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  #751  
Old 01-22-2009, 08:33 PM
spork spork is offline
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Quote:
Originally Posted by Guillermo View Post
Talking about the open air test, how is it possible for the rotor to change sense of turning? .
I guess I'll just have to learn to accept that you'll never understand that the "open air" test and the "treadmill" test are THE SAME. In fact they're both in "open air". It just happens that in the "treadmill" test we have MUCH better control of the wind speed and direction, the grade of the surface, etc.
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  #752  
Old 01-22-2009, 09:51 PM
Guest625101138 Guest625101138 is offline
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Quote:
Originally Posted by Guillermo View Post
Thanks Rick,
Talking about the open air test, how is it possible for the rotor to change sense of turning?

Cheers.
Guillermo
If you are meaning the situation in the second open air trial where the rotor begins to operate as a turbine you will see it is actually spinning the wheels in reverse. The torque from the rotor is greater than the torque limit resulting from the coefficient of friction and weight on the two front wheels. About 3m after taking off with the rotor forcing the wheels in reverse you see the rotor reverses and becomes a propeller being driven by the wheels.

This is the situation I describe with the turbine boat in post #719:
"I have not done precise calculations on the very low boat speed condition. There is likely to be reverse rotation of the prop when the boat is stopped because the prop has a good reverse flow regime under this condition while the turbine is in a poor regime. But the downwind force will cause the boat to progress downwind such that the reverse flow regime on the prop gets worse and the forward flow regime on the turbine improves. Once this happens the mechanical advantage of the turbine takes over and away it goes, accelerating until there is a force and power balance somewhere higher than windspeed."

You will note by the speed of the litter past the cart there is a strong gust. This puts the rotor in a high power regime which is sufficient to overcome the wheels. Once the vehicle starts moving to reduce apparent wind the rotor torque is reducing and the wheels stop slipping.

Point is you can see the rotor change direction and become a propeller. Once this occurs it is bringing air backwards through the disc. There is clearly no stagnation point or vortex trap that it gets stuck in. It is a pity there is not more litter as you would be able to see it quickly reaches wind speed.

Maybe this is a simple test that could be done with some fine powder being dropped in the wind stream as the vehicle is taking off.

Rick W
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  #753  
Old 01-22-2009, 11:29 PM
spork spork is offline
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Quote:
Originally Posted by Rick Willoughby View Post
Maybe this is a simple test that could be done with some fine powder being dropped in the wind stream as the vehicle is taking off.
That particular self-starting demo ended up being a bizarre mixed blessing. On the one hand it underscores some interesting aspects of how this cart operates, but on the other it seems to be the source of a whole new laundry list of concerns.

Rick is absolutely correct that the critical aspect of this particular demonstration is the gust and the wheel slip. I also did a self-start demo with a fan indoors. Perhaps repeating that demo with smoke might show what we want to see.
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  #754  
Old 01-23-2009, 01:46 AM
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Guillermo Guillermo is offline
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Quote:
Originally Posted by Rick Willoughby View Post
Once the vehicle starts moving to reduce apparent wind the rotor torque is reducing and the wheels stop slipping.
So the wheels slip initially. I see, thanks.
A pity the video is not in slow movement mode, to better realize the rotor and wheels turning.

Cheers
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  #755  
Old 01-23-2009, 10:12 AM
SamSam SamSam is offline
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Originally Posted by spork View Post
How can we convince you that the principle of equivalence of inertial frames is real - and applies to treadmills? This is one of the most basic tenets of physics.
Somewhere here someone mentioned Galilian Relativity or something so I looked it up in wiki. Eventually I was led to a site that showed Galileo's method of explaining it, which was a "narration" where you were to imagine various happenings going on in a ship's hold as it sailed along. Examples were weights dropped, people jumping in the air etc. Lastly was the example that smoke from a candle would go straight up in the air. At the end of the story/narration the fellow talking said all these "truths" held good below decks, but were not the same above decks in the airstream.

Reading one thing led to others and even though I don't grasp much of it, isn't below decks 'equivalent inertial frames' and above decks in the wind 'non-inertial frames', which require "ficticious" (not my word) equations to be right?

As far as a powered treadmill being the same as a blowing wind, in some cases that might hold true, but in a wind powered device, which together with DDW, is the priority claim in all this discussion, is it the same? A model sailplane tethered to the front of a running treadmill won't fly, but tethered to a running person it will.

Is it possible that in the treadmill powered cart you are not necessarily going faster than the wind, you are just going faster than the treadmill? Are they the same thing?
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  #756  
Old 01-23-2009, 01:17 PM
ThinAirDesigns ThinAirDesigns is offline
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Quote:
Originally Posted by SamSam View Post
As far as a powered treadmill being the same as a blowing wind, in some cases that might hold true, but in a wind powered device, which together with DDW, is the priority claim in all this discussion, is it the same?
Yes.

Quote:
A model sailplane tethered to the front of a running treadmill won't fly, but tethered to a running person it will.
I guess I don't follow what you are imagining, because tethered properly, the sailplane will fly in both cases.

Imagine a long airport people mover and two light styrofoam models with identical strings say 20ft in length. One string is taped to the floor of the moving sidewalk and one is place in the hand of a person next to the sidewalk . As the people mover is turned on and starts to move at 4mph, the person also starts moving at 4mph. The planes will both be pulled through the air at 4mph and will both fly.

You'll have to explain where my example fails relative to what you are imagining.

Quote:
Is it possible that in the treadmill powered cart you are not necessarily going faster than the wind, you are just going faster than the treadmill? Are they the same thing?
The cart traveling faster than the belt of the treadmill is truly traveling faster than the wind.

Imagine riding your bike down the road at 10mph with a 10mph tailwind -- you're pumping your legs, and your wheels are turning but you feel absolutely no wind on your face. Now ride on a treadmill set at 10mph -- you're pumping your legs, and your wheels are turning but you feel absolutely no wind on your face.

Without external reference, it is *impossible* for you to tell whether you are on the treadmill or on the street in those above scenarios. This is the point of Galilean relativity.

JB
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  #757  
Old 01-24-2009, 01:14 AM
spork spork is offline
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SamSam, it looks like JB beat me to the answers. So I'm definitely not ignoring you, but I think JB covered it. If you'll direct me to the site that said things are different above deck I'll be happy to have a look. I suspect they were saying that things would be different above deck because of the relative wind (in other words a ball thrown straight up wouldn't land in the same place), but that simply means they haven't set up truly equivalent inertial frames. If the ship had a tailwind equal to its speed things would work exactly the same above or below deck. Make sense?
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  #758  
Old 01-24-2009, 02:50 PM
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chabrenas chabrenas is offline
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It really does look as if the prop on the roadcart autorotates a small fraction of a turn before being driven. Seems too slow for an artefact of the frame rate. Is it taking up slack in the drive train? Maybe even winding up the drive shaft?

Guillermo: I don't accept your proposition that a sail harvests energy from a bigger cross-section of an airflow by traversing across it. In my view, only a bigger sail does that.

I believe that the sail gets more energy from the air (the wind does more work on it) because it is rigidly attached to an object that is immersed in a fluid (or in contact with a solid) that has motion relative to the air mass, and which can apply reactive forces that cause the sail to move in such a way as to increase the relative velocity between the sail and the air mass (i.e. to create an apparent wind which is the vector sum of the transverse motion and the relative motion of the air mass and the second medium).

In a boat, the keel or centreboard provides that force, and on a cart the friction between the wheels and the surface of the solid do so.
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  #759  
Old 01-25-2009, 04:40 PM
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Guillermo Guillermo is offline
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Quote:
Originally Posted by chabrenas View Post
.... In my view, only a bigger sail does that....
Mike,

For a given size of sails and wind speed, the overall energy gotten from the wind depends on how much mass of wind the sails are using and their efficiency in altering its direction/speed of movement. Mass used depends on relative directions and speeds of craft and wind, and change of wind direction/speed on the sails efficiency for such specific relative movement.

Once discounted all loses in the sails, the net amount of captured energy can be more or less effectively used depending on the ability of the interface system (underbody, wheels, skates, etc) in keeping the craft moving in the desired direction with the less possible loses (heel, drag, friction, etc), so how efficient such interface system is in maximizing the desired use of energy. Of course this influences the relative direction and speed between craft and wind, so influencing the amount of mass of wind used, as said before.

I hope to have been able to make myself understand.


Cheers.
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  #760  
Old 01-26-2009, 07:38 AM
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chabrenas chabrenas is offline
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Guillermo: I agree with both paragraphs of your response.

I also believe that this means that, given a specific wind velocity relative to the water or land mass, the only way to make available more energy than that available from the flux through a projection of the sail profile on a plane normal to the relative motion of air and water/land, you have to increase the velocity of the sail relative to the air mass.

A traditional sailing boat (or ice or land yacht) does this by developing a velocity component normal to that between air and water/land, and hence a resultant velocity relative to the air mass that is greater than the relative motion of the air and the water/land (and we usually call this resultant velocity the 'apparent wind').

Hence, it is this increase in velocity of the sail relative to the air that is the source of the extra energy, not the fact that the sail harvests energy from an area greater than its projection on the plane normal to the relative motion of air and water/land.
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  #761  
Old 01-27-2009, 05:34 PM
Guest625101138 Guest625101138 is offline
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Some have difficulty understanding where the power comes from with the propeller. The following shows what energy the air loses in two cases.

Case 1 is a propeller in still air with a vehicle doing 1m/s.

Consider the propeller accelerating the incoming air stream by 0.5m/s so it goes through the disc at 1.5m/s. At this flow rate we will select the prop area to have a mass flow of 100kg/s.

In this case the air is at rest before the vehicle passes but is moving in the reverse direction at 0.5m/s after the vehicle passes.

Energy/s (power) = 0.5 x mass/s x (VAafter^2 - VAinitially^2)
= 0.5 x 100 x (0.5^2 - 0^2)
= 12.5W
So the air has gained 12.5W as the vehicle passed. The only way this can be done is to provide an external power source as the air has gained energy. This energy is what the prop must produce if it could work at 100% efficiency.

Case 2 is the vehicle operating at 4m/s in a tail wind of 3m/s such that the propeller sees EXACTLY the same operating conditions. Air flow into the prop is 1m/s and the air is accelerated rearward at 0.5m/s so passes through at 1.5m/s as before. Mass flow through the disc is the same. The airflow ahead of the vehicle is 3m/s and reduces to 2.5m/s within the prop disc after the vehicle has passed.

Energy/s (power) = 0.5 x mass/s x ((VAafter^2 - VAinitially^2)
= 0.5 x 100 x (2.5^2 - 3^2)
= -137.5W
In this case the air is losing energy at a rate 137.5W. The prop only needs to deliver 12.5W to achieve this result. So there is 125W available to overcome system losses and vehicle drag.

Hopefully this analysis shows in very simple terms where the energy comes from.

Rick W
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  #762  
Old 01-27-2009, 11:37 PM
markdrela markdrela is offline
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Quote:
Originally Posted by Rick Willoughby View Post
Some have difficulty understanding where the power comes from with the propeller.
I have an even simpler way to explain it. The attached PDF shows the DDWFTTW situation boiled down to its absolute base minimum:
* The prop and turbine powers are equal.
* The prop sees a slower velocity and therefore has a proportionally larger force than the turbine.
QED.

The naysayers can now throw at this all the losses, inefficiencies, etc that they want. But the simple fact is that if these losses are made low enough, as they always can be, the prop's thrust force will always be greater than the turbine's retarding force. So there's some excess thrust left over to overcome hull drag.
Attached Files
File Type: pdf fvp.pdf (4.0 KB, 1618 views)
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  #763  
Old 01-28-2009, 02:15 AM
Joakim Joakim is offline
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Quote:
Originally Posted by markdrela View Post
I have an even simpler way to explain it. The attached PDF shows the DDWFTTW situation boiled down to its absolute base minimum:
* The prop and turbine powers are equal.
* The prop sees a slower velocity and therefore has a proportionally larger force than the turbine.
QED.
Now we have made a loop and came back to where we were 50 pages ago. This didn't help then, probably won't help now, since it seems too hard to understand for most naysayers.

Joakim
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  #764  
Old 01-28-2009, 03:03 AM
Guest625101138 Guest625101138 is offline
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Joakim
It seems we now have full agreement that it is possible. The nay sayers have finally been convinced.

If they had have given it some clear reasoning in the first place rather than just offering ill-thought opinions and trying to justify their position it would have been a much shorter thread.

Rick W
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  #765  
Old 01-28-2009, 10:17 AM
spork spork is offline
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Quote:
Originally Posted by Rick Willoughby View Post
Joakim
It seems we now have full agreement that it is possible. The nay sayers have finally been convinced.

If they had have given it some clear reasoning in the first place rather than just offering ill-thought opinions and trying to justify their position it would have been a much shorter thread.

Rick W
I'll be happy to provide nay sayers from other forums if you like. We have one guy that is absolutely impervious to reason or logic.
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