The Wind Powered Sail-less Boat

Just dynamics, please. Not vectors, not power, not torque, etc. Just energy balance for a given (arbitrary) volume, in other words. Thanks.

Cheers.

 

Guillermo, I'd still appreciate an answer to my question at #364, but try this attachment on for size -- it addresses DWFTTW a bit, but primarily focuses on the DDWFTTW cart.

JB

 

As I'm sure you're aware, dynamics has no real meaning in a steady state system. But we can evaluate forces, torques, or even energy. Since you want "just energy balance" I'll be happy to oblige:

Let's do our analysis in the frame of the prop-cart for simplicity (we can choose any other frame if you prefer - but I'll start with this one).

There are only two significant forces on the cart that is going a bit faster than the wind (yes I know I used the "f" word - but that's where energy comes from). Those two forces are the horizontal force of the ground on the wheels and the horizontal force of the air on the prop-disk. There is also the weight of the car on the road, but that doesn't figure into the energy analysis in this frame, and the aerodynamic drag on the frame of the cart - but that can be made arbitrarily small, and I'll address it later if you really want.

So... for steady state (faster than the wind or any other speed) the horizontal force of the road on the wheels must be equal and opposite to the horizontal force of the air on the prop-disk. Now let's consider the power at each interface. The road is doing work on the the cart at a rate given by F * Vc (where F is the horizontal force of the road on the wheels, and Vc is the velocity of the cart relative to the road). On the other end, the prop-disk is doing work on the air at the rate of F * (Vc - Vw) (where Vw is the velocity of the tailwind).

So we can see the net energy into the cart is given be E = F * Vc - F * (Vc - Vw). The difference (F*Vw) is the excess energy needed to overcome all innefficiencies and power losses. These include parasitic drag on the prop blades, transmission losses to bearing friction and gear mesh etc., rolling friction, and we can go ahead and throw the aerodynamic drag on the vehicle frame into this category as well.

We can see that depending on the wind speed, and the cart's velocity beyond the wind speed, there can be a very significant amount of excess energy to overcome these losses.

 

Let me explain myself a little bit more clearly:

To simplify your demonstration, in order I can understand it, please consider the following simple model:

- Just two dimensions, to simplify things.
- Flux of an steady and 'perfect' wind from left to right, parallel to floor. Not viscosity, compresibility, etc, to be taken into account.
- Cart standing over floor, whatever dimensions, with an arbitrary mass M.
- Cart friction loses, etc, not to be taken either into account.
- A control 'box' (what I called a volume, if we were in three dimensions), let's say a parallelogram with two sides parallel to floor and the other two perpendicular, going from:
Left to right:
* side A ('entering gate'): cart at rest.
* side B ('way out gate'): cart at speed higher than wind
Up to down:
* side C, located at a big enough distance over the cart to guarantee the wind there is not disturbed by the cart (can be infinite, if necessary; concept remains valid), so flux is parallel to floor and at the same speed as it was when entering the 'volume' at the side A,
* side D, located at whatever arbitrary distance you want under the floor level.


Now, please, work out for me the energy balance of the system, this is: energy at 'gate' B minus energy at 'gate' A.

Thanks again.

Cheers.

 

I want to make sure I understand your scenario correctly. I think your describing a situation in which the wind is blowing through the volume at constant speed, and the cart is placed inside the volume at the left edge with no velocity. From that point the cart will accelerate to greater than wind speed before exiting the volume on the right. Do I understand your setup correctly?

Assuming I do, I'll offer a couple observations:

1) This is clearly not the steady state case - but that's fine. You want to analyze the cart from stationary, up to and greater than wind speed. I've never done that analysis from an energy perspective, but I'd be happy to.

2) Thinking of the steady state condition at any speed above or below the wind speed, the cart leaves a "tube" of wind in its wake that is moving slower than the free-stream wind. Interestingly enough, this wind in it's wake is moving slower than the other wind relative to the ground, but has been sped up relative to the cart. This tube of wind satisfies both the momentum and energy balance requirements. It's easy to show how it does this.

Let me know if my assumption about your scenario is correct and I'll attempt an energy analysis of the transient case you describe.

 

Yes, your assumption is correct.

Cheers.

 

O.K. I've given it a little thought. What you would need is a plot of energy as a function of time for both the inlet and the exit of your control volume. Since this is not steady state, neither would be constant, nor would I expect them to be equal. There will almost certainly be a net energy flux into and out of the control volume during the time the cart is accelerating up to and beyond the speed of the wind. To be honest I can't see what value these two plots would add.

In general people are usually concerned with three aspects of the cart:

1) Can the cart self-start in a tail wind?
2) Can the cart achieve faster than the wind travel directly downwind?
3) Can the cart maintain that rate of travel steady state?

Each can be demonstrated and analyzed fairly readily. Analysing the situation you describe would certainly be possible, but frankly I'd be more likely to do that by writing a simple computer simulation rather than attempt it in a closed-form fashion (primarily due to laziness on my part). If I did this I actually don't expect I'd learn much from the results.

My apologies for not wanting to spend too much energy going down that road.

 

I assure you that if my article is published any work that is not purely my own will be properly credited.

My responses to most of Boston's canned rants can be found in post #326 on page 22 of this thread.

 

JB
Thanks very much for the file attached to #367. I found it very enlightening, especially regarding the motion of sailboats, of which I've had a poor comprehension.

As to the cart, my math is rusty, but I was able to follow enough of it to see where it goes.

 

No problem clmanges. Glad it was useful to you.

JB

 

Moderator Note: A number of posts containing personal attacks had to be removed from this thread. As a result the posts are renumbered so some of the references to post numbers are now off by a few. I apologize for that.

 

seems reasonable that an engineer would be able to identify this formula




there was a guy named Socrates anyone remember him
had a knack for asking questions
got him in trouble but did manage to very efficiently prove a point
any one know what that point was

not really expecting an answer
just pointing out the obvious

cheers
B

 

Ok, a little quid pro quo here Boston:

I'll identify your equation and you'll go back through this thread, count and indentify the number of times you you insultingly claimed that DDWFTTW was impossible and defied the laws of physics -- something you have now been forced to admit was a miserably failed conclusion.


Just for fun I'll get you started with just a few of your select quotes:

All from Boston on this tread:
--------------------------------

no system is 100% efficient and so it is not possible to even reach the equivalent of actual wind speed on a dead down wind course let alone exceed it in a wind powered craft

... your talking about a system that is more than 100% efficient and it doesnt exist its called perpetual motion

not trying to piss any one off just not a believer in perpetual motion

if this thing were real we could quit drilling oil or burning coal and we cant I wish we could but it doesn't work

the basic problem of all over unity devices how to blink and derive the extra power

if you need to ask questions dont this is pathetically obvious

is the word perpetual anathema around here, cause its what this thing represents

Im just frustrated that the basics of physics are being so flaunted

I just am not going to explain basic physics

I kicked there buts on there considerations of the physics involved

kicked all your asses huaaaa

hell Ive already shot this down in very short order with little more than a basic conservation of energy proof

----------------------------

Of course as it turns out, the one that needed the "basic physics" explanations was Boston as the DDWFTTW cart does just that and all *without* breaking a single law of physics.

Remember, quid pro quo.

JB

 

The fact that Boston is wrong doesn't make ThinAirDesigns and spork right. Sure, they have the technical stuff right, or right enough for me.

What they don't have right is any manners or taste. They will now, mostly spork, keep this thread alive largely with antagonistic moaning about how stupid Boston is, how impossible it is to communicate with such dullards, and how frustrated they are at having to deal with this. They will be SO very VERY frustrated with this impossible business that they will keep this thread alive, apparently forever.

I'm tellin' ya, IGNORE these guys. Think about it on your own if you like, go look it up on the internet, it's all there. Talking to these gentlemen accomplishes nothing but the stroking otheir egos.

To invoke a phrase from the days of yore: Don't Feed The Trolls.

 

Jeff
how would you like us to handle people that by any reasonable standard appear to be being dishonest

Im sorry you got dragged into this but at some point a little judicious intervention is required
I regret having to ask the following
as it is seldom that group thug spamming occurs

if a small group of people comes on here and clearly is lying and thug spamming
how would you like us to call em on it

please read through from thinairs first appearance
so you can see what I'm talking about
he asked if he could answer any questions

soooo
all I did was ask one
"can we please see a complete computational analysis of the device in question"

that was a completely honest question
and the kind of question engineers love
it cuts through all the bull and gets right to the point

the avalanche of subterfuge and antagonism that ensued rather than answer the question is clear for all to see
just go read it

so I fed it right back to em and when it got to hot
they run and complain to the moderator
thats amazing

I thought Ild seen the cheepest shot they could take
but that tops it

the only other question I asked was if they could identify a simple equation
one easily recognizable to any real engineer
but these guys dont recognize it
why
are they not what they claim to be

please
how would you like us to proceed
and I will kindly proceed in the specified manor
how can we meet the rules and still warn people
I believe these guys are lying about there credentials
as it is clear to me that these guys are thug spammers
and being dishonest in there claimed credentials
engineers speak the language of math
not philosophy and insult

I like the forum and for the most part just read around here
I have been active in only a few threads and mostly just ask questions about the engineering and physics of sailing

these guys come in as a group and start a fight

all I did was do to them what they did to me
I started out joking around and having a great time
next thing I new
these guys were comparing themselves with the Write brothers

Jeff if you do have time for it
I think its most telling to consider the following

after offering to answer any questions we may have these guys then refused to answer the only valid question
for five pages
heaping distractions and insults the hole way
( and yes I fed back to them whatever crap they fed to me )
I finally looked it up myself because these guys couldnt speak the language of engineering and found some math that said it could be possible
if these guys were trying to show it would work rather than just start a fight why do they refuse even now to show the math
so I immediately wrote a note to that effect
stepped up like an adult and noted my error while calling em on there lack of civility
I had previously given the idea a hell no
the gloating and childish insults that ensued was amazing
I could tell right away that these guys were acting without any scientific integrity and so I began to dig
finnaly
although they said they were engineers and had been working on this thing for years
they admitted having no computational analysis of this device

now I ask you
how can these guys be engineers if they dont do any math

it is really obvious that these guys were on a mission to deliberatly piss off the competition rather than be found out
and I felt like accommodating them was the best answer
I regret having violated any forum rules
but the conclusions I have and others have come to, concerning there integrity and honesty appear by any reasonable standard
to be valid

these guys are thug spammers clearly on a mission to cause trouble
they are not being honest
and they are working as a team

I respectfully ask that they be banned from the site
thank you for listening
sincerely
B


if you were to even just go back to there last post you will find it full of antagonism

I would also note that these guys didnt invent that thing yet intend to write a paper on it
it screams of a lack of scientific integrity bordering on the illegal
had I not exposed them
they might have gotten away with it