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Questions about DDWFTTW

by Opus_723
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jduffy77
#163
Jan6-12, 04:50 PM
P: 40
Quote Quote by kmarinas86 View Post
Edit: You added:

By net energy, I mean there is a reference frame in which the net transfer of energy across the intersection between the depicted air stream and the propeller is zero.
I wish you would explain how you can think this.
kmarinas86
#164
Jan6-12, 04:51 PM
P: 1,011
Quote Quote by jduffy77 View Post
I do indeed. You seem to be having some problems with it at the moment though. You have made a correct statement here which should lead to an enhanced understanding of the cart mechanism, but instead it is leading you to incorrect conclusions.



Your diagram is incorrect. That is what I am trying to help you with. I think talking "deflection of existing energy" in the context of the cart is nonsensical.
Doesn't the angle of deflection of existing energy matter?

Quote Quote by A.T. View Post
The best way to avoid confusion is to be precise:

- Make clear which reference frame you are analyzing (power/kinetic energy are frame dependent quantities)
- Distinguish between "air" with "wind" (movement of air relative to something)
- Distinguish between true wind (relative to ground) with relative wind (relative to cart)
- Distinguish between work done by the cart chassis on the air, with work done by the propeller on the air.

Being precise in formulating the questions, often makes the answer obvious.


That is not necessarily true. Depending on the propeller pitch the acceleration can be not maximal at WS but rather above it. So the increase in KE (seen from the ground frame) is maximal there. But the power transmitted though the vehicle always increases with speed. Here I posted some simulated values:

http://www.physicsforums.com/showthread.php?p=3352297


From the ground frame: Some of the air is doing negative work on the cart chassis. But there is more positive work done on the propeller blades by the air.

The propeller always slows down air relative to the ground. The faster you go, the more volume of air you encounter, that you can draw KE from. But that increase is linear, while chassis drag and transmission inefficiency increase non linearly with speed.





Have a look at the table below that shows different settings for a variable blade pitch propeller, coupled to the ground via wheels. What you describe above is starting out in CASE A (that gives you maximal initial acceleration) and then at some point below 1WS switching to CASE C that allows you to go faster than wind.



Note that the Blackbird didn't have that ability (even when it had variable pitch later). They didn't want the ability to turn the wheels with the prop, to avoid confusion about using stored energy. They used CASE C only.

But Andrew Bauer was using his propeller as a turbine below windspeed. Here is video where you can see him starting in "windmill mode" and change the blade pitch later.
http://www.fasterthanthewind.org/201...f-ddwfttw.html
See also the graphs on page 15 in Bauer's paper.
http://projects.m-qp-m.us/donkeypuss...-Interface.pdf
"Make clear which reference frame you are analyzing (power/kinetic energy are frame dependent quantities)"

I probably wasn't clear enough as my analyses were often switching from one reference frame to the next. Most people on this forum can only think about one reference frame at a time. It's easy for me to switch them, but I guess that hard it's to notice the reference frame switching from my dense way of writing.

BTW: I am from the point of view that potential energy is itself a frame-dependent quantity that balances exactly the changes in kinetic energy (dKE/dPE = -1), such that things like inertia and their effect on gravity are frame-invariant. But that is a totally different subject anyway.
jduffy77
#165
Jan6-12, 04:53 PM
P: 40
Quote Quote by kmarinas86 View Post
Saying this means you deny that the cart involves the deflection of existing energy. That's like saying that no matter can be changing direction in this system.
Does burning a piece of wood or hitting a baseball involve "deflection of existing energy"? If that is the sense you mean it in then ok. It seems a strange way of putting things.
jduffy77
#166
Jan6-12, 04:57 PM
P: 40
Quote Quote by kmarinas86 View Post
Doesn't the angle of deflection of existing energy matter?
The energy for the ddwfttw cart comes from slowing down the wind with respect to the ground. Is that the "deflection of existing energy" that you are talking about?
kmarinas86
#167
Jan6-12, 05:07 PM
P: 1,011
Quote Quote by jduffy77 View Post
The energy for the ddwfttw cart comes from slowing down the wind with respect to the ground. Is that the "deflection of existing energy" that you are talking about?
By deflection I mean an angular change of the direction with respect to some surface. If by deflection I meant the actual thing deflected, then I should really call this "deflected matter". So what you are taking about is "deflected matter" as seen from the ground point of view.
jduffy77
#168
Jan6-12, 05:15 PM
P: 40
Quote Quote by kmarinas86 View Post
By deflection I mean an angular change of the direction with respect to some surface. If by deflection I meant the actual thing deflected, then I should really call this "deflected matter". So what you are taking about is "deflected matter" as seen from the ground point of view.
Air is certainly being deflected by the prop. It is being shoved out the back which gives an equal and opposite reaction. That's pretty much the definition of thrust.

This does not help your first post today make any sense however:

Quote Quote by kmarinas86 View Post
So it does not even seem to me that the kinetic energy of the wind even needs to be extracted by the cart in order for this to work. All there needs to be is a deflection of existing energy. After all, if we discount the hypothesis that potential energy is somehow relevant in this system, while at the same time we assume the conservation of energy, then it would stand to reason that "kinetic energy+heat" is conserved both before and after the force interactions. If the air does not lose kinetic energy in the reference frame of the intersection, then neither could the rest of system gain it.

Finally, I would also like to point out that at from 1:07 to 1:22 in the video, the amount that the energy in and the amount of energy out is calculated to be different, even though the friction was not modeled! Clearly the video is not taking into to account the work done tangentially. It turns out that the video as a whole takes into account the work done along the path of the wind and the cart (horizontally that is), but not the tangential work. For example, consider how much faster the prop moves cuts up through the air than the air itself does. If they took into account the work done by the propeller onto the air tangentially to the wind, it turns out that net work that the wind does on the rest of the system, and vice versa, combining both horizontal and tangential components, is zero from the frame of reference of the intersection between the air propeller.
A.T.
#169
Jan6-12, 06:21 PM
P: 3,900
Quote Quote by kmarinas86 View Post
How then does this cause the kinetic energy of the combined mass of the cart and the ground to increase relative to the intersection point?
In the frame where the air's KE doesn't change, the carts KE increases while the grounds KE decreases.
Quote Quote by kmarinas86 View Post
The air bounces off the surface of the airfoil at the intersection point.
Keep in mind that this is strongly simplified picture. The real airflow is quite different, but the result in terms of force on the blade is the same.
Quote Quote by kmarinas86 View Post
If the air does not lose kinetic energy in the reference frame of the intersection, then neither could the rest of system gain it.
See first comment.
Quote Quote by kmarinas86 View Post
Clearly the video is not taking into to account the work done tangentially.
Work is done only along the direction of movement, at the rate P = F dot v (the dot product cancels the tangential force component). It is an idealized model that assumes just a short interaction with the blade. The inefficiencies of a real world propeller (like swirling the air tangentially) constitute the losses.
kmarinas86
#170
Jan6-12, 07:03 PM
P: 1,011
Quote Quote by A.T. View Post
If the frame where the air's KE doesn't change, the carts KE increases while the grounds KE decreases.

Keep in mind that this is strongly simplified picture. The real airflow is quite different, but the result in terms of force on the blade is the same.

See first comment.

Work is done only along the direction of movement, at the rate P = F dot v (the dot product cancels the tangential force component). It is an idealized model that assumes just a short interaction with the blade. The inefficiencies of a real world propeller (like swirling the air tangentially) constitute the losses.
Thank you for spending the time to answer.
kerosene
#171
Jan26-12, 01:58 PM
P: 1
I did the animation of spiral track sail to prop.

2 things I think will help people to understand what's going on.

1. Sail carts can frequently "beat a balloon" downwind with a significant factor. Downwind velocity component being 2-2.5 x wind speed. This is well documented and can be studied in many places. People follow their intuition and think that the dw component (for some reason) cannot exceed wind speed. If you argue against this fact then read about it 1st.

2. The wind cart works by using "gearing" and the relative speed differences of the two different interfaces. You can can turn a resisting force to a larger pushing force as long as the speed differences support this.

The breaking power at wheels (ground to wheel) could be for example 10N at 10m/s =100w. Same 100w at 5 m/s apparent wind speed can generate 20N thrust.

Obviously there are efficiencies at play in real application but by using above thinking you can throw different numbers to check what is going on at different wind and cart speeds. You will see that there is plenty of "excess" to be wasted - and thus the cart does work.
kmarinas86
#172
Feb11-12, 08:40 AM
P: 1,011
Quote Quote by kerosene View Post
People follow their intuition and think that the dw component (for some reason) cannot exceed wind speed.
The one and only thing that first got me to accept that this is possible was to think of the effective velocity between the air mass and the blade. The difference between the wind velocity vector and the effective velocity vector can have a sign opposite of that between the wind velocity vector and the craft velocity vector. Because of this reason, both DDWFTTW and DUWFTTW are possible.

Evidence is now easy to come by: youtube.com/results?search_query=Dynamic+Soaring

Thinking about tacking wasn't helping that much. As evident on this thread, I and many others wasted a lot of time on that issue. I suggest that next time someone doubts that this is possible, one can skip the whole discussion about tacking and go directly towards talking about the effective velocity between the air mass and the rotating, pitched blade in the sense of vectors (velocities), not scalars (speeds). It's the only thing that should really matter here.
spork
#173
Feb11-12, 09:01 AM
P: 203
Quote Quote by kmarinas86 View Post
Thinking about tacking wasn't helping that much. As evident on this thread, I and many others wasted a lot of time on that issue. I suggest that next time someone doubts that this is possible, one can skip the whole discussion about tacking and go directly towards talking about the effective velocity between the air mass and the rotating, pitched blade in the sense of vectors (velocities), not scalars (speeds). It's the only thing that should really matter here.
We have found that there is no single explanation that works for everyone. Whenever someone finally "gets it" they usually ask why we wasted their time with all the other useless or even wrong explanations. But the fact is, we have a whole bunch of different explanations. They're all accurate, and each person seems to respond to a different one - and think the rest are nonsense.
kmarinas86
#174
Feb11-12, 09:38 AM
P: 1,011
Quote Quote by spork View Post
We have found that there is no single explanation that works for everyone. Whenever someone finally "gets it" they usually ask why we wasted their time with all the other useless or even wrong explanations. But the fact is, we have a whole bunch of different explanations. They're all accurate, and each person seems to respond to a different one - and think the rest are nonsense.
Ohhhh welll.......
kmarinas86
#175
Apr24-12, 05:54 PM
P: 1,011
Quote Quote by kmarinas86 View Post
Saying this means you deny that the cart involves the deflection of existing energy. That's like saying that no matter can be changing direction in this system. That has everything to do with blue and maroon arrows in the diagram below:



http://www.physicsforums.com/attachm...1&d=1325884076
physics.aps.org/assets/1e41c2ebe02d4468

Quote Quote by Focus: Getting an Extra Bounce
Focus: Getting an Extra Bounce
Published October 4, 2004 | Phys. Rev. Focus 14, 14 (2004) | DOI: 10.1103/PhysRevFocus.14.14

Computer simulations and experiments show that a ball can rebound from a surface with more vertical speed than it had initially.
Anomalous Behavior of the Coefficient of Normal Restitution in Oblique Impact

Hiroto Kuninaka and Hisao Hayakawa
Phys. Rev. Lett. 93, 154301 (2004)
Published October 5, 2004

Figure 1
NASA-JPL



Quick sand. Computer simulations agree with experiments suggesting that a disk can hit a surface and rebound with a surprisingly vertical trajectory. Research on such impacts can help improve models of the flow of granular materials–such as these Martian sand dunes (shown in false color).

Figure 2
M. Louge/Cornell Univ.



Pop-up. In previous experiments [2] the trajectory of a ceramic ball hitting an elastic surface made a larger angle with the surface after impact than before. (See computer simulation video below.)
Animation courtesy of H. Kuninaka, Kyoto University.

http://physics.aps.org/assets/0d26d4...8/video-v1.ogg

High ball. This two-dimensional simulation shows how a ball can deform an elastic surface when it bounces. The surface becomes in effect a ski jump, which redirects the ball’s velocity skyward.

Like a gymnast who runs toward a vaulting horse and then hurls herself skyward, a ball can, under certain conditions, rebound from a glancing impact with a surprisingly vertical trajectory. It’s a phenomenon that’s been observed but never fully explained–and at times even doubted. But now researchers report in the 8 October PRL that they have developed a theory that explains the phenomenon and have tested it with computer simulations. Their explanation–which hinges on the ball’s impact deforming the surface it hits–could help refine models of the flow of granular materials such as sand dunes, cement, and soil.

The coefficient of normal restitution compares the vertical component of the velocity of an object before and after it has bounced. Conventional wisdom says it’s less than one–that is, a ball can’t leave the ground moving faster than when it arrived, because that would require extra energy (in the case of the gymnast, her body creates energy). But since the early 1990s several research groups have reported experiments with oblique impacts in which they found what seemed to be absurd results: A hockey-puck-like disk glanced against a wall and then appeared to pop away with an increased perpendicular velocity [1] and a ceramic sphere rebounded off a softer surface with a noticeably more vertical trajectory [2].

“They first thought I was crazy!” says Michel Louge of Cornell University, who performed the sphere-bouncing experiment with student Michael Adams. But after carefully ruling out experimental error, he concluded that the ball must deform the surface in such a way that it changes the trajectory of the ball. In that way, he thought, some of the horizontal component of the velocity could be transferred to the vertical component. He wasn’t sure exactly how this would happen, although it was clear that the effect was limited to special situations. “My conjecture in the [experimental] paper was just that–a conjecture,” he says.

Now Hiroto Kuninaka and Hisao Hayakawa of Kyoto University in Japan report that they have simulated the small-scale interactions between a disk and an elastic surface that can lead to a greater-than-one coefficient of normal restitution. Their computer simulation calculates a coefficient of 1.3 when the disk strikes the surface at an angle of about 11 degrees; at that angle, their simulated ball rebounds at about 15 degrees. The simulation results resemble Louge’s experimental data, according to the authors.

The simulation allowed the team to see the virtual disk denting the surface when it hit at oblique angles. Bill Stronge of Cambridge University in England describes the indentation as a kind of ski jump, which redirects the sphere’s velocity skyward. Because of this phenomenon, the coefficient of normal restitution can be greater than one without breaking the laws of physics. “The point is that the target material is softer than the ball,” says Kuninaka.

But Stronge doubts that the impact could make as vertical a ski jump as the researchers’ model suggests. It may have some effect, he says, “but I think that it’s certainly not nearly as dramatic as they have portrayed.” Computer models of granular materials such as cement and soil must account accurately for the collisions between grains, which can be treated like collisions with walls. So the work could contribute to practical advances in industries that manage and transport these materials, says Louge–and add to the understanding of the physics of ball sports.

–Chelsea Wald

References

J. Calsamiglia, S. W. Kennedy, A. Chatterjee, A. Ruina, and J. T. Jenkins, “Anomalous Frictional Behavior in Collisions of Thin Disks,” J. Appl. Mech.66, 146 (1999).
Michel Y. Louge and Michael E. Adams, “Anomalous behavior of normal kinematic restitution in the oblique impacts of a hard sphere on an elastoplastic plate,” Phys. Rev. E 65, 021303 (2002).
From this, we can see that "DDWFTTW" phenomenon isn't limited to sails or propellers:

Anomalous behavior of normal kinematic restitution in the oblique impacts of a hard sphere on an elastoplastic plate
http://masters.donntu.edu.ua/2010/fi...em_1/nem_1.pdf
spork
#176
Apr24-12, 06:22 PM
P: 203
Quote Quote by kmarinas86 View Post
From this, we can see that "DDWFTTW" phenomenon isn't limited to sails or propellers
The very fact that these are "anomalous" tells us that they're not all that similar to DDWFTTW. DDWFTTW may be counter-intuitive, but it's very easily explained with very traditional physics.
kmarinas86
#177
Apr25-12, 08:56 AM
P: 1,011
Quote Quote by spork View Post
The very fact that these are "anomalous" tells us that they're not all that similar to DDWFTTW.
I'm not sure that follows.

Quote Quote by Focus: Getting an Extra Bounce
Bill Stronge of Cambridge University in England describes the indentation as a kind of ski jump, which redirects the sphere’s velocity skyward.
The rotation and pitch of a propeller could create an interface between incoming wind that, due to changes in the propeller's position and the change of wind's angle of attack with the propeller's surface over time, the propeller may appear to the wind as if it were a "ski jump". That might be going beyond the minimum to explain DDWFTTW, but it also would explain why a different propeller design might be better at DDWFTTW than another.
jduffy77
#178
Apr25-12, 09:02 AM
P: 40
Quote Quote by kmarinas86 View Post
From this, we can see that "DDWFTTW" phenomenon isn't limited to sails or propellers:

Anomalous behavior of normal kinematic restitution in the oblique impacts of a hard sphere on an elastoplastic plate
This article has absolutely nothing to do with ddwfttw whatsoever. Your apparent desire to over complicate the simple lever which is the ddwfttw cart is fascinating to me. I believe your confusion stems from the fact that you still wish to see the cart as being pushed along by the wind. You need to think about the cart's prop exactly as you would the prop in a powered airplane. In this case the cart is powered by the wheels but it is generating thrust in exactly the same manner as the airplane.
kmarinas86
#179
Apr25-12, 09:15 AM
P: 1,011
Quote Quote by jduffy77 View Post
Quote Quote by kmarinas86 View Post
From this, we can see that "DDWFTTW" phenomenon isn't limited to sails or propellers:

Anomalous behavior of normal kinematic restitution in the oblique impacts of a hard sphere on an elastoplastic plate
http://masters.donntu.edu.ua/2010/fi...em_1/nem_1.pdf
This article has absolutely nothing to do with ddwfttw whatsoever. Your apparent desire to over complicate the simple lever which is the ddwfttw cart is fascinating to me. I believe your confusion stems from the fact that you still wish to see the cart as being pushed along by the wind. You need to think about the cart's prop exactly as you would the prop in a powered airplane. In this case the cart is powered by the wheels but it is generating thrust in exactly the same manner as the airplane.
"Powered by the wheels" makes no sense. They're not the source of energy.
The relative motion of the wind with respect to the ground is also not necessary, otherwise, dynamic soaring would not work. Dynamic soaring could also work in the upper atmosphere bordering the vacuum of space, by dipping in and out of it, but you can't give a vacuum a "velocity" with respect to the ground. So you don't even need to reduce the relative velocity of two masses to accelerate a third with respect to the first and the second.

Replace the "hard sphere" with the propeller and the "elastoplastic plate" with the wind (the wind deforms faster than the propeller). That seems analogous to me.

And do you think that DD"W"FTT"W" can only happen with sails and propellers? I think just about any two interfaces will do. One of them doesn't have to be "wind". That's my point. I'm saying that DD"W"FTT"W" could done with anything, even with two "solids" if the angles are just right. DD"W"FTT"W" could also happen inside a fluid, where particles in the fluid can be likened to "ships" or "wind".
jduffy77
#180
Apr25-12, 09:31 AM
P: 40
Quote Quote by kmarinas86 View Post
"Powered by the wheels" makes no sense. They're not the source of energy.
No, in fact the only thing that does make sense to say about the props power source is that it is the wheels. The ground is in fact the source of energy if you choose to analyze the cart in a frame other than that of the ground.

Quote Quote by kmarinas86 View Post
The relative motion of the wind with respect to the ground is also not necessary, otherwise, dynamic soaring would not work.
This is completely wrong. The relative motion of the wind with respect to the ground is central to the carts functioning and the cart has nothing to do with dynamic soaring.

Quote Quote by kmarinas86 View Post
And do you think that DD"W"FTT"W" can only happen with sails and propellers? I think just about any two interfaces will do. One of them doesn't have to be "wind". That's my point. I'm saying that DD"W"FTT"W" could done with anything, even with two "solids" if the angles are just right. DD"W"FTT"W" could also happen inside a fluid, where particles in the fluid can be likened to "ships" or "wind".
This is correct but completely contradicting what you said earlier about relative motion.
As long as you have two surfaces which are in motion with respect to each other, you could design a vehicle which used leverage to travel faster than its power source.


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