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Sea Lotus said..

Gestalt said..
something i wrote in a previous thread

Say you have a feather and a brick travelling at 15knts each. They are both doing the same speed but the brick being more dense will hit you with more force.

So. 15 knots is 15 knots regardless of density. But as cooler air has more density it also has more force.

The confusion is the terminology used. We should be talking about wind energy and not wind speed.

To measure wind energy you need to know both temp/density and wind speed.

Those cups/blades are hit with energy (speed x density) to turn them, if more molecules (dense air) with same speed hit the blade it will apply more force therefore it will spin the turbine faster and you will read higher wind speed than molecules are moving at same speed but less dense air.
So that logic tells me i would read wind energy from an anemometer.
Can you explain how its otherwise please?

Calculated wind predictions maybe a different story, cold humid 15kts maybe stronger than a hot dry 15kts at same spot.

Cup system takes net force of two opposite forces due to different aerodynamics of cups sides, but blade system has one direction and same aerodynamics all around.
Is it possible cup style and blade style give different results depending on density and cup shape?

For me if include foiling, light: 10-16kts, if only fin light: 13-18kts, med: 18-25kts, strong: 25+kts, 61kg

Good point about speed x density, but I am not sure it would affect the wind speed with a blade anemometer, at least. Blade measures speed of wind while offering essentially no resistance, if it was waterproof and you put in water it would measure water speed too, without taking into account density of the water. But when wind hits a sail it generates pressure on the sail, since sail does not spin out of the way like a blade, and the denser the wind the more force is applied to the sail. Thinking that as the wind hits a sail it compresses, and denser air compresses less so energy transfer occurs faster than thin air.

As for light wind, 8-10 knots on freeride/freerace foil.

Lets say for water turbine, does it measure speed of water or debi? Thats where i am getting confused if knot is a wind speed measurement or something else like debi in air terms.
Other extreme would be very high in atmosphere close to space, would the anemometer outside show the baloons real speed (assuming no wind there)?

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Paducah said..

Gestalt said..
i looked into buying a wind meter that measured density but they are a bit costly.

At least in the US, one can get the density altitude readings from local airports which gives a good index. You can also work it out with some online calculators:
www.weather.gov/epz/wxcalc_densityaltitude
airdensityonline.com/free-calcs/

and I've used the second link to get the air density at a race track near where we sail. Air density is a big deal for tuning race cars.

The effect of cold air can be huge. Where I live, the difference between summer and winter can be a sail size (about 15%). In altitude terms, like going from sea level up to 1400m.

will do some digging of local airport data and see what i can find. thx.

If you think the blades of the device as windsurfers, if they feel more power in their sails when its cold they will go faster, therefore device will show high wind speed.

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Sea Lotus said..
Lets say for water turbine, does it measure speed of water or debi? Thats where i am getting confused if knot is a wind speed measurement or something else like debi in air terms.
Other extreme would be very high in atmosphere close to space, would the anemometer outside show the baloons real speed (assuming no wind there)?

Space is a vacuum, so no atmosphere or wind like on Earth, but there is solar wind made up of particles that can be caught by a sail coated with a very thin metal film and accelerate the sail and whatever it is attached to.

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Gestalt said..
at the moment in qld you can have a wind meter at the shore and it still won't help.

one minute 20 next minute 10.

A rain guage would be handy on the wind meter lately.

I get going in lighter winds to when it's really cold. saw a documentary on sail GP a few weeks back and one of the teams were talking about how they had to re trim their boat when the temp dropped to account for the density of the air.

Hang on a sec. Isn't the wind speed measured with the force that propels the wind gauge?

No, force is only measurable if there is some resistance, then force is needed to overcome it.
As others have explained, a good anemometer is designed to be friction less, so no force is need to turn it.
If you connect the blades to a small generator and connect the output of that to a load of some sort, you can measure the watts produced, which will be a measure of wind force.
As said this is also what windsurfers need to sail.

So what we really need to know is how many watts we need to get going and then have a way to measure that.

wouldn't be to hard to put sensors on the boom and mast foot and even under the deck of the board to measure power going into it.

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Ben1973 said..
So what we really need to know is how many watts we need to get going and then have a way to measure that.

wouldn't be to hard to put sensors on the boom and mast foot and even under the deck of the board to measure power going into it.

Now I think we are getting force and power mixed up.

You know what I mean

The force pushing the board forward is coming both from the mast base and the feet.
Is the force on the boom head, the same as the feet?
Not quite sure where the force on the fin fits into this.

What is light wind ????

Only eating half the can of baked beans,

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lotofwind said..
What is light wind ????

Only eating half the can of baked beans,

Username checks out.

Im the expert on the subject .

Just need to water proof this
www.sciencedirect.com/science/article/pii/S187770581400561X?ref=pdf_download&fr=RR-2&rr=70fee7d95bcd3308

Doesn't sails/blades create force due to different pressures in both sides of wing? That force is then redirected to motion (less friction is more efficient ofcourse), after motion appears we are eble to calculate rotation speed of mill then apply a "knot" value (with calibration from another rpm values) to that irrelevant rpm value.
At core what is causing that rotation is net force or torque over the axis, created by pressure difference on wings and blades opposing air resistance while spinning+mill resistance.

If these are true, then we are measuring a force created by a flow, like in water, more flow more spin. Then we use that measurement to calculate speed.
If device is reading flow, which already includes air density, then 15kt reading in cold humid should hive same power on your sail as 15kts reading in warm and dry.

Don't get me wrong i am not arguing, just trying to understand this, maybe wrong thread to discuss sorry.

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Sea Lotus said..
Doesn't sails/blades create force due to different pressures in both sides of wing? That force is then redirected to motion (less friction is more efficient ofcourse), after motion appears we are eble to calculate rotation speed of mill then apply a "knot" value (with calibration from another rpm values) to that irrelevant rpm value.
At core what is causing that rotation is net force or torque over the axis, created by pressure difference on wings and blades opposing air resistance while spinning+mill resistance.

If these are true, then we are measuring a force created by a flow, like in water, more flow more spin. Then we use that measurement to calculate speed.
If device is reading flow, which already includes air density, then 15kt reading in cold humid should hive same power on your sail as 15kts reading in warm and dry.

Don't get me wrong i am not arguing, just trying to understand this, maybe wrong thread to discuss sorry.

A brick hitting you at 10kph is gonna hurt more than a sponge as it's more dense
cold air is more dense than warm air so that to will be more powerful.

as for your water flow idea, more water travelling at 10kph is still travelling at 10kph, same speed but more of it so more powerful. To increase the power you need to either increase the speed or weight.

to measure the power you would have to know the speed of rotation and resistance of the device used to measure it.

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decrepit said..
The force pushing the board forward is coming both from the mast base and the feet.
Is the force on the boom head, the same as the feet?
Not quite sure where the force on the fin fits into this.

The fin (and board to a degree) is resisting those forces, how much depends on wind angle. It would be easy to fit strain gauges on a fin to measure the loads there but that wouldn't tell us how much force is needed to get planing.

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Ben1973 said..

Sea Lotus said..
Doesn't sails/blades create force due to different pressures in both sides of wing? That force is then redirected to motion (less friction is more efficient ofcourse), after motion appears we are eble to calculate rotation speed of mill then apply a "knot" value (with calibration from another rpm values) to that irrelevant rpm value.
At core what is causing that rotation is net force or torque over the axis, created by pressure difference on wings and blades opposing air resistance while spinning+mill resistance.

If these are true, then we are measuring a force created by a flow, like in water, more flow more spin. Then we use that measurement to calculate speed.
If device is reading flow, which already includes air density, then 15kt reading in cold humid should hive same power on your sail as 15kts reading in warm and dry.

Don't get me wrong i am not arguing, just trying to understand this, maybe wrong thread to discuss sorry.

A brick hitting you at 10kph is gonna hurt more than a sponge as it's more dense
cold air is more dense than warm air so that to will be more powerful.

as for your water flow idea, more water travelling at 10kph is still travelling at 10kph, same speed but more of it so more powerful. To increase the power you need to either increase the speed or weight.

to measure the power you would have to know the speed of rotation and resistance of the device used to measure it.

Yes, if you throw a brick at 10km/h to one of the anemometers cup, device will show higher value than if you throw a sponge at it at 10km/h, because brick will spin it faster due to more momentum has hit it. So when you see a value on the device its not about real speed (10km/h) but its momentum, device is not measuring items speeds directly. If you are reading 15kts with both sponge and brick on the device then sponge (hot air) must be moving much faster, but you feel same momentum hitting you in both cases if it is showing 15kts.

I am guessing devices are calibrated from a sample which is calibrated in a wind tunnel. So they have simple list of x rpm= y knots to get calibrated.
They don't need to measure every friction or resistence to make a calculation, they just need to check consistency.
To measure speed we need to know distance travelled, there is no such measurement at this device either.

Not really as the force (wind) applied last for a long enough duration to spin the cups up to the same speed it's just cold dense air would spin it up faster.
If a particular of air is a gold ball and on a hot day there's 1 per meter cubed but on a cold day there's 4per meter cubed they are al travelling at 10kph but the 4balls are going to have a bigger effect when they hit something

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olskool said..

Gestalt said..
at the moment in qld you can have a wind meter at the shore and it still won't help.

one minute 20 next minute 10.

A rain guage would be handy on the wind meter lately.

Sunshine Coast my arse .

All I wanted to know was what people define light, med and strong wind as and what they would use in those conditions.

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Ben1973 said..
All I wanted to know was what people define light, med and strong wind as and what they would use in those conditions.

I'd also say that weird strength and density are related.

Therefore a strong wind in the UK for example, might be a few knots less than here.

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Sea Lotus said.. >>Yes, if you throw a brick at 10km/h to one of the anemometers cup, device will show higher value than if you throw a sponge at it at 10km/h, because brick will spin it faster due to more momentum has hit it. So when you see a value on the device its not about real speed (10km/h) but its momentum, device is not measuring items speeds directly. If you are reading 15kts with both sponge and brick on the device then sponge (hot air) must be moving much faster, but you feel same momentum hitting you in both cases if it is showing 15kts.

I am guessing devices are calibrated from a sample which is calibrated in a wind tunnel. So they have simple list of x rpm= y knots to get calibrated.
They don't need to measure every friction or resistence to make a calculation, they just need to check consistency.
To measure speed we need to know distance travelled, there is no such measurement at this device either.

This is an incorrect assumption for a friction less device, the cup will spin at the speed of the object hitting it regardless of the objects density. It will measure a feather travelling at 10km/hr the same as your brick.

The brick will of course have a lot more kinetic energy and will do more damage to anything that resists it.

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Wind Smurf said..
I'd also say that weird strength and density are related.

Therefore a strong wind in the UK for example, might be a few knots less than here.

Yes wind strength is related to density, but not wind speed.

Density altitude is probably the correction factor that you are looking for en.wikipedia.org/wiki/Density_altitude

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Ben1973 said..
All I wanted to know was what people define light, med and strong wind as and what they would use in those conditions.

Well that's what happens on this forum, one question leads to a whole pile more interesting ones.
Just like a family discussion, you have to keep butting in to get it back on topic.

But there's very few off topic police here.

And to prove it here's another very rare but related wind strength subject.
Lamina wind, is more effective on a sail than turbulent.
I've experienced this at Lake George, in light wind. Couldn't get planning where there were small ripples on the water, but just flowed onto the plane when we hit the glassy smooth, then dropped off the plane when we hit the small ripples. A really weird experience, totally opposite to what you'd expect.
The only explanation I can come up with, is where there's ripples, even small ones, there's turbulence, where it's glassy smooth there isn't. I think this means that effective wind is over the whole sail in laminar flow, where as in turbulent flow the bottom of the sail doesn't work as well.