wind turbines blades

[michel123456]

Well I was wondering watching a wind turbine park newly installed on a mountain here close. The shape of the blades look like that of an airplane, about like in the following picture.

 

There is an obvious difference with the shape given by ancients who had only empirical knowledge, like in this windmill here below

 

Empirical evidence says that you get the stronger force at the largest radius, and the Greek has constructed the windmill with the largest surface of the 'blades" far from the centre.

Why are the blades in a modern wind turbine with the shape getting smaller to the edge? It is not the same principle as in an airplane where the motor gives the impulse, here we want to get the impulse from the wind. So why?

Edited August 9, 2011 by michel123456

[CaptainPanic]

Why are the blades in a modern wind turbine with the shape getting smaller to the edge? It is not the same principle as in an airplane where the motor gives the impulse, here we want to get the impulse from the wind. So why?

I think that there is a limit to how thin you can make the blade: if you make them any thinner they would break.

Similarly, there are limitations to how fat you can make the outer part.

 

But I am sure that there are other arguments too (a combination of aerodynamics in different winds, noise, and of course investment costs).

 

Remember, those blades are pretty huge, so one of the primary problems is to keep them in 1 piece.

(source of picture)

[michel123456]

Right. But i still don't understand why they are shaped this way. I think they could be cut as an oar. Why getting thinnner like a knife?

 

Why not like this:

Edited August 9, 2011 by michel123456

[Externet]

The aerofoil profile determines the 'rotatability';

the blade thickness is made thicker near the hub for structural stiffness;

the lenght of blades determines the swept 'disc' area which is proportional to the power capture capability;

the narrowness of the blades allow some change in the wind speed while passing trough the swept disc yielding more uniform power extraction. If they were too narrow, wind would not slow down enough to get optimal power out of it; if made too wide wind slows down too much and diverts toward outside of the swept 'disc' yielding less power.

Economics, manufacturing , transporting, assembly and even aesthetics have a role in the shape of blades.

A wide tip blade as you mention would turn with a much greater force when hit by wind, but when gets to speed, the extractable power becomes less.

-As far as I understand the thing...

[Cap'n Refsmmat]

Generally, increasing the number of blades or the blade area gives relatively little extra energy but makes the turbine heavier and more expensive:

 

http://en.wikipedia.org/wiki/Wind_turbine_design#Blade_count

[J.C.MacSwell]

Right. But i still don't understand why they are shaped this way. I think they could be cut as an oar. Why getting thinnner like a knife?

 

Why not like this:

 

This is not just a difficult shape structurally,given the dynamic load of the placement of the greater surface area, but terrible aerodynamically. The induced drag at the tips would be very high as the high pressure air on the windward side is drawn around the tip toward the low pressure leeward side. Better is a smaller, thinner, slightly feathered tip that not so much harvests power as reduces the pressure gradient around the tip and thus reduces the induced drag which robs efficiency and reduces power generation.

[michel123456]

This is not just a difficult shape structurally,given the dynamic load of the placement of the greater surface area, but terrible aerodynamically.

 

it is the shape chosen by Greek, Dutch, Spanish windmills that last for centuries.

 

The induced drag at the tips would be very high as the high pressure air on the windward side is drawn around the tip toward the low pressure leeward side.

That is the reason why the structure will rotate.

 

Better is a smaller, thinner, slightly feathered tip that not so much harvests power as reduces the pressure gradient around the tip and thus reduces the induced drag which robs efficiency and reduces power generation.

 

i thought the main goal is to harvest the most power available. It is not an airplane where the motor must produce thrust, it is the opposite procedure.

 

The triangular shape of my last picture is nothing but half a regular blade, like below:

 

 

Ships design (hydrodynamics) has also been through the same principles. Ancient hull design was based on the belief that a regular and complete gentle curve would make the boat glide over the water in the best way. Contemporary design produce chamfered shapes because it has been understood that the second half of the shape is not necessary. see below

 

 

[InigoMontoya]

I've no knowledge of wind turbines so I'm not saying anything towards the larger thread. However...

 

Ships design (hydrodynamics) has also been through the same principles. Ancient hull design was based on the belief that a regular and complete gentle curve would make the boat glide over the water in the best way. Contemporary design produce chamfered shapes because it has been understood that the second half of the shape is not necessary.

This is simply not true.

 

What you see in your chamfered shape is the waterline. Look at modern ships and yes, you'll see a "cut off" stern, but the cut off point doesn't extend below the waterline and as such is irrelevant to the ship's hydrodynamics. Your rendering shows the shape, but it doesn't show how such a shape sits in the water. Try this one on for size...

 

 

The chamfer is there, but it's not in the water. *IN* the water, you absolutely want "both halves" of the shape.

 

 

 

edit: One comment: It's true that you'll see less than optimal hull designs in pleasure craft, however, this is done knowingly by the boat designers who are trying to build the biggest boat that you can still easily get on a trailer and drag home. For that reason - and the fact that efficiency isn't a huge deal for the pleasure craft crowd - you will see boat designers accept the efficiency hit incurred by chopping off the stern. But in ships? Where efficiency is a very big deal and you're not trying to trailer it? Nope. The only exception I'm aware of are amphibious assault ships, but again, it's a design trade off the engineers are knowingly making.

Edited August 10, 2011 by InigoMontoya

[michel123456]

This is simply not true.

 

I am ready to go back and believe you.

 

But I see the same chamfering in modern cars.

 

 

A kamm tail is a aerodynamic profile which has a section cut off and it tricks the air into thinking that the section is still there. So if you look at the Honda Insight the rear end has a tiny lip and the tail of the car is dead flat. This lip and the cars cut off end trick the air into thinking that the swoopin shape continues to the ground and greatly improves the aerodynamic properties of the car and this helps a lot with pushing your fuel efficiency up and engines stress levels down. Below is a classic example of the Kamm tail on a classic race car.

Edited August 10, 2011 by michel123456

[insane_alien]

Wind turbines are the shape they are because of over century of propeller design, aerodynamics and the advent of computational fluid dynamics.

 

vintage windmills were based on the aerodynamics of sailing ships.

 

there is a big difference in knowledge now, we can optimise for the most efficient shape rather than guessing a bit.

[InigoMontoya]

I am ready to go back and believe you.

 

But I see the same chamfering in modern cars.

Sure, and the Kamm tail is better than a blunt tail, but it's still not as good as a fully contoured tail.

 

Too, the cars you're showing in your examples are being measured by more than just aerodynamic drag.

 

The Honda Insight has a trunk. People want that trunk to be useful and easy to get at. Fully tapered tails don't lend themselves to such. So.... Honda makes a design trade. They get improved functionality at the cost of less than optimal aerodynamics.

 

The race car folks want to be able to corner well. In the simplest terms, the car's moment of inertia (ie, inertial resistance to cornering) is related to the CUBE of the car's length (there's more to it than that, but I'm simplifying for the purpose of this conversation). Cutting 10% off the length of the car allows your car to corner 25% faster (again, a simplification). The larger point being that once again a designer has sacrificed aerodynamics for another design goal.

 

Now look at a car whose design is more heavily weighted towards aerodynamics....

 

(BTW: The above car gets 112 mpg)

 

And think about actual aircraft. Find me a modern aircraft with a blunt tail. With weight being so important with aircraft, wouldn't they like to lose the weight of an elongated tail? You bet they would! But a truncated tail kills 'em in the drag department and makes the long tapered tail worth it.

Edited August 10, 2011 by InigoMontoya

[J.C.MacSwell]

it is the shape chosen by Greek, Dutch, Spanish windmills that last for centuries.

 

 

That is the reason why the structure will rotate.

 

 

 

i thought the main goal is to harvest the most power available. It is not an airplane where the motor must produce thrust, it is the opposite procedure.

 

The triangular shape of my last picture is nothing but half a regular blade, like below:

 

 

Ships design (hydrodynamics) has also been through the same principles. Ancient hull design was based on the belief that a regular and complete gentle curve would make the boat glide over the water in the best way. Contemporary design produce chamfered shapes because it has been understood that the second half of the shape is not necessary. see below

 

 

 

Induced drag resists rotation. That is why it is called drag. (not being funny, that is the definition in this case)

 

The goal is trying to harvest the most power. Adding extra blades would contribute power as well, yet make the rest less effective, to the point they cost more than they contribute, but at low speed they are more effective... so the Greek, Dutch and Spanish were right for their time.

 

You are right about the truncation of the ships and cars. The energy has been dissipated due to friction and form drag and cannot be recovered (or barely worthwhile for practical reasons)...but the airflow/waterflow is not even in the same direction in this case.

 

I've no knowledge of wind turbines so I'm not saying anything towards the larger thread. However...

 

 

This is simply not true.

 

What you see in your chamfered shape is the waterline. Look at modern ships and yes, you'll see a "cut off" stern, but the cut off point doesn't extend below the waterline and as such is irrelevant to the ship's hydrodynamics. Your rendering shows the shape, but it doesn't show how such a shape sits in the water. Try this one on for size...

 

 

The chamfer is there, but it's not in the water. *IN* the water, you absolutely want "both halves" of the shape.

 

 

 

edit: One comment: It's true that you'll see less than optimal hull designs in pleasure craft, however, this is done knowingly by the boat designers who are trying to build the biggest boat that you can still easily get on a trailer and drag home. For that reason - and the fact that efficiency isn't a huge deal for the pleasure craft crowd - you will see boat designers accept the efficiency hit incurred by chopping off the stern. But in ships? Where efficiency is a very big deal and you're not trying to trailer it? Nope. The only exception I'm aware of are amphibious assault ships, but again, it's a design trade off the engineers are knowingly making.

 

It is in the water at cruising speed, the speed the hull was designed to be efficient at.

 

Sure, and the Kamm tail is better than a blunt tail, but it's still not as good as a fully contoured tail.

 

Too, the cars you're showing in your examples are being measured by more than just aerodynamic drag.

 

The Honda Insight has a trunk. People want that trunk to be useful and easy to get at. Fully tapered tails don't lend themselves to such. So.... Honda makes a design trade. They get improved functionality at the cost of less than optimal aerodynamics.

 

The race car folks want to be able to corner well. In the simplest terms, the car's moment of inertia (ie, inertial resistance to cornering) is related to the CUBE of the car's length (there's more to it than that, but I'm simplifying for the purpose of this conversation). Cutting 10% off the length of the car allows your car to corner 25% faster (again, a simplification). The larger point being that once again a designer has sacrificed aerodynamics for another design goal.

 

Now look at a car whose design is more heavily weighted towards aerodynamics....

 

(BTW: The above car gets 112 mpg)

 

And think about actual aircraft. Find me a modern aircraft with a blunt tail. With weight being so important with aircraft, wouldn't they like to lose the weight of an elongated tail? You bet they would! But a truncated tail kills 'em in the drag department and makes the long tapered tail worth it.

 

Exactly... and this is why the ship was truncated where it was also. Extending the ship a little further still above the waterline would improve calm water hydrodynamics at cruising speed (and especially above) , but the increased radius of gyration of the ship in a sea (similar to your "cornering" above) and dock length in port would not make it worthwhile.

[michel123456]

Wind turbines are the shape they are because of over century of propeller design, aerodynamics and the advent of computational fluid dynamics.

 

vintage windmills were based on the aerodynamics of sailing ships.

 

there is a big difference in knowledge now, we can optimise for the most efficient shape rather than guessing a bit.

 

I suppose so. But I would like to know on which principle we came from the one to the other.

 

 

The old design puts surfaces far from the center, which seems logical. The new design: what is its principle?

Edited August 11, 2011 by michel123456

[Pantaz]

It's helpful to think of them as propellers. The basic principles are very similar. With that in mind, note that the cross-section of a propeller blade is an airfoil -- a wing, and since the air speed over the blade increases as you move farther toward the tip, the chord (distance from leading edge to trailing edge) and angle of attack gradually decreases (that's the twist you see along the length of a propeller blade) to distribute loading evenly over its length.

 

The "Blade Design" section of Wikipedia's wind turbine design page provides a good overview of why modern blades look the way they do. The wind turbine aerodynamics page goes into much more detail.

 

Also, NASA's airfoil simulator may help you visualize changes in drag and lift with different shapes (including a simple flat-plate) and angles.

[J.C.MacSwell]

I suppose so. But I would like to know on which principle we came from the one to the other.

 

 

The old design puts surfaces far from the center, which seems logical. The new design: what is its principle?

 

One principle is that the more efficient you can make the blade, the faster it will go and the more swept area it will cover. Of course it all has to rotate at the same rpm so the outer part of the blade will be moving faster and sweeping more area. There is a maximum theoretical amount of energy you can harvest given the energy available in the approaching disc of air, the Betz Limit or 59.3%: http://en.wikipedia.org/wiki/Betz'_law If you attempt to harvest more you would get less. If you tried to get it all you would get none as you would have choked everything to a halt.

 

The old design is great if you want high torque at low speed, but as it increases speed each blade interferes with the next. Same reason you would not put one turbine in front of, or even in too close proximity to another. It does self starts a lot more readily. The newer ones take forever and are often powered to get them going.

Edited August 11, 2011 by J.C.MacSwell

[InigoMontoya]

[deleted]

Edited August 12, 2011 by InigoMontoya

[Enthalpy]

And because blade tips move much faster (almost 100m/s) than the harvested wind (15m/s) they do use all the swathed area. In fact, blades are built to run fast so that they need little material to swath the huge necessary area. As well, it allows to use moderate wind but resist storms just by tilting the blades. These are the big advantages of modern turbines, with narrow blades covering a big area.

 

Presently, wind turbines are at Betz's limit (or even little above, because this limit is approximative), which proves they don't waste area at their tip. So they wouldn't need to be wider near the shaft to harvest wind there; I believe to understand they must be thicker at their root to resist the bending moment, and their width only improves streaming around this big thickness.

 

It's the drawback of blades running faster than the wind: their lifting force increases a lot and exceeds the useful forward force by a big factor - for instance because the blades run nearly flat. The resulting moment stresses the blades near the shaft, and the lifting (=downwind) force stresses the bearings and the pole.