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Is it possible for the two blade sections to experience the same angle of attack?

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Old   September 16, 2020, 13:02
Default Is it possible for the two blade sections to experience the same angle of attack?
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Hi,


I did RANS simulations for a wind turbine blade. I took two blade sections and compared with the experimental results of the same sections. In the wind tunnel experiments, one blade section was tested under different angles of attack ranging from 2 -18.



Now, when I compare my simulation results to those of experiments, I notice that results for both sections agree well but at the same angle of attack. I was wondering to know whether it is possible for two different blade sections to experience same angle of attack?



I would highly appreciate your comments.
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Old   September 16, 2020, 17:32
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Quote:
Originally Posted by mazhar16823 View Post
I was wondering to know whether it is possible for two different blade sections to experience same angle of attack?

Hi, I'm sorry but I don't really understand your question?


What angles of attack did you simulate? I would expect that at low angle of attack you have better results than high (due to separation), is this what you are asking?
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Old   September 16, 2020, 19:57
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Quote:
Originally Posted by DeadLee View Post


I would expect that at low angle of attack you have better results than high (due to separation), is this what you are asking?

What you said above is true, but my question is different. Let's say I have two airfoils taken from the wind tunnel experiment. Now, the geometry of these airfoils i.e. section-1 and section-2 is same as that of two airfoil sections namely section-1 and section-2 on a wind turbine blade deployed at a site. (The section names are given in such a way that they correspond to a specific location on the blade).



However, it is known that the two airfoils (wind tunnel) were tested under several angles of attack (say 2 - 18 degrees). On the other hand, the wind turbine blade is simulated using numerical approach.



Now, I want to make a comparison of section-1 &nd 2 (wind tunnel results) and the section-1 and 2 of the blade. As both sections have different position on the blade they should undergo different angles of attack right?



OR it is possible that two airfoil sections positioned at different station can experience same angle of attack?
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Old   September 16, 2020, 20:22
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Quote:
Originally Posted by mazhar16823 View Post
As both sections have different position on the blade they should undergo different angles of attack right?


OR it is possible that two airfoil sections positioned at different station can experience same angle of attack?

You're talking about the twist of the blade? I mean, the angle of attack changes as you go along the blade? If that is what you are asking..



Well that all depends on your blade geometry. Usually for horizontal axis wind turbines it changes but it doesn't have to. For vertical axis wind turbines it depends.
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Old   September 16, 2020, 20:28
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Quote:
Originally Posted by DeadLee View Post
You're talking about the twist of the blade? I mean, the angle of attack changes as you go along the blade? If that is what you are asking..



Well that all depends on your blade geometry. Usually for horizontal axis wind turbines it changes but it doesn't have to. For vertical axis wind turbines it depends.

Yes exactly. I am a little concerned about whether it is possible. Because if I present my simulation findings saying that two different blade sections compare well with wind tunnel experiment at the same angle of attack. I might be questioned on how is it possible for two blade sections to undergo same angle of attack since a 3D blade undergoes several degrees of angles of attack.
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Old   September 17, 2020, 06:23
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Originally Posted by mazhar16823 View Post
Yes exactly. I am a little concerned about whether it is possible. Because if I present my simulation findings saying that two different blade sections compare well with wind tunnel experiment at the same angle of attack. I might be questioned on how is it possible for two blade sections to undergo same angle of attack since a 3D blade undergoes several degrees of angles of attack.
Ok good, firstly some points to make your sentence clearer now I understand.

Section : I think it would be better to say spanwise position or location instead. When you talk about blade sections it's easy to imagine you are using different blade profiles (NACA 0015 etc) which is definitely possible too. Of course if you have an image this will be even clearer.

Undergo : We say a blade has several angles of attack (along it's spanwise direction). Undergo is when something happens to something.

Possible: Everything is possible, we can make the blade how we want The real question is why the blade is made that way.

So I think the question you are asking is: why does the blade have the same angle of attack along it's length?

Or even shorter: why does the blade not have twist?


Answer: This depends on a lot of things

Did you have a choice in setting the twist of the blade?

Or did you just get given a geometry and have to run the simulation?
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Old   September 17, 2020, 10:20
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Quote:
Originally Posted by DeadLee View Post
Ok good, firstly some points to make your sentence clearer now I understand.

Section : I think it would be better to say spanwise position or location instead. When you talk about blade sections it's easy to imagine you are using different blade profiles (NACA 0015 etc) which is definitely possible too. Of course if you have an image this will be even clearer.

Undergo : We say a blade has several angles of attack (along it's spanwise direction). Undergo is when something happens to something.

Possible: Everything is possible, we can make the blade how we want The real question is why the blade is made that way.

So I think the question you are asking is: why does the blade have the same angle of attack along it's length?

Or even shorter: why does the blade not have twist?


Answer: This depends on a lot of things

Did you have a choice in setting the twist of the blade?

Or did you just get given a geometry and have to run the simulation?

Yes. I just got the geometry and ran the simulation. However, the imported blade geometry already comes with following information (pls refer the image):


- The rotor was pitched say "x" degrees towards the pressure side around the Z-axis as in the picture.



- And the blade is tilted "x" degrees towards the pressure side around the X-axis in given coordinate system.
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Old   September 17, 2020, 10:22
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Quote:
Originally Posted by mazhar16823 View Post
Yes. I just got the geometry and ran the simulation. However, the imported blade geometry already comes with following information (pls refer the image):


- The rotor was pitched say "x" degrees towards the pressure side around the axis as in the picture.



- And the blade is tilted "x" degrees towards the pressure side around the x-axis in given coordinate system.

Sorry. Here is the image attached for the blade orientation.
Attached Images
File Type: png blade.png (12.8 KB, 5 views)
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Old   September 17, 2020, 12:48
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1) You were given a blade which has twist/pitch/tilt, whatever we call it.

2) You ran simulations and compared results at 2 places to the wind tunnel test (at same angle of attack between CFD and test)

3) The comparison between the CFD and wind tunnel is good.

I don't really see the problem/question, you did what you were asked and got good results! No?
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Old   September 17, 2020, 12:55
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Quote:
Originally Posted by DeadLee View Post
1) You were given a blade which has twist/pitch/tilt, whatever we call it.

2) You ran simulations and compared results at 2 places to the wind tunnel test (at same angle of attack between CFD and test)

3) The comparison between the CFD and wind tunnel is good.

I don't really see the problem/question, you did what you were asked and got good results! No?

Got good results,yes but my confusion is not in 2D airfoil simulations and wind tunnel comparison. But in the comparison of two positions on the wind turbine blade and wind tunnel. If those two positions in wind tunnel were tested under different angles of attack, does it make sense that two positions on the wind turbine blade can have same angle of attack?



Earlier, you said it is possible but how and under which conditions?
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Old   September 17, 2020, 14:28
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Originally Posted by mazhar16823 View Post
If those two positions in wind tunnel were tested under different angles of attack, does it make sense that two positions on the wind turbine blade can have same angle of attack?
You are comparing a 2D/3D wind tunnel airfoil/wing experiment to a 3D CFD wind turbine simulation at a specific location, right?

We can make the design however we like.
If it's a good design, it makes sense. That's it.

Quote:
Originally Posted by mazhar16823 View Post
Earlier, you said it is possible but how and under which conditions?
If I knew I would be very rich
These kinds of questions take years to answer and are the whole point of CFD.
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Old   September 17, 2020, 14:32
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Quote:
Originally Posted by DeadLee View Post
You are comparing a 2D/3D wind tunnel airfoil/wing experiment to a 3D CFD wind turbine simulation at a specific location, right?

We can make the design however we like.
If it's a good design, it makes sense. That's it.



If I knew I would be very rich
These kinds of questions take years to answer and are the whole point of CFD.

Alright thanks. So, I won't worry about it anymore and will leave to my future research lol!. Actually the main thing which I was confused with, is about the Large eddy simulation which was carried out on 50% span of the same wind turbine blade in a restricted domain in which results of one position has an angle of attack higher than mine (RANS). For instance, if my results agree with wind tunnel at 4 degrees AOA, in LES they agree at 8 degrees AOA.
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