Thursday, April 26, 2007

Apparent Winds, Shear and Twist

I love the folk who comment on my confused posts on the theory of wind and sails, such as the one on Gradients, Tacking Angles and Coriolis, itself an attempt to clarify some questions raised in an email from a Dutch reader. It is pure joy to me to see how an apparently simple question can send my readers off in all directions with such a varied mix of answers.

Carol Anne and Ant shared with us their experiences of wind variation sailing on inland lakes, while JB and Litoralis gave us a technical briefing on wind shear, gradients, viscosity and friction - not to mention fake forces. Fred gave us an insight on how wind varies with height on open water and even managed to explain how this whole issue is related to why there was so little sailing in Valencia last week. Tim and Milo focused in on one area that I had hoped somebody would pick up: why the apparent wind direction at the top of the mast will be different from that at the bottom.

But wait. I see some disagreement here. Tim believes that "the wind will be more on the beam the higher up the sail you go" whereas Milo claims that "the apparent wind is more beamy at the bottom."

Hmmm. They can't both be right, can they? What do you think?

But none of these comments support the suggestion in the original email that the Coriolis effect (fake or otherwise) can cause an asymmetry between port and starboard tacks such as in the claim that "on starboard tack .... sailors experience more need for twist high in the main then on the port tack." It would seem to me that this would be true if there were some consistent change in true wind direction (as well as speed) between the top and bottom of the mast. Are we all sure that this is never seen? Are higher altitude winds generally shifted the same direction from low altitude winds? If so, does this effect apply at the scale of a boat's mast?


Kevin said...

The only Coriolis effect I see in my sailing is the one that determines in which direction my chances of winning will swirl down the toilet...

M Squared said...

When I have observed differences on port vs. starboard tack, it's usually attributed to tuning differences. There are tuning adjustments that can be made on the water and those better left to dockside. I've never noticed that one side is predominantly favored.

As for wind... it's certainly not a fluid with behavior limited to two dimensions. Certainly there are times when we all experience a puff arriving before the cat's paws. Or the breeze that doesn't die in a slick spot...

At any rate, the whole discussion is jam packed with too many variables to reach any conclusion. The race course is anything but a controlled environment... :D

Carol Anne said...

The very rough general rule is that if you have a tall mast, and the winds at the top of the mast are different from the winds at the surface of the water, most of the time, the winds at the top of the mast are a precursor for the winds that are eventually going to exist at the surface of the lake. So what's going on at the top of the mast can give you a clue about what's going to happen.

Where I sail, the general trend is for the higher altitude winds to be more from the right as the day gets warmer. But I believe this wind shift is more driven by the local terrain than by any bigger effect -- the sun heats the land, the air over the land rises, air from the lake flows to fill the vacuum, and since there's a lot of variation in the terrain, there's a lot of variation in what the wind does.

Also, even on a relatively calm day, the sun heating the land can cause a dust-devil, essentially a small tornado, that will sometimes wander out onto the lake and slam a boat just totally out of the blue.

When that kind of thing happens, it's good to have a boat whose keel is about 60% of the boat's weight.

windsend said...

In your original post, it seems to me you were asking about whether there is a difference between true wind direction aloft and alow. The answer is that there is, although it's usually pretty slight over the height of a laser mast.

Picture looking down at the earth from above the North pole as though it were a turntable rotating counter clockwise. Now picture holding a pen above it indicating the direction of the wind aloft where it feels little friction from the ground. Now drop the pen onto the turntable where it feels the friction. It turns left. So the wind alow is always slightly backed compared to the wind aloft. (of course the opposite direction applies in the southern hemisphere.)

The wind feels more friction over land than over water. This leads to some interesting corollaries like what direction shift to expect as you approach a weather shore and which side to pass an island...

Tim said...

If you use vectors it becomes easier to see.

To visualise this, if you use the idea of a set square as an example (use the 30/60). The base represents the boat speed, the upright is the true wind then the hypotenus is the apparent wind.

If you then visualuse the upright getting longer ( to represent an increase in wind speed) then visualise how the angle of the hypotenus has to change you can see that the apparent wind is more on the beam.

This all changes as the true wind is aft of the beam (between on the beam and dead astern). Then the apparent wind moves further aft as the true wind increases in strength.

Tim said...

In regard to a difference in the true wind direction at different hights; this does happen but the effect is small for a dinghy and I suspect not easy to see even on a big vessel.

I do know that a gust is caused by wind at a higher altitude coming down to the sea level and as the wind speed and direction at altitude is going to be different, the gust will have a shift from the predominant true wind direction experianced at sea level. So it depends on what the upper wind direction is relative to the sea level wind direction as to which way the gust shifts the wind direction. That may be something to do with the earths spin but more likely the weather system in the area; Cyclone or anti cyclone.

I don't know if this is the same as the normal oscillating wind pattern that is usually experianced. I don't think it is.

JB said...

Plenty of things affect wind shear,and apparent wind shear. Like real estate, location is possibly the most important, since it will affect the differecnce between the macro scale gradient wind (typically higher level) and the meso scale local winds (sea breezes, wind bending around headlands etc.) which are typically closer to sea level.

If you really want an answer about coriolis effects then you calculate the rossby number.
In the case of a sailing boat lets' assume a velociy (AWS) of around 20kts~10m/s, and a length scale of around 100m (big but lets be generous) the Rossby number for a boat on the surface of the earth is then around 10(m/s)/(10-4(/s)*100(m))=100 (The 10-4(/s) is the corilois frequency)
This means that on the scale we're talking about the only time the corilis effect would be more important that a thousand other geographical effects would be in a flat calm, at which point you can probably still generate more wind shear by walking accross the deck and making the sail flap.

milo said...

Well, when I said the wind was more beamy at the bottom, I contradicted what the numbers in my example said. The numbers were correct, the language of the conclusion was incorrect. The apparent wind at the top in my example was 26 degrees off the bow and 22 degrees off the bow at the bottom. This is actually more abeam at the top, which is what I meant. Twist is good. My example is an exaggeration of the relative difference in velocities. Sorry for the confusion.

Tillerman said...

Thanks for clearing that up milo. You had me a bit confused there for a while.

Aus673 said...

Tillerman, you ought to get your hands on a copy of Frank Bethwaite's book "High performance sailing". It is very good on this sort of thing and has some great pictures by way of explanation.

Tillerman said...

Sshhhh aus673. Don't tell anyone that Frank's book is sitting right next to my computer.

But this conversation is not about me looking up answers in a book and copying them into the blog. It's all about exploring how sailors think about wind and sails in so many different ways and trying to tease out the truth from the discussion.

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