by Opus_723
Tags: ddwfttw
P: 206
 Quote by kmarinas86 If so, then some of this phenomenon could be related even to General Relativity, as it appears that the fundamental microscopic non-inertial, vibratory/rotational motions involved must some how have changed course to some small degree (in this non-relativistic example) as a result of the force interactions involved.
Nothing fancy -- simple straightforward fluid mechanics involved and it's not even anything new, having been done for hundreds (thousands?) of years. A simple flip of the environment (viewed from the perspective of a fish for example) easily shows that the keel of the very first boat to ever tack its way upwind was achieving a downfluid VMG faster than the fluid, absolutely steady state.

JB
P: 3,182
 Quote by kmarinas86 I will now concede that in this there is a source of energy that would make for the illusion that the apparent wind can accelerate a sail craft in the opposite direction that it is blowing
It is not "an illusion". It is a well verified empirical fact and in full agreement with Newtonian physics. The "source of energy" is the velocity difference between the air & surface which is always being reduced.
 Quote by kmarinas86 ...matter's energetic motions were somehow deflected internally ... some of this phenomenon could be related even to General Relativity...fundamental microscopic non-inertial, vibratory/rotational motions involved ...
LOL. It's just simple mechanics as the vectors diagrams show.
P: 1,011
 Quote by ThinAirDesign Nothing fancy -- simple straightforward fluid mechanics involved and it's not even anything new, having been done for hundreds (thousands?) of years. A simple flip of the environment (viewed from the perspective of a fish for example) easily shows that the keel of the very first boat to ever tack its way upwind was achieving a downfluid VMG faster than the fluid, absolutely steady state. JB
 Quote by A.T. It is not "an illusion". It is a well verified empirical fact and in full agreement with Newtonian physics. The "source of energy" is the velocity difference between the air & surface which is always being reduced. LOL. It's just simple mechanics as the vectors diagrams show.
Without positing pre-existing "hidden" momentum inside the mass of the apparent wind and/or the craft itself, I cannot at all see how something that blows at you can pull you forward. It seems to make more sense to imagine that the apparent wind is simply allowing this "hidden" momentum (which we know exists in the form of the $\vec{p}$ in the equation $(m_{whole}c^2)^2$$=E_{whole}^2=(m_{parts}c^2)^2+\left\|\sum \vec{p}\ c \right\|^2$ to appear visible to a human observer, than it is to believe that apparent wind would be gaining energy by doing work on the sail. After all, if the apparent wind is moving to the left and pushes the sail to the right, this would mean that the apparent wind would have to accelerate both itself and the sail, which would violate the conservation of energy, if it were not for this hidden momentum. It is no coincidence to me that so many think incorrectly that the DDTFTTW craft is impossible. They see the apparent wind as a source of energy, and they cannot imagine how it would increase speed relative to the craft by being thrusted by the propeller tailwards while at the same time having that power of the propeller being explained by the same incoming headwind. The reason why this is so prevalent is that there is something wrong with apparent wind being able to do that with the cart, if you don't accept that there is a hidden source of energy! Yes, the forces can explain conservation of momentum, but from the inertial frame of the craft at time $t$, it is very clear that without a hidden source of energy, we cannot explain why the head wind and the craft with increase respect to the frame once $t$ has passed. In this case, it is not hidden because of deception, but rather, it is hidden because the energy is that of atoms and molecules. So the skeptics of DDTFTTW are not entirely wrong in their skepticism. There must be a hidden energy source (It's nature's energy!). :)
P: 3,182
 Quote by kmarinas86 I cannot at all see how something that blows at you can pull you forward.
Sailors use this of ages.
 Quote by kmarinas86 ...hidden momentum...
Very creative, but there is no need for such obfuscatory nonsense. All the momentum is clearly visible all the time and is being conserved.
P: 206
 Quote by kmarinas86 I cannot at all see how something that blows at you can pull you forward.
Before I respond I really must clarify something.

Sailboats sail upwind all the time. They can leave a point downwind and readily arrive at a point directly upwind of where they were by simply sailing towards a point situation to the right (or left) of the upwind goal, and then once halfway there, they turn and sail directly towards that point.

A: If taken literally and without context, your above quoted statement would make it seem as though you don't see how a boat can sail upwind such as the above. I'm pretty sure that's not what you mean but I do want to ask the question.

B: If you believe boats can make upwind progress by sailing at an angle to the wind, but are having difficulty believing that *anything* wind powered can make steady state progress directly into the wind using basic Newtonian physics then I need to know that.

A or B or other?

Thanks

JB
P: 1,011
 Quote by A.T. Sailors use this of ages. Very creative, but there is no need for such obfuscatory nonsense. All the momentum is clearly visible all the time and is being conserved.
The net momentum is indeed conserved.

2 + (-2) = (3) + (-3)... etc.

That makes it look like there is no mystery.

What does not make sense to many still-skeptical skeptics is how would the Blackbird DDTWFTTW sand yacht conserve energy. Neither you, nor them, seem to have the explanation.

Note that:

(2)^2 + (-2)^2 is not (3)^2 + (-3)^2... etc.

Would you mind explaining where the energy comes from to allow the wind to do work on the DDWFTTW vehicle (in the time between $t$ and $t+\epsilon$) at the same time the DDWFTTW vehicle accelerates, with respect to the initial inertial frame of the vehicle at time $t$? The work is done in opposite directions, conserving momentum even macroscopically, but not the kinetic energies of both (both increase as far as the initial inertial frame is concerned). My "very creative" resolution addresses this problem by bringing up the point about the true and factual existence of the below-macroscopic energy of atoms and molecules as being the entity that accounts for this apparent gap.

My explanation is not that of a hidden net momentum, but a hidden set of vector momenta which sums to zero in the frame being evaluated (i.e. the momentum whose energy is identical to the rest mass of a body, as evaluated from the system frame in question, times the speed of light squared). I have a hunch that somehow this is either the static P-V energy that was already present in the air mass prior to vehicle operation, and/or the vibrational and rotational energy of the vehicle's particle makeup. Probably both.
P: 1,011
 Quote by ThinAirDesign Before I respond I really must clarify something. Sailboats sail upwind all the time. They can leave a point downwind and readily arrive at a point directly upwind of where they were by simply sailing towards a point situation to the right (or left) of the upwind goal, and then once halfway there, they turn and sail directly towards that point. A: If taken literally and without context, your above quoted statement would make it seem as though you don't see how a boat can sail upwind such as the above. I'm pretty sure that's not what you mean but I do want to ask the question. B: If you believe boats can make upwind progress by sailing at an angle to the wind, but are having difficulty believing that *anything* wind powered can make steady state progress directly into the wind using basic Newtonian physics then I need to know that. A or B or other? Thanks JB
Neither. Potential energy must be extracted from the system. Either the static P-V of the wind and/or the energy from the particle make-up of vehicle (and/or even that of the ground, if need be). A correct explanation cannot be found by trying to conserve "kinetic energy+heat" while ignoring potential energy.

The quote (taken out from a sentence after a comma) is taken too far out of context. It doesn't even relate to what I am saying.
P: 206
 Quote by kmarinas86 Neither. The quote (taken out from a sentence after a comma) is taken too far out of context. It doesn't even relate to what I am saying.
Ok, got it. I was pretty sure from your previous posts that "A" wasn't what you meant, but didn't want to move forward without confirmation.

Thanks.

It appears that what you are saying the examples I have in that post can and do happen, you just don't believe they can be explained through simple Newtonian physics.

Would that be a fair representation of your position?

JB
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P: 6,779
 Quote by kmarinas86 Potential energy must be extracted from the system.
For a fluid or gas, potential energy is used to describe the gravitational potential energy of a gas or fluid. For a wind driven vehicle, the energy extracted from the air affected by the wind driven vehicle corresponds to the reduction in kinetic energy (wrt ground) of the affected air. (Pressure effects are short term and only exist in the immediate vincinity of the propeller.)
P: 1,011
 Quote by ThinAirDesign Ok, got it. I was pretty sure from your previous posts that "A" wasn't what you meant, but didn't want to move forward without confirmation. Thanks. It appears that what you are saying the examples I have in that post can and do happen, you just don't believe they can be explained through simple Newtonian physics. Would that be a fair representation of your position? JB
I guess that depends on what you call simple, what you call Newtonian, and what you mean by explain. You can explain things using forces without highlighting the apparent non-conservation of "kinetic energy+heat". That is simple (to me), but ignoring where this kinetic energy comes from doesn't do it for me, so I would disagree that it is somehow an adequate explanation. (If a claimed-to-be explanation is inadequate, does it really explain what needs to be explained?) Potential energy latent inside matter isn't exactly something that I would call part of "Newtonian" physics. It's not included in most of the (simple enough to be convincing to most) analyses that have been offered to explain the Blackbird. Certainly it can be explained using classical physics.

The lack of significant mention of potential energy when discussing how tacking can allow sails to move ahead of the wind, in addition to the absolute absence of this point in many of the videos that I have seen that try to explain DDTWFTTW, has (I bet) contributed much confusion for people (including skeptics and naysayers) who wonder where the energy comes from and who, like myself, have for a time not been able to see how tacking would be of any benefit to it.
P: 206
 Quote by kmarinas86 The lack of significant mention of potential energy when discussing how tacking can allow sails to move ahead of the wind, in addition to the absolute absence of this point in many of the videos that I have seen that try to explain DDTWFTTW, has (I bet) contributed much confusion for people (including skeptics and naysayers) who wonder where the energy comes from and who, like myself, have for a time not been able to see how tacking would be of any benefit to it.

If by "potential energy" you mean something other than the mass of one fluid moving relative to another (or surface) and the kinetic energy contained thus, then the reason it isn't used in an explanation or video is that it would be flat wrong. There is NO other energy involved in accelerating the craft.

If by "potential energy" you mean the mass of one fluid moving relative to another (or surface) and the kinetic energy contained thus - meaning the power of the wind, I can't imagine how you have missed such explanations.

JB
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P: 6,779
 Quote by kmarinas86 where this kinetic energy comes from
You could consider the source of energy for the true wind to be the heat from the sun.

 Quote by kmarinas86 Potential energy latent inside matter isn't exactly something that I would call part of "Newtonian" physics.
I don't recall any mention of potential energy in the descriptions of how wind driven vehicles operates (sail boats, DDWFTTW vehicles, DUW vehicles, ... ). Extracting potential energy within matter involves a chemical or nuclear reaction, which doesn't occur with the wind powered vehicles being discussed here.

 Quote by kmarinas86 The lack of significant mention of potential energy when discussing how tacking can allow sails to move ahead of the wind. ... where the energy comes from
I'm not sure what you mean by potential energy. Wind driven vehicles extract kinetic energy from the wind (using a ground or water based frame of reference).

 Quote by kmarinas86 how tacking would be of any benefit to it.
Tacking isn't required for a DDWFTTW vehicle. A DDWFTTW vehicle could connect the wheels to a treadmill geared so the upper surface of the treadmill moves upwind at some fraction of the vehicles speed, for example 1/2 of the vehicles speed (an advance ratio of .5). The treadmill could pull parachutes along the upper surface and then collapse them (perhaps pull them through a tube) along the lower surface. It wouldn't be as efficient as a propeller, but if the losses could be reduced enough, it would work.

A sail can't generate thrust from an apparent headwind component, so it needs an apparent crosswind component which it diverts to aft of the boat's heading to generate thrust, which is why a sail boat needs to tack in order to acheive vmg downwind greater than true wind.
P: 1,011
 Quote by rcgldr I'm not sure what you mean by potential energy. Wind driven vehicles extract pressure energy and kinetic energy from the wind (using a ground or water based frame of reference).
Pressure energy to me is a form of potential energy, though I tend to look at it from a "molecular" perspective where electric forces reign supreme over gravitational ones.
P: 203
 Quote by kmarinas86 Pressure energy to me is a form of potential energy, though I tend to look at it from a "molecular" perspective where electric forces reign supreme over gravitational ones.
Even pressure energy doesn't come into the equation. The vehicle simply slows down the air relative to the ground beneath it. That's all. For pressure energy to benefit the cart it would have to leave a volume of air in its wake that has been expanded to greater volume and lower pressure. It doesn't do this.
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P: 6,779
 Quote by spork Even pressure energy doesn't come into the equation.
There's a pressure differential in the immediate vincinity of the propeller, but eventually the affected air's pressure returns to ambient and it's velocity is changed. From a DDWFTTW vehicle's frame of reference, the pressure and flow near the propeller corresponds to the description in this NASA article:

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

From a ground frame of reference, the pressure differential at the propeller results in the affected air from the true wind to be slowed down.
P: 203
 Quote by rcgldr There's a pressure differential in the immediate vincinity of the propeller...
Of course there is. But you said: "Wind driven vehicles extract pressure energy and kinetic energy from the wind"

And this isn't the case. Sure they create a lower pressure in front of the disk and a high pressure behind the disk, but ultimately, they don't take any "pressure energy" from the wind. That would be equivalent to saying that an airplane's exploits the "pressure energy" of the air to stay aloft. The fact that it has a local effect (both positive and negative) on air pressure does not imply that it extracts pressure energy from the wind.
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P: 6,779
 Quote by spork But you said: "Wind driven vehicles extract pressure energy and kinetic energy from the wind"
Yeah I worded that badly and corrected my previous posts. The pressure effects are short term and only exist in the immediate vicinity of the propeller.
P: 203
 Quote by rcgldr Yeah I worded that badly. The pressure effects are short term and only exists in the immediate vicinity of the propeller.
I figured it was a simple mis-statement. That's one of the reasons I didn't mention it when you posted it, but only when kmarinas86 talked about it as potential energy. I got the distinct idea he saw it as something it's not.

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