How Sails Really Work - Arvel Gentry

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Silkie
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How Sails Really Work - Arvel Gentry

Postby Silkie » Wed Jan 25, 2012 9:49 pm

Why have I not read this stuff already?

http://www.arvelgentry.com/index.html

I make no apologies for having found the link in another place.
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Re: How Sails Really Work - Arvel Gentry

Postby ubergeekian » Wed Jan 25, 2012 9:53 pm

Silkie wrote:Why have I not read this stuff already?

http://www.arvelgentry.com/index.html

I make no apologies for having found the link in another place.


At a first read it looks OK, but he goofs big time with this

If we can reduce the 3-D effects, then "deflecting the air downward" is not essential to the origins of lift.

The principle of Conservation of Momentum says otherwise. You cannot exert an upward force on the aerofoil without exerting a downward force on the air stream, and you cannot exert a downward force on the airstream without deflecting it downwards.
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Re: How Sails Really Work - Arvel Gentry

Postby Silkie » Thu Jan 26, 2012 8:00 pm

That's pretty impressive speed reading.
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Re: How Sails Really Work - Arvel Gentry

Postby ubergeekian » Fri Jan 27, 2012 11:01 am

Silkie wrote:That's pretty impressive speed reading.


I taught fluid dynamics for years, so I know what to look out for.

Plus ... a fundamental skill of the academic is finding one mistake in a documents so it looks as if you've read the whole thing carefully, thereby giving weight to whatever half-baked and ill-informed opinions you then voice on the subject.
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Re: How Sails Really Work - Arvel Gentry

Postby Gardenshed » Fri Jan 27, 2012 12:18 pm

So do you agree with Gentry's views or not, minor error aside?

Seems that Gentry's views are not his alone.

http://www.telegraph.co.uk/science/scie ... -myth.html

What's your critique of these theories/opinions?

BTW, I liked the story about him going into a yard to look at a mast being told "that's the Gentry mast that won 2 americs cups" and being able to say, "I'm Arvel Gentry"

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Re: How Sails Really Work - Arvel Gentry

Postby ubergeekian » Fri Jan 27, 2012 2:54 pm

Gardenshed wrote:So do you agree with Gentry's views or not, minor error aside?


He's spot on. No problems there.

There are basically three wrong theories about how wings work. One says that they deflect the approaching airstream down, just as the bucket on a pelton wheel deflect the water jet. That's wrong because it can't explain what happens above the wing, and it's dead simple to demonstrate that what happens above matters more (roughly twice as much) than what happens below.

So wrong theory number two pops up, which is the old "air has further to travel over the top" claim. It's wrong because once two adjacent particles of air have split up acrimoniously at the leading edge, one to go over and one to go under, there is absolutely no reason why they should ever want to get together again, unless they look each other up years later on Facebook and wreck new relationships with an illicit affair.

Wrong theory number three says that Bernouilli's theorem can't explain lift. Well, Bernouilli's theorem doesn't try to explain lift. It gives a very simple and very reliable relationship between fluid velocity and pressure ina smooth (non-turbulent) flow, and if you apply it to the flow round a wing it gives a very accurate prediction of the lift. It's not in the business of saying "why", though, just "how much". Most people who can't get Bernouilli to work don't realise that you need to take momentum transfer into account as well - possibly because they are trying too hard to avoid Error 1.

Forces on fluids arise in two main ways, not counting viscous effects. You can either change the pressure of the fluid or you can change its momentum (by changing its speed or direction). At the surface of a wing, it's pressure that matters, because that's the only thing the wing "feels". A long way (typically 10x chord or further) from the centreline the pressure variations have faded to nothing and only the disturbance in the flow (= momentum change) remains. In between it's a mixture: around a circle of about 1 chord radius from the centreline half the lift force is being transmitted by pressure and half by momentum change. The Bernouilli-is-wrong-boys almost always ignore the momentum changes.

"Why" is a much harder question and, I would argue, not a scientific one. If you put a solid object in the path of a moving viscous fluid, there will be a force on the object. Any component along the direction of undisturbed flow we call "drag" and any at right angles we call "lift". Things which produce a lot of lift are called "aerofoils", but almost any non-symmetrical obstacle will produce some lift, so asking "why" can get a bit metaphysical. "Some shapes are just better than others" suits me fine as a response.

Finally we know that a side effect of producing lift (or a cause - who cares - they always come together) is circulation: flow around the wing superimposed on flow past the wing. Lanchester worked this out in the 1890s but nobody really believed him for another 40 years. It's an incredibly simple relationship: lift = density x free stream velocity x vortex strength. That's how the Flettner Rotor Ships worked: instead of using an aerofoil wing to make the air circulate, Flettner just spun a big vertical cylinder. Viscosity dragged the air round, producing circulation and therefore lift.

Sorry, that's a bit long. Summary: if you want a wing or a sail to work well, try to keep the longer curved side as smooth as possible.
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Re: How Sails Really Work - Arvel Gentry

Postby Gardenshed » Fri Jan 27, 2012 4:25 pm

Thanks for taking the time to write that out. I did some fluid dynamics during my degree course but it was fairly basic stuff and circulation if mentioned, wasn't retained in my cranium.

Sail trim on the other hand has always interested me but being a mechanical engineer, I've more appreciation for the FEA programmes and the construction and stability of the flying shape of the sails than the fluid dynamics. On board, it's all about tell tales, heel and feel with the speedo or adjacent boats as the judge, not an exam paper!

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Re: How Sails Really Work - Arvel Gentry

Postby Silkie » Fri Jan 27, 2012 6:02 pm

Thanks also from me Ian. The bit about circulation was completely new to me too and the main reason for putting up the link in the first place although it's also nice to have a better understanding of what's going on at the leading edge of the main when it's just starting to luff. As Neil says it's probably not directly relevant to what we actually do on the water although I might fit some "Gentry tufts" next year just for fun.
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Re: How Sails Really Work - Arvel Gentry

Postby Ghillie » Wed Feb 08, 2012 9:50 pm

Yes, thanks also for the clear explanation, being a mere civil engineer (ret) I too had not appreciated the vortex strength effects. I note that many thin chimneys have spiral spoilers to mitigate the loading at right angles to the wind direction caused by vortex shedding. Anyway do you (uberkeekian) have an explanation for the fact that a baseball (or cricket ball) curves to the left when thrown with an anticlockwise (when viewed from above) spin (about a vertical axis), when it seems from every view point (mine, anyway) that it should curve to the right. (air is going faster over the starboard side, front of the ball is pushing air to the left etc etc. A golf ball well struck by a driver will tend to rise (backspin) and a topped one dive in the same way. Perhaps a sphere has special qualities in fluid flow.

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Re: How Sails Really Work - Arvel Gentry

Postby ubergeekian » Sun Mar 11, 2012 12:26 am

Ghillie wrote:Yes, thanks also for the clear explanation, being a mere civil engineer (ret) I too had not appreciated the vortex strength effects. I note that many thin chimneys have spiral spoilers to mitigate the loading at right angles to the wind direction caused by vortex shedding. Anyway do you (uberkeekian) have an explanation for the fact that a baseball (or cricket ball) curves to the left when thrown with an anticlockwise (when viewed from above) spin (about a vertical axis), when it seems from every view point (mine, anyway) that it should curve to the right. (air is going faster over the starboard side, front of the ball is pushing air to the left etc etc. A golf ball well struck by a driver will tend to rise (backspin) and a topped one dive in the same way. Perhaps a sphere has special qualities in fluid flow.


Sorry, I've only just spotted this question ...

It's not the velocity of the ball surface relative to the air that counts, it's the velocity of the air relative to the centre of the ball. In the example you give the rotating ball drags air forward on the right hand side and backwards on the left hand side. Without the rotation the air would be moving backwards past the ball on both sides, so the effect of spin is to increase the backwards air velocity on the left and decrease (or even reverse) the backwards air velocity on the right.

Greater velocity on the left -> lower pressure on the left -> ball veers left.

It can get awful confusing when the object you're studying is moving as well, which is why we often try to make the system "quasistationary" which is a posh way of saying "hold the ball/wing/missile/squirrel stationary and let the fluid move past it.
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Re: How Sails Really Work - Arvel Gentry

Postby Ghillie » Mon Mar 19, 2012 11:09 pm

Many thanks uber for coming back with an explanation. Hmmmm not too sure I follow, but its getting harder to follow much these days. If the ball was spinning at an angular velocity such that there was no relative airflow on one side , but twice the airflow on the other, the ball would still curve the wrong way according to Bernoulli. The dragging of the air would have to reverse this somehow for it to curve the correct way...

I resorted to Dr G which cites a site describing differential airflow boundary separation on each side of the ball as the cause; the separation occurring sooner with increased airflow speed. So on the backspinning side more laminar flow results but on the forward side separation occurs sooner along the ball's surface and thus the wake is deflected with a resulting sideways force that curves the ball. (Magnus force apparently).

Enough of this, back to anti-fouling.........the yard assures me that the last-second anti-fouling of the bits masked by the pads as the boat is in the lift is ok.................................... for ensuring a good harvest of mussels!

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Re: How Sails Really Work - Arvel Gentry

Postby ubergeekian » Tue Mar 20, 2012 12:53 am

Ghillie wrote:Many thanks uber for coming back with an explanation. Hmmmm not too sure I follow, but its getting harder to follow much these days. If the ball was spinning at an angular velocity such that there was no relative airflow on one side , but twice the airflow on the other, the ball would still curve the wrong way according to Bernoulli. The dragging of the air would have to reverse this somehow for it to curve the correct way...


This is horribly difficult to do without diagrams, but let me try. Supposing you are looking down on a ball moving at 10 m/s through the air. Your point of view is moving with the ball, so air is moving from top to bottom of your field of view. Its velocity when it appears at the top is 10 m/s, it speeds up a bit to pass the ball, hitting 20 m/s at the widest part, and then it disappears out of th ebottom of the frame again at 10 m/s.

The flow is symmetrical, so the pressure distribution is symmetrical and there is no net sideways force on the ball.

Now imagine the ball spinning anticlockwise in still air. It starts a vortex going, with air moving around it in an anticlockwise direction. Velocity will be a maximum at the surface of the ball and fall off linearly as you go further away. On the left the induced flow will be from top to bottom and on the right the induced flow will be bottom to top.

Now start the original ball spinning the same way. That top to bottom induced flow on the left increases air speed there and the bottom to top induced flow on the right decreases air speed there. The flow is now non-symmetrical. It's faster on the left and slower on the and as a result Bernouilli tells us that the pressure is lower on the left and higher on the right.

Bingo, net force from right to left and
a baseball (or cricket ball) curves to the left when thrown with an anticlockwise (when viewed from above) spin (about a vertical axis)


Sure, the airspeed relative to the surface of the ball is less on the left and more on the right (ignoring boundary layer effects) but that really doesn't matter. It's the air velocity pattern in the framework fixed on the ball's centre which has the effect.

Similarly
A golf ball well struck by a driver will tend to rise (backspin) and a topped one dive in the same way.
When it's got back spin it speeds up the air going back over the top and slows down the air going back over the front. Lower velocity underneath -> higher presure underneath -> ball rises.
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Re: How Sails Really Work - Arvel Gentry

Postby Ghillie » Sun Mar 25, 2012 11:52 pm

Thinking about this, my head is now spinning so much (anticlockwise from above) creating a vortex around it so making collisions with the left had side of door openings inevitable if I try to walk briskly through them.

Thanks uber, as you say the key to understanding is considering the wind speed relative to the body, not just its surface.....

I dimly remember enjoying the use of dimensional analysis in deriving relationships.

I guess that in this case, the out of balance forces normal to the direction of the the flight of the ball and its axis of spinning will depend inter alia on the speed of the wind/angular rotation of the ball, the air viscosity (temp and density) the surface roughness,g etc.

Thus by rearrange the above factors and others to make the units = to ML/T2 will give the equation (with constants) for the force causing the ball to curve .............maybe when I have an idle moment.......I will clean the anchor.

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Re: How Sails Really Work - Arvel Gentry

Postby cpedw » Mon Mar 26, 2012 5:51 pm

Ghillie wrote:.......I will clean the anchor.


Delving into Buckingham's Pi theorem will have that anchor gleaming.

Derek


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