I'm not quite following, since you quoted my entire post: what is only unconditionally true for the free stream? Look guys, I know there are secondary effects here that I (and they) have not discussed, but the most obvious thing about a Venturi is the fact that velocity differences between points are proportional to the area differences: At a spot where the area is halved from another spot, the velocity is double that of the other spot. That's all I'm saying and that's all that is claimed here. Agreed. It must be happening otherwise the device would have to be inflating like a balloon or pressurizing like a scuba tank! The shape's explicitly stated purpose is to funnel the air to increase the airflow speed. The pressure drop is merely a consequence. What improvement? I just told you that when you look at their numbers, the actual claim is a reduction in energy extracted of 25%! Their language is just a smokescreen comparison of apples to oranges: you have to look at the actual numbers. They are claiming that by funneling the air to increase its velocity, they get a 6-fold increase in power versus a turbine of the same size in the freestream. The part in bold and what that means is what they gloss-over in the language. Now, the aerodynamics of that is definitely more complicated, but only marginally relevant. Still, we can discuss: -Bernoulli's equation/conservation of energy applies. -Venturi effect applies. -Air outside/in front of the device is slowed by the drag from the device, converting velocity pressure into static pressure via Bernoulli's principle. That's the trick: air entering the cone is not freestream air, it has already been decelerated and had its pressure increased before it even enters the device. It is this increased static pressure that drives the air through the device. -The result is that this device captures less air than its dimensions imply it should. The variable speed ratio in their graph is a result of that issue: a 2:1 speed ratio implies the area ratio is 2:1 (Venturi equation), but in reality, it has to be larger than that and be variable since pressure is a square function of velocity.
[quote='billy"] Good link. Shows how little wind power interferres with other use of the land (or sea): [/quote] Which is at least as true of this tunnel setup as of propeller designs. Because, as in all photos, inherent presumptions mislead without warning: the massive foundations (400 - 500 yards of high quality cement braced with rebar) are invisible, the road access and so forth is invisible behind the cows, the extra sophistication and expense and fragility of a turbine generator made light and mobile and placed dozens of meters into the air on a pole is invisible, the side effects of flicker and bird strike and noise are invisible, and so forth. Again, the particulars of this structure are not significant, and the photographic comparison misleads in the extreme. Let's base our windmill arguments on this photograph, why not: https://www.google.com.au/search?q=...com%2Fwallpaper%2Fholland_wall1.htm;2560;1920 How nonsensical propeller windmills are - just look: It's below the tree height! It's bulky and vulnerable to storms! Billy can calculate for us how much more tipover force the base must withstand. Or we could just quit arguing from the details of postcard photos of research models. As far as I can tell from what seems a confusing, confused, or quite possibly deceptive description, the venturi is there to provide extra pressure drop - the speed gain that the increase in power extraction is based on comes basically from the funnel, just as one can get extra hitting power via a nozzle on a hose.
If air pressure is (say) 14.6psi, then if you double the airspeed, you get 8 times the power. If you decrease that to 7.3psi then you get half the power (at the same speed.) However, if you then double the airspeed, you still get 8x the power.
correct, in free air. you will have to provide links to prove your case with the venturi because you are proposing a perpetual motion machine by suggesting a venturi ADDS power.
Energy is conserved. The energy in an airstream that can be extracted by turbine to do work, however, increases with added windspeed. Funneling the wind through a smaller area increases the windspeed, which allows a turbine placed in the stream to extract more energy to do work than it could from the slower free wind - in principle, because it's a nonlinear function of the speed which is a linear function of the funnel dimensions, more energy than could have been extracted by turbine from the intercepted airstream at its original speed and area. But actually doing that in the real world incurs complications.
The last sentence, that power is proportional to the cube of wind velocity. Inside the venturi, you have to account for the robbing of Peter to pay Paul: the thing held constant in the freestream wind power calculation, density, is now a variable. And it will drop to conserve energy as velocity rises. Power (energy) will remain the same (ideally) even though the speed goes up, because since there are so many fewer air molecules to transport that energy, the speed becomes irrelevant. Hence I said the statement "power goes as the cube of velocity" is unconditionally true in the free stream where density remains constant. Inside the venturi, the statement is conditioned on introducing a way to overcome the pressure (density) drop. And that requires an external power source. Well, since their claim relates to power, I was just bringing the issue of pressure (density) home to roost. It can't produce a net gain without energy added to drop the backside pressure. There is no definitive data that suggests that anything of the kind (power boost) ever happened. They give one data set that looks to me like anemometer readings in front of and inside the venturi. Those consistently measured velocity increase but no work was being done. The second case was with the turbine installed, in which the data is all over the place. And in that set the velocity differences are between the pole mounted turbine and the one in the venturi. So there's no way to confirm the idea that the body of the structure blocks the wind sufficiently to get a net energy gain. My point is you can't have one without the other. There is not a venturi effect without a pressure drop. And if that pressure drop is not sustainable due to backpressure, then nothing happens. Air just flows through at nominal speed, right? I'm saying 'ideally', ignoring compressibility and everything else. Apples and oranges. Yes, that part of the claim (2:1 speed should correspond to 8:1 power, not 6:1 power) -- that's the apples. Here are the oranges: on one day during a week of testing the turbine inside the venturi showed a 9:7 velocity improvement over the pole-mounted one. The enclosure therefore showed an improvement, but only briefly. There is no power data given, hence no way to calibrate oranges to apples. Besides, that's wind power, which has to be derated according to efficiency to even hope to get the apples to line up with the oranges. It's virtually useless. All we really need to see are the electrical power data. I'm on page with you as far as the smokescreen, but after looking a little more closely at the data than you may have done (I'm only guessing; it looks like you didn't) I'd have to disagree in the sense that it was not consistent, there was one day when the freestream turbine was better, etc. I was just trying to clear that up, recognizing that you're on top of your game here as far as the science goes. That last statement is certainly true. Pressure at any cross-section is ½ρv². The pressure drop as you know is the difference of those two quantities, at the mouth of the venturi vs. inside. I'm only saying that if the back pressure won't sustain that lower amount, nothing happens. Or else nothing much happens. For this reason, if they were designing for net gain, they would need some way to create a vacuum at the tail end (like some posters magically produce in perpetual motion machines). You suggested that this is possible from the aerodynamics of the outer surface of the contraption, and I doubted that, since it wasn't designed for that purpose. My point is that lower backpressure has to be designed into this to get a net gain in wind power in a venturi that runs exclusively from wind energy. Therefore the venturi can only produce more power than the pole mounted turbine if some vacuum-creating aerodynamic surface can be designed in. Theoretically, that is, and subject to the limited scope of what I just laid out.
thank you. if you have a 100 watt wind at the input of the venturi then you will have 100 watts or slightly less at the throat. betz will limit this to about 60 watts, and that's assuming perfect conditions. the improvements will have to be better blades or alternator/ gearbox. for homebrew designs you can expect very low blade efficiency and moderate to high alternator/ gearbox efficiency.
Well then no, you have that wrong: it is true anywhere that velocity is the only thing in that equation is changing. You have the first part right and the second part wrong: Density is constant in this calculation because the pressure change is very small. The thing that is not constant in that equation is area. We need to stop here and get this issue cleared-up because it is critical: for low speed applications, density is constant. See: http://en.wikipedia.org/wiki/Bernoulli's_principle#Incompressible_flow_equation In other words, for speeds below about Mach 0.3, an incompressible flow assumption is valid. Now, Venturi: http://en.wikipedia.org/wiki/Venturi_effect Note that the Venturi equation contains only one density term. Though I still need to respond to this, to make sure we're even talking about the same issue of what they claim: I guess I need to keep repeating this until it sinks in: Their numbers do not claim perpetual motion. They are not claiming to extract more energy from the air than it already has in it. ...of the same capture area. They are not claiming that it does that. They are not claiming it captures more energy, they are claiming it captures less.
density IS NOT constant when moving through a venturi. to claim any significant increase in airspeed while claiming no decrease in density is proposing, well, it ain't gonna work. ANY increase in airspeed will result in a corresponding decrease in density.
Power=Work/Time. A unit of measure of power is HP. 1 HP=550 ft-lb of WORK per second. HP=Torque*RPM/5252.
Yep. It can be measured by work over time, or by force times speed (as in your example of horsepower) or by torque times rotational speed. It can even be measured by volts times amps; indeed, kilowatts and horsepower are now used almost interchangeably by motor manufacturers since they represent the same thing (power.)
That's how it DOES work. If density increased in a venturi there would be no increase in airspeed. Since the air does not change density, it speeds up to get the same air mass through the narrower opening.
They said they arrived at this design by computer modeling. To me that suggests that the overall structure was optimized to give max. power at min. cost. And since they need an external power source to boost the power above nominal, then that would require their model to try to induce a low pressure region near the tail end. But no such aerodynamic surfaces are present. Of course all of this confusion would be cleared up if they had simply been more open about the core facts needed to clear up the technical questions going on here. I also looked on the sketchy and problematic data they posted. It seems to suggest that one one day the ground wind velocity was higher than up on the pole. So much so, they they got a whopping 9:7 speed improvement, which amounts to more than double the wind power (729:343). It seems odd that this would happen. I even wondered if it was a glitch in their data. I've thought about some "poor man's" energy devices, ways to give renewable energy devices to impoverished people. One idea that occurred to me was something along the lines of wind schooners, only tethered to a shaft that would convert tension in a rope to torque, and from there drive a generator. For all the low efficiency concerns, it's conceivable that this could be done with scrap materials. Then if a benefactor would provide the generator/battery set in a ruggedized box, they'd have at least a few watts to work with. Some of the places I had in mind also experience prevailing winds so I was thinking it might be feasible. I suppose it would be like giving them a donkey that doesn't consume food and water. Something along that line, anyway. I was reminded of this when I saw that picturesque windmill. That's right. And the designers knew going into this that wind power increases as the cube of wind speed. It looks to me like they naively believed that turbine electrical power should increase simply by placing it in a venturi. That's why I wondered if this weren't an air conditioner repairman's solution to the energy crisis. I say that because they work with venturis without knowledge of fluid mechanics. I could see how such a misconception might occur to such a person. The reason why it can't increase the power (energy) is the constraint due to conservation of energy. The density of the air has to drop in compensation for the increase of velocity of air molecules. Kinetic energy is ½mv², so if the velocity doubles, then the mass must drop by 4 as fewer molecules are able to get through the restriction. There's no change in net energy even though the air is moving faster. Therefore no power gain is possible. Of course we knew that going into this, with or without a venturi. Exception: They may have improved the coupling of those air molecules to the blades, if ever so slightly. If that's the case, then it suggests that this particular model of a turbine might run more efficiently in a cowling of some kind. At least a short duct might help; I see nothing to suggest that this elaborate ductwork is necessary, if anything it calls for a better designed turbine.
why don't you stalk and harass someone else. it was russ watters that said density remained unchanged in a venturi density decreases as airpeed increases. i might add that this assumes no heat transfer.
Look at any sailboat tacking (against the wind at an angle) upwind. This is the advantage of sails which can be shaped to form the most efficient veturi effect. In airplanes it is called "lift" I understand the argument of size and windloss from other structures, but if you have ever sailed, you will understand the awesome power of wind wherever it is present and low height makes "trimming" the sails for maximum efficiency easy. While a sail rotor never turns as fast as a blade type rotor, its power can easily be used to step up speed at the generator.
Definitely! And as sails have gotten more advanced they look more and more like airfoils and less like big flapping pieces of canvas. Some modern sails are now rigid wing masts; several America's Cup winners have used this design to get more performance out of their boats. Now look at a modern wind turbine. Note how similar the rotor looks to a wing mast (and to the wing of a sailplane.) Modern turbines are getting very close to the best possible efficiency. I started sailing when I was 14! I learned to fly when I was 22, and got my seaplane rating a few years after that. Seaplanes especially are a fascinating hybrid of the two - an aircraft that can "sail" due to the potential to set water rudders and air rudders differently. Modern turbines do this by changing the pitch angle of the rotor blades, allowing the system to change the angle of attack of the blades to the wind to optimize power production. Agreed. And I often fly an aircraft made of fabric (a ram-air parachute.) No parachute or paraglider will ever equal the efficiency of a rigid-wing sailplane, though - nor will a flexible rotor system ever equal the efficiency of a well designed rigid one.
Because you, not someone else, said "density IS NOT constant when moving through a venturi." It is. You might be confusing pressure with density. They are not the same thing.
you said it, i didn't. less pressure means more rarefied, as in less dense. i'm not going to haggle with you over this.
