CALCULATION OF THRUST FORCE

[Kanwarosama]

HI,
I am doing a project where i need to provide to do the structural anlysis of a wind turbine tower. For this purpose, I calculated the thrust force using the actuator disk theory. The problem with this is it provides me a huge point load of 800 kN. and when i apply this load on the tower it creates a high deflection like 2 meters or so. In my opinion, the problem lies in the use of actuator disk and then applying a huge load as a point force.
Does anybody who has worked with wind turbine analysis provide me with a better idea about how should i apply/calculate the loads so i get reasonable deflection etc.

 

[GregLocock]

I suspect you'll get much better answers if you supply your calculations. However the assumption that it will be a point load on the mast is correct, and St Venants principle says that the details of the loading on the tip of a cantilever won't affect the deflection much anyway.

Cheers

Greg Locock


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[Kanwarosama]

Hi,
I have a mast that is 150 meters, the diameter of rotor is 162 m, the thrust coefficient is 0.635 and wind speed is 10 m/sec.
Now, by the equation
T= 1/2* density of wind*area of rotor*thrust coeffcient* speed of wind^2.

So I get around 800 by this. Applying such huge force obviously causes very heavy deflections. But my question is that although this actuator theory is present in literature everywhere, is this a reasonable estimation of load? Or if this is applying it as a point load, does this make sense?

 

[rb1957]

are you using the right density ? mass density, not weight density.

what density number are you using ?? ... yeah 1.225 kg/m3 gives 800kN ... I don't see anything obviously off.

is your model correct (stiffness wise) ? you're off by a factor of 1000 ... hummm, metric system ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Kanwarosama]

I am using air density as 1.25 kg/m3.

 

[GregLocock]

So your load looks about right, what are the structural properties of your mast? What equation are you using for deflection?

I'd have thought the masts were tapered, so analysing them is going to be a bit trickier than just the plug and play one liner WL^3/3EI

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[jhardy1]

162 metre diameter is a BIG wind turbine - quite a few megawatts generating capacity! 800 kN doesn't sound unreasonable to me. By way of comparison, a "typical" 3.5 MW offshore wind turbine might have a 100 metre diameter rotor, with a design operating horizontal wind load of around 1 MN.

If the mast deflection is excessive, you may well need a stiffer mast.

http://julianh72.blogspot.com

 

[Kanwarosama]

Yes,
I understand this. I have the fem model which I built from an already exisiting wind turbine. The fem model has the tower but not the blades. The model works fine for modes of vibration, etc but just this deflection is a problem. 800 kN also sounds reasonable but thats what i dont understand that actual wind turbine tower has a deflection less than a meter, while I have it at 3 meters.
So, something isnt adding up for me. I am assuming thats its due to the fact that I am applying a point load but all structural models I see do apply this as point load. So thats where i am stuck.

 

[Kanwarosama]

My structural model is that there is the tower which has diameter of about 6.5 m at base and 3.5 meters at top. Thickness about 45 mm at bottom and 18 mm at top
Yes, its tapering as well.
The deflection calculation i did theough fem and simplified by hand. Both match reasonably. So I am sure if that equation lets say is right (which it is) , and I have calculated the right value then may be the only thing is how to put this load on tower. What do you think, the assumption that thrust from the blades acts a point load on tower, does it make sense?

 

[rb1957]

I think it s very reasonable ... except, as I think of it, wind turbines are not, AIUI, designed to produce thrust, but rather torque. Sure there'll be a thrust component, but I think most of the "thrust" is in the tangential direction.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[GregLocock]

I disagree, that axial load on the motor must be present. That is force *(wind velocity-exhaust velocity)=power (roughly) So now we're down to an error of just 300%. Aluminum or steel? Bad bcs?



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[rb1957]

ok Greg, I'll defer to your greater (but not "greta" !!??) knowledge on the subject (almost everyone has more knowledge than me on the aerodynamic design of wind turbine blades).

But please explain to me why a wind turbine blade would "want" to produce thrust like an airplane propeller ? Sure there'll be some thrust component but this is wasted on a turbine blade. Don't turbine "want" torque (rather than thrust) ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[BridgeSmith]

I'd have thought the masts were tapered, so analysing them is going to be a bit trickier than just the plug and play one liner WL^3/3EI

From the AASHTO LRFD Sign spec:

 

[GBTorpenhow]

But please explain to me why a wind turbine blade would "want" to produce thrust like an airplane propeller ? Sure there'll be some thrust component but this is wasted on a turbine blade. Don't turbine "want" torque (rather than thrust) ?

It is wasted in the sense that it doesn't generate power, yes. OP is using 'thrust' but a better analogous term might be 'drag'. It happens whether you want it to or not. As in, why would a plane "want" to produce drag?

 

[rb1957]

an airplane propeller is intended to produce thrust, a turbine propeller is intended to produce torque; at least that's what I think when I look at the "weird" propellers they use on turbines. Rather than being designed to produce fwd (thrust) force, I see these blades producing tangential force (that'll create torque on the motor) and (as I've said) some fwd force will be created but this is not useful (much like drag on an airplane).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[GBTorpenhow]

To produce thrust, a propeller must have torque applied to it. To produce torque, a turbine must have thrust applied to it.

 

[Gr8blu]

I have climbed more than a few wind turbines over the years. Most have been 90-110m for the tower portion, with another 5-10 m to get to the top of the nacelle. In these, a deflection (or movement if you prefer) of 2 m at the yaw deck (where the vertical tower column meets the horizontal orientation of the nacelle) is not all that uncommon. You don't always see it from outside on the ground because of the relative size of the components but it IS there. To make matters worse - it isn't always constant, either. It's a bit ... umm ... eccentric in its orbit.

As an aside - it's pretty easy to get the same motion at the top of a 35 m mast on a sailing ship, not even accounting for wave action on the hull.

Converting energy to motion for more than half a century

 

[Kanwarosama]

Hi,
Thank you all for your nice commemts. Bit Gr8blu, I have asked the operator that what kind of deflection they usually get, i got the answer that its about something less than a meter even when the full load is on.
Also I want to test my model for very extreme situation, which will cause even more deflection. But if you have experience, can you please tell me what is the maximum you have seen. And what might be an alarm situation. As in the standards, there is no limit defined.

 

[Gr8blu]

The maximum I've seen was with a wind speed of something like 15 m/s. For the tower I was in at the time, I would estimate that we were moving about 3.5 m relative to a point at the tower base. That's a bit hard to judge accurately though since the actual movement is, as I mentioned earlier, a bit erratic/eccentric.

As far as alarming goes - it's not based on tower movement (directly), but more on the BLADE deflection and/or tip speed under specific wind conditions. Practically speaking, a modern wind tower generates power with "average" wind speeds between 7 and 15 m/s. In that average range, a gust might be up as high as 20-22 m/s. The reason that blade is used as the limiting factor is that the tower is - generally - more ruggedly built. Kind of like thinking "I can replace a blade if I have to, but I don't want to replace the whole tower (and possibly foundation)". Or in terms of a car - "I can replace the suspension components, but I don't want to replace the whole frame".

Converting energy to motion for more than half a century

 

[Kanwarosama]

But then the question would arise that if tower is such ruggedly built why it displaces like 3 meters or so. I mean if based on structural engineering, this is very very huge. I know that when I tell someone this they always say its not possible and i also believe it so. Also the ooperator told me thag it stays under a meter or so.