Driven by wind, downwind, faster than the wind...

[turboplanner]

16 hours ago, Bruce Tuncks said:

Thanks Ibob, I finally found  Drela's paper and followed the argument through. While it still seems counter-intuitive, I am forced to accept the logic.

Well I have an electric bike and this can of course be powered by wind energy and go faster than the wind, but this is not counter-intuitive for me.

 

In that case a huge wind farm produces the power to charge a big battery which drives a small motor for a finite time.

That's a huge energy source for a little job that can't continue indefinitely.

So I agree; easy to understand.

 

An electric bike carrying a battery plus a generation system small enough to be a sub component of the bike is a different story.

From the power output of the wind generation system you have to deduct gearing resistance, rolling resistence of the tyres, surface coefficient (concrete or sand or mud .gravel etc), Startability resistance, Gradability resistence (unless the trial was on level ground), wind resistence (which increases with frontal area including the prop diameter, and the prop resistance calculations, and exponentially increases with speed.

 

Not so hard to understand that on level concrete is it slowly going to come to a stop, on an upgrade its going to stop then run backwards, and on a downslope, well bugger me its going to get fater and faster.

 

I agree with you on the mercury balance, but civil engineers would probably understand that.  I have difficulty comprehending force reactions, such as why if you bury a paper thin culvert pipe the reactive forces from the soil allow it to carry an axle group of 30 tonnes, or how a keystone in a flimsy arch will still hold up a huge cathedral.

 

[IBob]

Turboplanner, I live in a 1920s bungalow, with a header tank lifted as high as it will go in the roof space (not very high, about 1.5m at the base, due to shallow roof). The outfeed goes horizontal across the width of the ceiling space, then down a cavity to under the floor, where it is then split out and piped to the various taps and hot water cylinder.
The header tank is supplied by a pump.

If we allow this system to drain during a power cut, it will not then prime itself once we have water back in the header tank: nothing will come out of any of the taps, including the lowest, which are bath taps approx 700mm off the floor.
To get the system going again, I go out to a tap I installed which is almost on the ground and connected to the lowest part of the system. Turning this on results in about a minute of  gurgling and spitting, then a small amount of water, then full stream.

 

On the odd occasions when house-sitters have been told what that tap is for, they listen to this with an expression of benign disbelief. And even though I figured out why this happens, it's still oddly counter intuitive to have a tank in a roof and taps down below, but no water coming out...

I found this situation instructive when it came to building my aircraft: I went to some lengths to remove any undulations in the fuel pipework.

Edited October 18, 2021 by IBob

[turboplanner]

Centrifugal pumps do my head in at times; you go right through the system again and again and then finally find a small piece of grass is creating ir pockets.

 

[IBob]

Yep, I have an evac tube solar hot water system, with the collector some distance from the hot water tank. Took a fair bit of tinkering to get it settled down.

The supplier kept insisting the (multispeed) circulation pump be turned up due to the distance, in the end I turned it down to minimise cavitation as the water temp rises.

It still grumbles sometimes, but it works, and has been for some years now.

[IBob]

BTW and if anyone is interested, the performance of evac tube solar water heating is outrageous: I have the smallest 20 tube system and it provides all our hot water for about 9months of the year, and some of it for the other 3 months. The operating principals are totally different to the old flat plate technology. This thing is actually a heat pump that works even on overcast days.

 

Heading into summer, I am (again) looking at ways to dump the excess heat.

 

Ours is a low pressure system. My only reservation in a high pressure system would be the long term durability of the header that takes the tubes.

[pmccarthy]

Back to the downwind device. I can follow the logic and maths, but at a higher overview level I get stuck on a simple point. If it is travelling downwind at wind speed then there is no net wind and the rotor will not spin. If it is going faster than the wind then the rotor cannot draw energy from the wind, it draws energy from the vehicle. I think this has been said before, but it seems so obvious.

[IBob]

Maybe part of the confusion is around what is driving what.

As I see it, the wind is pushing the car (downwind).

If the car is moving, the wheels are turning.

If the wheels are turning, this causes the prop to turn, since the two are linked.

And the prop is pitched to blow backwards , into the wind.

 

It's the wheels that are driving the prop, not the other way round.
And if the car is travelling downwind at wind speed, the prop is turning, pushing back against that wind, so causing the car to go faster than wind speed.

[Thruster88]

If the wind is 10knots and the vehicle travels down wind at 15knots that is a 5knot head wind I think can all agree. So in a dead calm the same vehicle can travel at 5knots except we can hopefully see that is not possible. 

I dont think that scientist lost 10k, they were just seeing how many would fall for it.

[IBob]

Weird that folk can accept a vehicle travelling crosswind and on various headings at well over the windspeed, but they can't consider a mechanism for doing that in one particular direction......)

[Thruster88]

9 minutes ago, IBob said:

Weird that folk can accept a vehicle travelling crosswind and on various headings at well over the windspeed, but they can't consider a mechanism for doing that in one particular direction......)

There are numerous videos of boats and land vehicles going cross wind with a sail, it is easy to understand that principle.  

[facthunter]

By utilising the wind properly and coupling it to a drive (propeller or wheels) you can go directly into the wind. . You aren't getting anything for nothing. The Wind is the energy source. Nev

[facthunter]

When you Tack into wind  you always have a max angle you can get to before the back of the sail starts to collapse but you still get upwind (eventually). Nev

[pmccarthy]

Yeah but directly down wind at faster than wind?

[facthunter]

It's gotta be possible.  The prop is reacting with the air more than a flat sail just sitting there.  Nev

[onetrack]

The wind contains gusts which are faster than the average wind speed. The wind-propelled vehicle has momentum, and the propellor/rotor has momentum, and is also supplying "lift" once it is spinning - i.e., forward propulsion that is faster than the wind speed. Newtons Law says that an object in motion remains in motion at constant speed, and in a straight line, unless acted on by an unbalanced force.

[IBob]

Onetrack, I'm  pretty sure the issue of gusts etc has already been dealt with in part of the wider discussion this caused.

However, on reflection, I'm with Facthunter on the treadmill issue: not because I think the idea of using a treadmill is invalid, but because what we see there is a momentary snapshot quite probably affected the momentum of the prop. What we would need to see (for the treadmill experiment to be convincing) is a very long treadmill and the vehicle continuing to make it's way along it after the initial burst introduced by the operator holding it still.

[Hongie]

19 hours ago, Thruster88 said:

If the wind is 10knots and the vehicle travels down wind at 15knots that is a 5knot head wind I think can all agree. So in a dead calm the same vehicle can travel at 5knots except we can hopefully see that is not possible. 

I dont think that scientist lost 10k, they were just seeing how many would fall for it.

Neil deGrasse Tyson would not lightly put his name to something.