micro-sailing, thrust from turbulence

[lunatic]

Could some gains made upwind in gusty and choppy conditions be from "extracting energy from atmospheric turbulence" [http://aero.stanford.edu/reports/Stanford-OSTIV08.pdf"]? It seems both sail and keel are in a flow and motion that could produce thrust. Performance in these conditions seems to vary, even between tacks, could be variation of wind and wave train, or even asymmetry of rig and hull construction, or is there a sweet spot for thrust? and if so how to exploit it?
An easily observable example of "microlift"soaring are gulls gliding faster than the boat they are trailing, especially larger powerboats heading upwind with their turbulence added to the natural gusts. Question of scale, bird to boat quite a jump, maybe sailboards and ice boats are best able to exploit this thrust, though in extreme weather larger craft might benefit from conditions that look so bad. Controls beyond a frightened mind might be needed, wand driven foils and wind driven selfsteering are controls with unsteady inputs, but is there enough useful energy to be gained? There is for the gull, try to catch that show, it is quite a performance of sailing dead upwind, but in turbulence where is dead upwind

 

[CT 249]

I'm no expert on this, but from a bit of reading about microlift I can't see how it's very different from what we already do. For instance we're already using the wind created by swell movement a lot of the time IMHO, which is one reason we tend to pinch as we head up a swell (because we are then encountering the faster-moving air being pushed by the swell as it moves) and then bear away to power-up in the lull.

This is (IMHO) the same sort of thing that surfers encounter in big waves, and which can often be seen in pics of big wave surfing. The "gust" on the front of a big pitching wave is very easy to spot. In boats, the effects of the "swell gust" are sometimes attributed to the effect of the circular motion of the water particles that make up the swell, if I recall Bethwaite correctly.

However, the "swell gust" effect is very noticeable when wavesailing on windsurfers, or in small dinghies in light winds. And the mindset of racing sailors makes them very aware of the possibilities and problems of turbulence; as an example I can recall back in pro windsurfing race days when we actually got a fair bit of practice in using the downdraft of the TV helicopters overhead for propulsion.

Furthermore, the normal gust response of a rig is designed to extract energy from turbulence, isn't it?

Finally, I'm not sure if many keen high performance racers have frightened minds in strong winds!

 

[lunatic]

Nice to see my speculations are real to more skillful sailors. By "beyond a frightened mind" I mean a feedback system to maintain full exploitation of this phenomena.

 

[tspeer]

Look for papers on the Katzmayer effect. It turns out that the drag of a stationary airfoil with an oscillating angle of attack due to a changing wind direction is not the same as an airfoil oscillating in pitch to produce the same variation in angle of attack with a stationary wind. Oscillating in pitch results in an increased drag, while having the wind oscillate results in a decrease in drag, and even thrust.

Here's a simple way to look at it: If you are at the North pole, any way you go is South. If you have a wing angled for zero lift, any change in the wind direction is a lift. When you are bouncing from one lift to the opposite lift, your performance is going to look great when referenced to the mean direction.

I saw a really good demonstration of this with the trimaran USA 17. This boat had the largest wingsail ever built - 68 m tall. When at its mooring, it didn't hang back like an ordinary boat. In gusty conditions, it surged forward, moving forward and back as much as 10 m, due to the Katzmayer effect. One blustery night, with gusty 30 kt winds, two RIBs pulling back all night long on lines from the stern were needed to keep it from breaking free of its mooring.

People have been trying to exploit the Katzmayer effect for a long time.

 

[tspeer]

Could some gains made upwind in gusty and choppy conditions be from "extracting energy from atmospheric turbulence" [http://aero.stanford.edu/reports/Stanford-OSTIV08.pdf"]? ...

There is a sailing analog to the gust soaring in the paper. It's called, "tack on the headers, and jibe on the lifts."

 

[Richard Woods]

From memory, I don't have the book to hand. In Tony Marchaj's second book is an account of a study where a Dragon? was tethered in a lake with the sails up. The idea was to measure the forces on the tethers and thus the wind loads on the sails.

It didn't work. The boat just rocked violently to and fro. So the researchers renamed their paper "How not to..." So, as Tony said, the surprise really is how stable a boat is when sailing in a gusty wind.

And, as I've asked before, How can a hawk hover motionless but not a glider?

Richard Woods

 

[lunatic]

Seems utilization of Katzmayer effect best on the stable platforms of foilers and iceboats. Does Stanford paper indicate possibility of a glider hovering? maybe this requires a very nuanced feedback system. This may not be a Katzmayer effect, but I have seen Monarch butterflies leisurely flapping upwind miles offshore, there is muscular input but seems disproportionate to the thrust.

 

[JamesG123]

It has to do with Reynolds Number. The smaller the scale the "thicker" air becomes. This is why insects and hummingbirds can fly the way that they do, when it isn't practical at larger scales.

And, as I've asked before, How can a hawk hover motionless but not a glider?

Because gliders have higher stall airspeeds (30-40 kt.) than typical avg. windspeeds. Without thermal updrafts to give them energy, even the best glider has to trade altitude for speed to remain in flight. Also they lack the control authority at those speeds to maintain proper orientation and angle of attack. Hawks OTOH have a much lower stall speed and have very dynamic control surfaces (their whole body).

Some STOL aircraft can effectively hover in light winds because their engines provide enough thrust, lift, and airflow to maintain control while the wind gives them enough airspeed to maintain altitude.

Or... was that a rhetorical question?

 

[DCockey]

A hawk, or any other flying object, will also need to be in an updraft to hover motionless (no propulsion by wing flapping, propellers, jets, etc) in the wind at a constant altitude.

 

[lunatic]

Seems few limits on extracting energy from wind. Gliders, losing altitude in a down draft can gain more kinetic energy than they lose in potential energy. The Stanford paper implies possibility of hovering flight but for the problem of control. Try using a 200 year old sail trim system on your boat. Aircraft have set some good examples before but could be leading us astrsy this time into the interesting but impractical, superstall might be one.