Merged DDWFTTW in a “Real” Wind

[windgrins]

A perfect Forum for Humber's expertise!

Humber,

I have a forum made just for you that just started up.

Under science, see "Merry go round on a train" on this forum. (No kidding)

PS, I'm not implying that you should leave this one. Check it out!

Regards,

WindGrins

 

[Myriad]

Great post. I am tempted to think it might be enough on its own to completely answer all questions regarding the ddwfttw cart. I can't see what else there is to explain. But I admit I don't wear a tinfoil hat...

Thank you -- but based on (my own and others') experience with this topic, I am certain that while (I hope) that explanation could help some people to better grasp the nature of the mechanism, it will leave others just confused, and still others doubting its validity in various ways. And that's just people with honest doubts and misunderstandings, before the trolls even chime in. No single explanation works for everyone -- or even, so far, for any majority.

One question my previous post doesn't answer at all is how do we know the impulse added by the total of all tennis ball collisions (or one single "average" collision) is sufficient to overcome the resistance to forward motion caused by the torque on the propeller from the same collision(s), at any operating speeds above wind speed. You could work that out with elastic collision calculations and a lot of geometry using the tennis-ball-current model, but that question is more easily addressed in other ways (e.g. examining thrust to drag ratios of real propellers under various conditions).

That question didn't seem to be ynot's concern, though; he appeared to be interested specifically in how the cart could slow down the wind (and hence get pushed by the wind) while moving faster than the wind, as observed from a ground reference frame. That question is usually approached by mechanical analogies such as the ruler cart or parachute cart, which are fairly clear as to how they work and are good for conceptually establishing that DWFTTW is possible in principle, but can leave it a little difficult to go from there to understanding how the propeller accomplishes something similar in the actual working wind-powered carts.

Respectfully,
Myriad

 

[ynot]

Imagine the wind as a stream of tennis balls, // . . .

cut to save space​

. . . . //which the individual particles are always moving at all different speeds and directions, so the cart at wind speed cannot run out of air molecules.

Respectfully,
Myriad

I agree this is an excellent “For Dummies” description. Not sure it contains anything I didn’t already understand but perhaps it puts it all together in a better way. Thanks to yourself and others that have given the time and effort to explain things without unnecessary arguments and insults.

 

[A.T.]

The propeller is spinng at a right angle to the wind.

The wheel(s) only need to turn inline with the wind.

How can something turn in line?

Seriously ynot, you don't make any sense. Most parts of the wheel are not moving parallel to the vehicle velocity vector (pointing DDW) but at an vertical angle to it.

 

[A.T.]

That question didn't seem to be ynot's concern, though; he appeared to be interested specifically in how the cart could slow down the wind (and hence get pushed by the wind) while moving faster than the wind, as observed from a ground reference frame. That question is usually approached by mechanical analogies such as the ruler cart or parachute cart, which are fairly clear as to how they work and are good for conceptually establishing that DWFTTW is possible in principle, but can leave it a little difficult to go from there to understanding how the propeller accomplishes something similar in the actual working wind-powered carts.

To address that question I added a red trace of an air molecule in the below animation. Wider spacing means higher speed. In the ground frame you can easily see that the air molecule is slowed down by the collision with the blade.



It is still a simplified model but closer to the actual aerodynamics than the gear analogies you mention above.

 

[windgrins]

Nice Illustration

To address that question I added a red trace of an air molecule in the below animation. Wider spacing means higher speed. In the ground frame you can easily see that the air molecule is slowed down by the collision with the blade.



It is still a simplified model but closer to the actual aerodynamics than the gear analogies you mention above.

Now that (I think) I understand the cart in reasonable detail. This is an excellent video. I have never been accused of being the fastest to catch on!

Good Work!

WindGrins

 

[ThinAirDesigns]

Nice job A.T. and Myriad.

JB

 

[recursive prophet]

@Myriad: I'll add my kudos on your tennis ball analogy. It did give me some new insights. This is not to say I'm still not a bit confused-my default-but it was an interesting way to show the slowing of the wind process. Did you catch John Freestone's retard tennis reply mender posted a week or so ago?

I just sent an email to I Ratant asking if he had ever tried out his cart on a TM. I'm hoping he will do such a test and post a video of it. Perhaps he could also get shots of his cart racing the popcorn on a longer stretch, having an assistant release both while he films at the far end. I wonder what the carteers will say if the foam pieces still beat a cart that has successfully advanced on the TM?

 

[windgrins]

An Interesting test

@Myriad: I'll add my kudos on your tennis ball analogy. It did give me some new insights. This is not to say I'm still not a bit confused-my default-but it was an interesting way to show the slowing of the wind process. Did you catch John Freestone's retard tennis reply mender posted a week or so ago?

I just sent an email to I Ratant asking if he had ever tried out his cart on a TM. I'm hoping he will do such a test and post a video of it. Perhaps he could also get shots of his cart racing the popcorn on a longer stretch, having an assistant release both while he films at the far end. I wonder what the carteers will say if the foam pieces still beat a cart that has successfully advanced on the TM?

I would say stick the cart in a actual wind that's blowing as fast as the treadmill is set for and it will probably beat the foam pellets if it is able to advance on the treadmill. We do need to put our money where our mouth is so that's my prediction!

An interesting test, RC.

Regards,

Windgrins

 

[Subduction Zone]

The cart on a treadmill demonstrates that the cart can break the speed of the wind. Ratant should be able to tell whether his cart was still accelerating or not at the end of its run. As spork said on their guided run with the small carts, it was not a long enough parking lot for the carts to get to full speed. Ratant may have the same problem. He should give his a test on the treadmill, but he may have a gearing problem that I believe JB saw that means his cart could not ever get above wind speed,

 

[spork]

Warning! - Written in “Dummies” speak. . . .

It seems to me that the only frictional advantage of a cart moving at the speed of a wind compared to a cart being pushed up to the same speed in still air is that the cart moving at the speed of a wind doesn’t experience any frictional losses to a headwind. Other than that however both carts experience pretty much (if not exactly) the same energy sapping frictional losses from rolling resistance, mechanical actions and the propeller spinning and thrusting in air.

In the same way that a cart pushed up to a speed in still air can’t sustain that speed merely by a propeller hitting and accelerating air molecules backwards to gain forward momentum from an equal and opposite reaction, a cart moving at the speed of a wind can‘t either as both carts are essentially in still air.

The only way a cart moving at or faster than the speed of a wind can constantly retain that momentum is if the energy losses due to friction are constantly being replaced by energy from the wind. As all parts of the cart are moving at or faster than the wind this energy can’t be transferred DIRECTLY to any part of the cart from the speed of the wind. This includes the propeller regardless that’s it’s spinning at a right angle to the wind. As it’s only the wind that provides energy to the cart it seems to me that there must be some part of the wind that that is essentially moving faster than wind so it‘s able to interact directly with the cart to enable the energy transfer.

I realise this directly contradicts claims that the cart can go faster because it slows part of the wind down.

If you want to understand it, you should ask - and then listen.

 

[spork]

The claim DDWFTTW implies ONLY directly downwind.

If you claim that two sailboats are travelling directly downwind when they are also moving from side to side as well as downwind then I don‘t accept your definition of directly downwind. Don’t know why joining two boats to a common bar is any different than claiming a single sailboat is going directly downwind even though it’s also moving from side to side.

As the propeller is also essentially moving from side to side when moving in a circle it’s also not going directly downwind so I guess I also don‘t accept the claim that the entire cart is going directly downwind and the question of how the cart can travel directly downwind faster than the wind is moot.

Perhaps I might have to see if I can build a cart that uses something other than a propeller to see if actual DDWFTTW can be achieved.

Your cart better not have wheels then, because parts of them will always be going directions other than DDW.

 

[spork]

When the very part of the cart that causes the rest of the cart to travel DDWFTTW isn’t travelling directly downwind itself then I don’t think it’s “tough” at all.

The very thing that makes your car go is the pistons. When you're driving North, the pistons almost certainly never move North. Does that mean your car can't be said to be going North?

Whether the part of the cart that doesn’t go directly downwind is a major part of the cart or not is merely a consequence of design and scale. If it were a very small cart with a very large propeller would you still say it was a valid DDWFTTW vehicle? Would a very small cart fixed to a rail going directly downwind being towed by a very large tacking land yacht be acceptable to you as a DDWFTTW vehicle?

It seems like you have to think about defining your terms in some objective fashion.

I think it may be possible to design and build a cart where everything on the cart is actually going directly downwind. Or at the very least the part that is causing the DDWFTTW travel is going directly downwind. I may have a go at building such a cart.

I very much look forward to that.

 

[humber]

Imagine the wind as a stream of tennis balls, all moving at close to the same speed. (This is in some ways a poor model for actual wind, where the velocity distribution of the component particles is much wider, and only net pressure matters on the macro scale. But it does simplify things. I'll point out, later, where the simplification might cause some trouble.)

Average molecular speed is 500m/s. So the balls are in 3D random motion at 500m/s.

What you're saying is, you want to see, from the ground reference frame, forward-moving tennis balls (wind) catching up to and hitting some part of the cart, being slowed down and so providing a push to make it go faster or at least maintain speed.

From the ground, there is an average drift velocity called "wind speed".

So, we start by building a cart with a big tennis racket mounted in the back, face-on to the wind. Put this cart into the tennis-ball current, and it will start moving, as tennis balls hit the racket.

The balls will decelerate against the tennis racket. The net force will drive the vehicles in opposition to that force.

But, when it gets close to wind speed, fewer tennis balls per unit time catch up to the racket and hit it, and when they do the relative speed is very small so they no longer give it much of a push. When the total net force is no longer greater than frictional losses, the cart won't go any faster. If you were to push the cart to wind speed, no tennis balls would hit the racket at all, and if you pushed it faster, tennis balls would start hitting the front of the racket and slowing the cart down.

The force will fall with increasing velocity wrt the wind. The cart will reach terminal velocity below WS, wheels or not, because the balls will loose momentum as a result of transfer to the racket, and so slow. The accumulation produces a high pressure zone in front of the racket.

The way to get around this involves two factors: one, the face of the racket has to be placed at an angle; and two, the racket has to be moving laterally relative to the cart; that is, in a plane perpendicular to the cart's trajectory.
We could rig some kind of mechanical forehand-arm to sweep the tennis racket side to side across the back of the cart while holding it at one angle when going left to right and a symmetrical angle when going right to left. But it's easier to just spin the racket around by the handle (on an axis perpendicular to the handle).

As the air has all but come to a stop at WS, to move the ball in any way will require energy from the device that accelerates them away.
If a static deflector were used, then less force will be derived from the former tailwind, as more of its KE is remained with the deflected balls.

If that sounds a bit unbalanced, we can start with a two-headed racket, with a head at each end of the handle, and spin it by the middle of the handle. The axis is perpendicular to the handle, and oriented parallel to the cart's trajectory.
That takes care of the motion, now we must consider the angle. One possible angle (call it zero degrees) is for the racket heads to be flat in the plane that they're spinning in. But that gives us no advantage (or any real difference at all) from the single stationary racket.

All need power for the same reason as above.

Another possibility is for the racket heads to be turned 90 degrees from the previous case. That turns the device into essentially a sideways paddle wheel. That would also be ineffective: it would bat any tennis balls that caught up with the cart (or that the cart caught up with) off in all different directions but it would only ever accelerate them in the plane perpendicular to the cart's trajectory, giving no benefit regardless of the apparent wind speed.

Nature is quite old and won't fall for tricks.

We can use Newton's impact theorem. Not accurate, but good enough.

Each air molecule strikes the prop over impact time, t

f =ma, so
f*t = ma*t (1)

V= at. ( the wind velocity derived from the acceleration)
f = (mV)/t (2)

During the impact time, N molecules will hit the prop, so the force is
Fd = N*f. (3)

To find the force, we need to know N, and the area hit by those molecules.

The prop has the disk area A. Instant deceleration is impossible, so as the molecules decelerate over time 't', the wind moves distance x. That is, deceleration takes place over a small but finite distance x.

Consider an area, A, of the blade.

Distance = velocity*time
x = V*t
Volume = V*t*x

Combining
N = A*V*t (Number/Volume)
Multiply N by the single molecule force, f.

Fd = m*(Number/Volume)*A*V^2

The density of the molecules (number/volume) can be found from p, the air density. The final product is:

So Fd = p*A*V^2.

That is the force that will accelerate the prop (cart) when in a tailwind, but slow it when a headwind, and does not depend upon "how far the prop moves relative to the air", but only on the amount of air over impact time t.


That equation makes many simplifying assumptions, but when refined as Rayleigh and other did, we get the familiar drag equation.

Fd = 1/2*Cd*p*A*V^2

 

[humber]

The air is a fluid. It is viscous. It flows over and sticks to surfaces.
A prop can't "move away from it". It is not a stick or tennis balls.
The above formula is simple case of momentum transfer, and will become drag and torque components on the rotor.

 

[Michael C]

First point to ponder: the sail of an iceboat does not point to the tailwind. It is at an angle to the wind.

That’s just the point I was making.

No, that's not the point you were making. You said:

As the sail of an iceboat points to the tailwind.

As I said, it doesn't.

So explain the physics of why such angles slow the cart down.

In order for you to understand that, you first need to produce a correct answer to my question. I asked:

And now here's a question for you: what is the direction of the thrust force of a propeller, in relation to the plane of rotation of the blades?

You answered.

It goes in the opposite direction into the wind hitting it.

That is not an answer to the question. I'm asking a question about the thrust force of a propeller. It could be any propeller: the one on Blackbird, one on a Cessna, one on a scale model of the Cessna... I want to know what is the direction of thrust in relation to the plane of rotation of the blades.

 

[humber]

No, that's not the point you were making. You said:
As I said, it doesn't.



In order for you to understand that, you first need to produce a correct answer to my question. I asked:



You answered.



That is not an answer to the question. I'm asking a question about the thrust force of a propeller. It could be any propeller: the one on Blackbird, one on a Cessna, one on a scale model of the Cessna... I want to know what is the direction of thrust in relation to the plane of rotation of the blades.

Air molecules move between 500m/s and 700m/s. Not much chance of a prop avoiding them. I wonder why all text books talk of laminar flow and viscosity, but not of tethered ice-carts?

 

[zooterkin]

Air molecules move between 500m/s and 700m/s. Not much chance of a prop avoiding them. I wonder why all text books talk of laminar flow and viscosity, but not of tethered ice-carts?

Ah, I see, so you're doubting the claim of 'faster than windspeed' because the cart is moving slower than 500m/s?

 

[ThinAirDesigns]

How can something turn in line?

Seriously ynot, you don't make any sense. Most parts of the wheel are not moving parallel to the vehicle velocity vector (pointing DDW) but at an vertical angle to it.

Yep -- by Ynot's own definition, there can be no rotating parts (no wheels, no axles, no rolling bearings, no pullys/belts, no chains nor sprockets, no driveshafts, etc, etc.) unless those rotating parts can be removed and the vehicle still function.

I wish him the best in his new quest.

JB

PS: Of course he won't meet (or even attempt to meet) his latest definition, but will work on something based on some *other* newly created definition and then once again explain to us how that definition is also somehow more technically correct than the one we used.

 

[jadebox]

Once you get past the point of no wind=WS, the cart itself feels a headwind so this clearly is not the wind that allows the cart to accelerate up to 2.8X windspeed if you think about it. In the description I gave, it is always true below WS, at WS, and above WS. Eventually one will reach a practical limit in that there is not enough "angle of attack" of the blades to overcome the losses.

I still think that talking about wind tends to confuses the issue. The cart is moving with the wind. You can think of it as a part of the wind. What's moving, to the cart, is the ground. The cart uses the moving ground to drive the propeller.

Think of an airplane taking off with the wind. The airplane's propeller doesn't function any differently as the airplane accelerates up to then faster than the "wind." It's the same for the cart.

Yes, there are limits to how fast a propeller can drive a vehicle, and, yes, the thrust of the propeller has to overcome the drag of the vehicle. But, there's nothing significantly different, in regards to how the propeller functions, when the cart is going below, at, or above "wind speed." In all cases, it's providing thrust to accelerate the vehicle.

-- Roger