there hitting it at the same speed therefore it feels like less wind?????

At 2000m height the air density is 20% less than at sea level. So you would definitely feel less force in the sails at the same wind speed compared to sea level.

The opposite applies to a lesser extent with cold air. If are hardy enough to sail at 3C, the air is 10% denser than on a hot summer day of 30C. So the same wind speed will feel "heavier" in winter. Lots of people claim to observe this but I'm not really certain a 10% difference in force would be that noticeable. The force exerted by the breeze is proportional to the square of the windspeed, so a 5% change in windspeed causes a 10% change in force (more or less). 5% change in windspeed is the difference between 20 and 21 knots, and how many of us would really be able to tell the difference between a steady 20 and 21 knots?

 



Posted By: Scooby_simon
Date Posted: 17 Nov 05 at 10:10pm
Ahhhh,

But there is also more to it than this.

We don't use the "force" of the air to move us along; we use the pressure difference as a result of the speed differences across the front and the back of the sail(s) (well on most boats anyway) on our boats that creat low pressure that "suck" us along....

Heahache time in terms of actually "calculating" if colder = faster and hotter/higher/thinner = slower.....  you never know, you might find that thinner (but faster air) might reward the more skillfull.


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Posted By: CurlyBen
Date Posted: 17 Nov 05 at 11:24pm
Wouldn't denser air have a greater pressure differential anyway though?


Posted By: Scooby_simon
Date Posted: 17 Nov 05 at 11:38pm
Originally posted by CurlyBen

Wouldn't denser air have a greater pressure differential anyway though?


Yes.....

but there was some tongue in cheek

Heahache time in terms of actually "calculating" if colder = faster and hotter/higher/thinner = slower.....  you never know, you might find that thinner (but faster air) might reward the more skillfull.


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Wanna learn to Ski - PM me..


Posted By: CurlyBen
Date Posted: 17 Nov 05 at 11:46pm
Oops.. I'm a bit slow today! However for an identical difference in pressure (I assume this is what you meant to calculate?) then wouldn't the lower density air have marginally less drag force on the rig/hull/crew? Can't see that itself for the variations mentioned would be a huge difference! If that wasn't the calculation you meant I'll go and sit in the corner..


Posted By: Stefan Lloyd
Date Posted: 18 Nov 05 at 7:42am

Originally posted by Scooby_simon

We don't use the "force" of the air to move us along; we use the pressure difference as a result of the speed differences across the front and the back of the sail(s)

And the pressure difference results in a force. Force = pressure x area. No force, no movement (Newton's 2nd law).

 



Posted By: Entyplod
Date Posted: 24 Nov 05 at 6:59pm
Stefan Lloyd is pretty much right on this issue:

Wind on a cold day will cause the sails of a boat to provide more force than on a warm one (assuming the atmospheric pressures both days are equal). The confusion lies in both that the difference is not that large, and also in what a cup anemometer is actually measuring.

The force exerted by wind on sails is determined by the kinetic energy of the wind; being proportional to the density of the air, times the square of the wind speed. For any given wind speed denser air has greater kinetic energy, and hence gives a greater force on the sails, and vice versa.

For a boat the differences are however small. Roughly; for a 3 degree change in temperature (or a 10mb change in pressure) the density of air changes by 1%. Take a 20kt wind on a day that is 20degrees C. On a 5 degrees C day (15 degrees colder) the same air would have a density 5% higher. Hence the cold air would exert the same force as the air on the warm day would if its speed is the square route of 1.05 (i.e. 1.025) times higher – :this gives 20.5kts, not a big difference!!

A cup anemometer is trying to measure the speed of the wind, not the force (more correctly the ‘dynamic pressure’) produced by it. Ideally the cups spin at the speed the air flows past them – in reality this does not quite happen due to friction in the anemometer. If you want to measure the force (dybamic pressure) generated by air you need something that works more like an aircraft's air speed indicator.



Posted By: redback
Date Posted: 24 Nov 05 at 7:56pm

An anemometer is measuring the speed of the wind not the force.  I'd rather be hit by a fly at 30mph than a car at 30mph.  The difference is caused by the mass.  It seems perfectly reasonable to me that if we bend wind of greater mass we get greater force even though the wind speed is the same.

Now somebody said we'd be unlikely to be able to notice a 10% difference in force - I'd have to disagree.  When competing against other boats the difference in performance between boats can be in the order of 50m in a kilometer - about 2%.  When you are truly in tune with the boat quite small differences feel quite large, for instance what is the difference between an old jib and a new one - just a couple of seasons use can make the sail feel "off".

Finally what difference to the mass is caused by humidity?  Heavy wind is often wet wind?



Posted By: Scooby_simon
Date Posted: 24 Nov 05 at 9:47pm
Originally posted by Stefan Lloyd

Originally posted by Scooby_simon

We don't use the "force" of the air to move us along; we use the pressure difference as a result of the speed differences across the front and the back of the sail(s)

And the pressure difference results in a force. Force = pressure x area. No force, no movement (Newton's 2nd law).

 



So Stefan,

Sailing upwind your sail area pushes you "up wind".  Nope.

It is the pressure difference between the wind that goes around the front of the mast and sail(s) and the back of the mast and sail(s) that moves you up wind - i.e. you are "sucked" upwind as the wind goes slower around the front(windward) vs the back.

Down wind, yes F = PressurexArea does have more of an impact, but most high performance boats are still "sucked" along downwind as the AW is forward of abeam.
 

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Wanna learn to Ski - PM me..


Posted By: Stefan Lloyd
Date Posted: 25 Nov 05 at 7:25am

Originally posted by Scooby_simon

Sailing upwind your sail area pushes you "up wind".  Nope.

You seem to think I said something different to what I actually said. As you say, on anything but a run, the pressure difference arises because of the aerofoil effect of the sail. I never said otherwise.

But if you think sail area doesn't come into it, try sailing upwind with a pocket-handkerchief and see how far you get. Why do you think yachts put up smaller sails upwind when it gets breezier?

Force = pressure * area is just basic physics. Not opinion and not a subject for debate. Actually it is the definition of what pressure means. One pascal = 1N per square metre.



Posted By: Jon Emmett
Date Posted: 25 Nov 05 at 8:53am
Hmmm. I am going to sail in Weymouth this weekend good old cold North breeze, maybe if I am lucky with snow. Next week I sail in Brazil in around 30 degrees. Funny the wind speed will be about the same. It will be interesting to see if I feel noticeably less overpowered!!!

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http://www.amazon.co.uk/Be-Your-Own-Tactics-Coach/dp/0470973218/ref=sr_1_1?ie=UTF8&qid=1312565831&sr=8-1 -


Posted By: Scooby_simon
Date Posted: 25 Nov 05 at 11:24am
Originally posted by Stefan Lloyd

Originally posted by Scooby_simon

Sailing upwind your sail area pushes you "up wind".  Nope.

You seem to think I said something different to what I actually said. As you say, on anything but a run, the pressure difference arises because of the aerofoil effect of the sail. I never said otherwise.

But if you think sail area doesn't come into it, try sailing upwind with a pocket-handkerchief and see how far you get. Why do you think yachts put up smaller sails upwind when it gets breezier?

Force = pressure * area is just basic physics. Not opinion and not a subject for debate. Actually it is the definition of what pressure means. One pascal = 1N per square metre.



Yes sail area is VERY important because this controls the pressure difference between the front and back of the sail. 

This difference is a result of the difference in the speed of the air across the front and the back of the sail. 
So yes the density of the air is small factor(but as someone above explained with a worked example is was a small one for just simple "push")   We then come to the point where we need to decide what the wind extra "push" in the wind can provide as "thrust" via the sails by creating greater low presure area behind the sail(s).

So I agree that colder air is heavier - obvious.
BUT how much effect does this really make ?
how much more thrust(low pressure) do you get by running  slightly denser air around the sails.

I don't know how to calculate this and IMO it's going to be almost not worth worrying about.  Yes cold air is going to blow you over slightly harder when you are standing still (or sitting on the start line waiting to go).  But once sailing along it will provide a litte more thrust (a very little).  But not much.

What we need is someone who's done an aeronautical degree(or somesuch)  who knows the answer to this by calculcuting the differential lift if hot/cold air over a wing.....



 

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Wanna learn to Ski - PM me..


Posted By: Stefan Lloyd
Date Posted: 25 Nov 05 at 12:54pm

Originally posted by Scooby_simon

What we need is someone who's done an aeronautical degree(or somesuch)  who knows the answer to this by calculcuting the differential lift if hot/cold air over a wing.....

Well yes, and that would be me, as it happens. Mathematical modelling of flows over aircraft wings was precisely the area I specialised in during my first degree. I have already done the calculations you suggest earlier in this thread. To summarise, you will get more lift from colder air but I doubt the difference is noticeable in practice.



Posted By: yellowhammer
Date Posted: 25 Nov 05 at 12:56pm

Aircraft cruise high (low dencity) and cold ... gives higher speed at design Mach No. for wing and reduced drag. Not sure it reads across to a 0.03 Mk No (20 kts) at sea level.

Dencer air = more molecules in contact with sail = more drag, another factor to consider.



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Laser 3000 @ Leigh & Lowton SC
www.3000class.org.uk


Posted By: Scooby_simon
Date Posted: 25 Nov 05 at 1:29pm
Originally posted by Stefan Lloyd

Originally posted by Scooby_simon

What we need is someone who's done an aeronautical degree(or somesuch)  who knows the answer to this by calculcuting the differential lift if hot/cold air over a wing.....

Well yes, and that would be me, as it happens. Mathematical modelling of flows over aircraft wings was precisely the area I specialised in during my first degree. I have already done the calculations you suggest earlier in this thread. To summarise, you will get more lift from colder air but I doubt the difference is noticeable in practice.

 

 So we were agreeing with each other

Excellent.

 

What I was trying to say is that the few degrees of them and thus density of the air was not really worrying about.



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Wanna learn to Ski - PM me..


Posted By: Entyplod
Date Posted: 26 Nov 05 at 4:08pm

Redback asks what difference moisture makes the the density of air. The answer is actually the oppopsite to what one would think - moist air has a lower density than dry air. Odd, but true - and here is the boring mathematical bit to prove it (remember Gas Equation from school):

Density = Pressure/RxTemperature

R= Gas constant, which for water vapour is 1.6 times dry air. Hence density of moist air less than that of dry air.



Posted By: Scooby_simon
Date Posted: 26 Nov 05 at 4:10pm
Originally posted by Entyplod

Redback asks what difference moisture makes the the density of air. The answer is actually the oppopsite to what one would think - moist air has a lower density than dry air. Odd, but true - and here is the boring mathematical bit to prove it (remember Gas Equation from school):

Density = Pressure/RxTemperature

R= Gas constant, which for water vapour is 1.6 times dry air. Hence density of moist air less than that of dry air.



I knew that bit.  Just did not want to complicate it any more for now.....


 

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Posted By: redback
Date Posted: 27 Nov 05 at 11:58pm
I'm impressed by you guys.


Posted By: Jon Emmett
Date Posted: 28 Nov 05 at 8:19am
The important part is how it affects the sailing... So is it more than measuring the wind speed, then using the loos gadge to get the correct tension. Perhaps you need a wind density correction factor (percentage wind weight???) when sailing in extreme hot/cold conditions!

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http://www.amazon.co.uk/Be-Your-Own-Tactics-Coach/dp/0470973218/ref=sr_1_1?ie=UTF8&qid=1312565831&sr=8-1 -


Posted By: Scooby_simon
Date Posted: 28 Nov 05 at 11:55am
Originally posted by Jon Emmett

The important part is how it affects the sailing... So is it more than measuring the wind speed, then using the loos gadge to get the correct tension. Perhaps you need a wind density correction factor (percentage wind weight???) when sailing in extreme hot/cold conditions!


Yep
 

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Posted By: Ian29937
Date Posted: 28 Nov 05 at 12:50pm

How does the temperature/altitude/humidity impact the wind gradient ie the difference in wind speed between the top and bottom of the mast? 

I always had a nagging feeling that the different weight of wind which some people report might be due to the different twist profile required in the rig as the gradient profile changed.

Any thoughts from the meteorologists on the forum to confirm or deny?

Ian



Posted By: Stefan Lloyd
Date Posted: 28 Nov 05 at 1:40pm

In summer, thermal mixing causes the breeze to pick up during the day (I don't mean sea-breeze, I mean inland). That's why on a fine summer day it is often calm first thing and in the evening. In some circumstances you can get a strong temparature inversion within a few metres of the ground. Often you will see mist trapped close to the ground by this effect.

However if there is a strong breeze driven by a pressure system, turbulence causes the mixing to take place anyway, so you don't get much diurnal variation.

I'm not really certain that this will cause wind weight differences between summer and winter though.

Personally, I think it is more psychological and physiological. It is just harder to sail in the same weight of breeze in winter. You are encumbered with extra clothing and your muscles don't work nearly as well when cold. I was very aware of the latter when I used to windsurf in the winter.

 

 




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