Do you still call this "boundary layer" on the big scale? I guess so.
I mean that the wind at 1m above the ground is less than the wind at 5m above the ground. - There is energy to be gained by mixing these two airmasses. (In this case with the propeller).
Now, the problem I have is working out how it still takes energy from the wind while travelling faster than the wind...
It's clearly possible, since you can tack downwind and arrive at your destination faster than the hypothetical balloon drifting in the wind - this is done every day by sailors.
Also there does not need to be a correlation between ambient windspeed and the velocity of your vehicle. Imagine a hypothetical device of non specific design for catching energy from wind. It could be very large and catch a lot of energy. However the vehicle itself could be very aerodynamic and not require much energy to slice through air.
So you see how energy harvested from wind is not necessarily directly linked to energy required to overcome wind resistance - unless you are a square rigged sailing ship.
@molgrips "You can tack downwind and arrive at your destination faster than the hypothetical balloon drifting in the wind"
.. Can you really? - even when the destination is directly downwind of the starting point?
I was not sure on that one, so I didn't suggest it!
I can see how the treadmill replicates the 'no wind' with velocity situation. But there is external power too
The treadmill video is the key. There's no wind to speak of but it simulates the point at which the real wind-driven vehicle reaches the same speed as the wind. So no relative wind. And you see the model accelerate.
Can you really? - even when the destination is directly downwind of the starting point?
Pretty sure you can. Did you read the comments after the original article/vid?
No I didn't read the comments. Thing is this actual vehicle is having it's blades driven by the wheels. There MUST be another source of energy input. Otherwise, when it reaches windspeed, this becomes a perpetual motion machine.
The corollary would be an aircraft which flies forever with one propeller (windmill) driving another (tractor), but in this case the "mill" is the wheels on the ground. Both clearly impossible, so it must be getting it's extra energy from the wind gradient.
Otherwise, when it reaches windspeed, this becomes a perpetual motion machine.
No it doesn't. It's getting energy from the wind. It just so happens that the energy harvested from the wind is greater than the energy required to push it forward. The two aren't linked necessarily.
The corollary would be an aircraft which flies forever with one propeller (windmill) driving another (tractor)
Not really the same. A plane travelling the same speed as the wind is effectively dead still. This car travelling at the same speed as the wind has ground travelling under it, which adds a different energy system.
Focus on the model on the treadmill...
I'm going for lunch now, and then I have to do some work. I'll have a think and get back to you with the definitive physical analysis later on 🙂
My thought is this: Consider the craft equalling the windspeed measured (for example) at the altitude of the prop hub. The top of the blade arc will still be in a small tailwind, whilst the bottom will be experiencing a small headwind
I love your analysis, mountaincarrot - unfortunately it's wrong, as can clearly be seen from the fact that the top of the blade arc doesn't have a small tailwind given the vehicle is travelling faster than the wind up there is. There's simply not that much wind gradient.
BTW where do you fly - I'm just getting into model aircraft and curious to see others on here.
I'm familiar with the idea of reference frames, though I'm not sure how they explain my underlying concern about how the car can accelerate past the 'no wind' situation, without an external motive force, be in stored inertial, or other.I can see how the treadmill replicates the 'no wind' with velocity situation. But there is external power too.
I figured you probably understood this stuff, hence the leading question. The point is that from the reference frame of the vehicle, the conditions are exactly the same for the cart on the treadmill as they are for the full sized vehicle when it's travelling at exactly the same speed as the wind. In other words the treadmill experiment is a perfect simulation of the real vehicle in wind.
You could if you wanted to put the treadmill in an enclosing bubble on the back of a vehicle. Accelerate the vehicle up to the same speed as the wind, and also make the treadmill go (backwards) at the same speed as the external wind. Nothing has changed for the treadmill experiment from it's inertial reference frame so it will still work just the same.
Clearly at this point you could remove the bubble and it would make no difference to the experiment as the apparent wind is zero. You could then take the vehicle off the treadmill and put it on the ground and that would make no difference either, as the top surface of the treadmill is stationary relative to the ground. Now you have the cart in zero apparent wind moving forwards at the same speed as the wind and wanting to accelerate...
Agreed aracer, hence you need another energy source!
With linear wind, once the craft reaches windspeed, it's effectively dead calm. There is simply no where for any energy to come from to accelerate it further - unless you accept that parts of the craft are experiencing differing windspeeds, ie wind gradient. But do look up "dynamic soaring". The propeller on this craft must effectively use "Captive dynamic soaring" (I just inventend that term). -Watch my vid and explain how that model flies in circles!
Once the craft is going faster than the wind, then I believe it [I]CAN[/I] be driven, and losses made up by using the wind gradient.
On the other point, I read the sailing comments link, and I'll certainly believe that a boat sailing broad reaches could get downwind faster than the same boat sailing directly downwind. But but I'm still not yet convinced it could do it faster than the windspeed.
I've thought about this long and hard for the last day or so. I can conclude that the actual answer is 'its magic'.
HTH
Wee bit of maths to put on it:
The power used by the propeller is the propeller thrust x the airspeed
Pp = T x Va
The power "generated" by the wheels is the resistance to forward motion x the ground-speed.
Pg = R x Vg
Now, the resistance to forward motion includes the torque required from the wheels to power the propeller and also the aerodynamic drag, rolling resistance of the tyres, etc. But hopefully we can agree, for the sake of argument, that these drag forces are relatively small. Let's lump them in with "transmission losses" and call it efficiency (m), which will be a number less than 1.
So, the power available at the propeller is the power "generated" at the wheels x the efficiency.
Pp = Pg x m
So T.Va = R.Vg.m
Now, if you're travelling just a little bit faster than the wind, what does that do for the equations above? Let's say there is a wind-speed of 19m/s and you're travelling at 20m/s. You therefore have a ground-speed of 20m/s and an airspeed of just 1m/s.
So T.Va = R.Vg.m gives you T = 20 x R.m
So, assuming m > 0.05 , then Thrust is bigger than resistance, so the vehicle accelerates.
We'd expect m to be pretty high, certainly much higher than 0.05, 'cos transmission losses in a mechanically-geared system will be pretty low, aerodynamic drag will be pretty low (you only have an airspeed of 1m/s remember) and rolling-resistance is never terribly big.
If there was no wind, then there would be no difference between airspeed and ground-speed, so there would be no unbalanced force available to drive the vehicle.
Clear as mud?
So hang one... This works with a propeller type arrangement, but would the same sort of motion/energy capture be possible with a cylinder-type turbine? (i.e. one that rotates around a vertical shaft, rather than blades around a horizontal one)
I only ask because I got blown to shit on the way home and back this lunchtime on my bike, and the idea of using some sort of wind-powered vehicle on this sceptered isle as a means of transport appeals to me.
Just put a sail on the bike! (BTW I did once and it worked OK. Quite exciting till I ended up in a ditch).
And a vertical axis mill would never allow you to go faster than the wind whilst downwind according to my concept of mixing the vertical wind gradient - Which is the main point being made here.
Stevo, power is a good way of looking at it, that's where I was struggling over lunch trying to use forces 🙂
There's a slight difference between the real world car and the model on the treadmill. The model is held still until it gets up to speed, which means it's forced into the situation where apparent wind = 0. The real car would never reach that condition because there'd need to be enough relative wind to match rolling resistance.
So I think that the tower the prop is mounted on is an aerofoil to provide a thrust vector with just enough forward component to overcome this. Wouldn't have to be much.
Mountaincarrot - it's got nothing to do with vertical wind gradient. Otherwise, how would the model accelerate?
The model initially accelerates because of prasictc drag from the superstructure.
The main point is that in a linear wind there is simply no energy input to make it move faster than the wind. It's a perpetual motion machine UNLESS you have another energy source. So what is that if it's not the wind gradient?
If this thing can work by any other means, then if you put it on a moving converyor belt, it would drive off the front on it's own. THAT won't happen.
There. I had to get a conveyor belt in somehow.
in a linear wind there is simply no energy input
Energy is extracted from the difference between the windspeed and groundspeed. If it worked with no wind (relative to the ground) that *would* be perpetual motion.
Something that might help get your head around it: as was pointed out above an ordinary sailing boat can outrun a tailwind by tacking across it. In this case the propellor blades are, in effect, also tacking across the wind as they rotate.
If this thing can work by any other means, then if you put it on a moving converyor belt, it would drive off the front on it's own. THAT won't happen.
Is that not exactly what the model video shows above?
The treadmill video is the key. There's no wind to speak of but it simulates the point at which the real wind-driven vehicle reaches the same speed as the wind. So no relative wind. And you see the model accelerate.
The problem with this is that, in this scenario, the treadmill is turning the wheels at a rate fast enough to generate lift on the propeller. I think at zero, wind or to be more accurate, when the balance of forces is such that there is no overall force (some balance between the motive force on the wind and system friction) I cant see where the acceleration comes from to 'tip' it over. The treadmill works, because it is already moving fast enough to provide the energy to propel the car forward. In effect the car is being dropped into headwind.
I'm not sure I see Mountai carrots reasoning, but it is consistent with the idea that the car would not work if there wasn't wind.
For comparison-
Speed recordThe world land speed record for a wind powered vehicle was broken on 26 March 2009 by Richard Jenkins in his yacht Greenbird with a speed of 126.1 mph (202.9 km/h).[b]Wind speeds were fluctuating between 30–50 mph (48–80 km/h) at that time[/b].
The previous record of 116 mph (187 km/h) was set by American Bob Schumacher on March 20, 1999 driving his Iron Duck vehicle. Both records were set on Ivanpah Dry Lake near Primm, Nevada, USA.
But this would not have been directly downwind but on a very broad reach. A kitesurfer has also just popped the magic 100kmh mark to set a new water based record. When I was windsurfing a lot 40mph had just been beaten - never thought I would see the day that record had been beaten by over 50% by a bloody tea-bagger!
Yet to get my head around this bladed nonsense.
I cant see where the acceleration comes from to 'tip' it over.
If you look at my worked-example above, you can see that you actually get the biggest thrust to drag ratio when the speed of the car is a very small amount greater than the wind speed (therefore biggest ratio of ground speed to airspeed).
In a very theoretical steady wind, then this might be an issue. In the real world it's not as the slightest drop in wind speed will give you a decent amount of thrust to accelerate away. Equally, if you're already accelerating as you approach windspeed, you may have enough grunt to carry you over.
I cant see where the acceleration comes from to 'tip' it over
That thing the prop is on is an aerofoil I reckon. I bet it's moveable to gain just a smidge of forward thrust.
That thing the prop is on is an aerofoil I reckon. I bet it's moveable to gain just a smidge of forward thrust.
Not in the direction of (or opposite) the wind, i don't think.
Yet to get my head around this bladed nonsense.
Do you mean aerofoils etc?
The best way is to think of an aeroplane wing. The flow over it generates enough lift to pick a jumbo jet up. now imagine that instead of a jumbo, you hand a windsurfer attached to it. The force would be pretty much the same but with much less mass, the acceleration is governed by F=ma. So there would be huge acceleration, till the drag (and weight) force, was equal (and opposite to the lift force). As such the speed the wing travels at (in general perpendicular to the flow direction) is sort of not generally dictated by the speed of the wind.
Ex pilot so happy with the concept of aerofoils - it's the possibility of a vehicle acting in the manner shown on the vid that I am yet to comprehend.
Ex pilot so happy with the concept of aerofoils
Phew!
Design team's own explanation here:
[url= https://docs.google.com/View?docID=0AdRsKX7aaZTPZGRnbjhkajdfMTY0aGRzNWtnaGM&revision=_latest&hgd=1 ]Thin Air Designs FAQ[/url]
That's almost identical to mine! 😀
5 years of Mechanical Engineering weren't wasted after all!
They keep saying 'tailwind' but they must mean 'headwind' surely? At least in our cyclist terminology.
They keep saying 'tailwind' but they must mean 'headwind' surely? At least in our cyclist terminology.
The whole concept relies on a "tailwind".
It's a tailwind when you are stationary, but when you are travelling faster than [s]light[/s] the wind speed you have a headwind, no?
It's a tailwind when you are stationary, but when you are travelling faster than light the wind speed you have a headwind, no?
In cycling terms, No. On a bike no matter which direction you go or how fast, there will always be a headwind.
I could think of nothing else as I rode home and I'll concede the error in mty previous theories of wind gradient. (Dynamic soaring remains lots of fun..).
The Thin Air explanation seems to have it.
Doing away with calculus because I'll confuse myself, a non-rigorous power explanation at least satisfies me.
Say the car is already moving at the windspeed V so there is zero relative windspeed. A force f at speed V extracted from the wheels as a propeller drive would tend to slow the vehicle and provide extracted power fV. In order to maintain the vehicle speed and ideally make it go faster, a larger force F must be applied by the prop. This power The prop applies it's available power relative to the local airspeed (currently zero). So it's easy for extracted power fV to apply a large push force F against the (already moving) air because v is small, and locally tending to~0.
So F>f and so overall the vehicle will accelerate beyond windspeed until they are equal.
QED? well it satisfies me anyhow.
ok, Here is the best explanation i have seen
Start with a vehicle where the spinning of wheels on the ground drive the propeller which moves the machine. Let's assume, at first, that it is a perfectly frictionless machine, perfectly aerodynamic, with 100% efficiency in the propeller.In dead air, if you pushed it forward, it would continue to move forward at exactly the speed you pushed it because the wheels drive the propeller which keeps it moving.
If there was a slight tailwind, then it would continue to accelerate. The wheels would always be pushing the propeller a tiny bit faster than what is needed for the speed it is currently going, because the air speed is slightly less than the ground speed. Suppose the tailwind is 1 kph. When the wheels are rolling at 2 kph, they are spinning the propeller fast enough to go 2 kph, but that is 2 kph relative to the air around it. Since the air is still moving at 1 kph, that means the propeller is trying to drive the vehicle at 3 kph relative to the ground. Once you reach that speed, the wheels are now pushing the propeller enough to go 3 kph relative to the wind, which means 4 kph relative to the ground. etc.
Now re-introduce reality. There are four energy drains to overcome: First is the internal friction of the vehicle, which can be made to be very small with good engineering. Second is the wind resistance, which is not significant until you have at least beaten the challenge of exceeding the wind speed, and can be reduced with good aerodynamics. Third is the internal efficiency of the transfer of energy from the wheels to the propeller (which their blog says was the biggest challenge). Finally (though they don't mention it), there is the need to salvage all the energy from the ground speed. If the wheels start to slip, for instance, you don't get all the energy pushed back into the propeller. They had to introduce friction here, but it is rolling friction, which is pretty tiny.
