Simulation of a rotor stator configuration

AS_Aero · October 30, 2018, 09:31

In addition to it here is the streamlines for 3 different rpms 50, 400 and 500 and I really cant tell actually which one is generating energy or consuming energy. Can someone infer from this about the rotor blades ?

ghorrocks · October 30, 2018, 17:56

The torque is the area integral of the pressure on the blades, plus the wall shear. In addition, your device has many blades some of which will have a net force in the positive direction and some of which will be pushing in the negative direction. The net torque on the entire blade array is the sum of the contribution of all the individual blades and in your case the individual blade loading will be varying greatly.

You can't estimate that from images. You just need to use the torque functions to see the net torque.

But you can see that your device has many large separations and some blades pushing in one direction and some the other - in other words blades are working against each other. This means this device is going to have a very low efficiency compared to things like horizontal axis wind turbines or windmills as these devices have all blades pushing in the same direction.

AS_Aero · October 31, 2018, 03:42

This was just to show the difference between two RPMs and I am still not sure about this negative torque value where it is coming and how. And I thought someone could give me a reasonable answer based on these streamlines why I would have a negative torque in RPM50 and possitive in RPM500. Because as per the explanation from previous thread other person told me that for RPM500 it should be consuming energy to rotate the turbine and wind speed wornt be enough. So am wondering.

Gert-Jan · October 31, 2018, 04:02

Don't look at streamlines since they don't drive your rotor. They are a result of pressure differences.
So don't solely look at the effect but at the cause......

AS_Aero · October 31, 2018, 05:20

So what exactly could give me some insight about this problem ? Lets say my setting is completly correct and its giving me negative power till RPM 400 and from RPM500 its positive. Does this mean that my turbine should work at rpm500 and above ?
Secondly how to know which blades are working and which not and for that I can see the pressure countour or ? There are some high pressure zones

ghorrocks · October 31, 2018, 17:05

If you want to know which blades are working and which are not then define a torque function on each blade. Then you will see which are working and which are not.

You seem hung up on the negative power. We have said many times that this is probably just due to the way you have defined the positive and negative directions, and the negative torque up to 400rpm for 10m/s wind is the device generating power and the positive torque above that is the device loosing power (and would therefore slow down to below 400rpm to be in the generating power region). If this is the case then your device generates power with a negative power from your model.

But as has been said many times, you need to check your rotation direction definitions to ensure that this is the case.

Gert-Jan · October 31, 2018, 17:19

Quote:

Originally Posted by AS_Aero

So what exactly could give me some insight about this problem ? Lets say my setting is completly correct and its giving me negative power till RPM 400 and from RPM500 its positive. Does this mean that my turbine should work at rpm500 and above ?

Didn't you understand what we were saying? No matter what sign your power has, your turbine can generate power at low speed and will consume power at too high speed. No way your case will work at rpm500 and above if the sign has changed between 400 and 500 rpm.

Quote:

Originally Posted by AS_Aero

Secondly how to know which blades are working and which not and for that I can see the pressure countour or ? There are some high pressure zones

I refer to the last post from Glenn, he clearly explains what is happening. You have to separate the blades to see what is happening on each individually.

To my opinion, in this forum we help you with the setup of this system, but are not going to do the whole analysis/postprocessing.

AS_Aero · November 1, 2018, 04:39

Hi Gert

I understood what you are talking about the power sign. But as I mentioned you already, I dont see an evident mistake in the rotation direction I have given. Thats where I wanted you to see the image atleast of my setup so that you can figure out where I went wrong, as you have more experience on this. The thing is I have mentioned many times that my rotor is rotating in anticlockwise direction, that means I have given -RPM in the rotor domain angular velocity. But does this be the reason for bringing the sign of power in negative ?
Again it would be really great if you could have a look on the image of the rotor domain setup which I posted long ago in this thread so that you can tell me whats going on actually. And for sure the rotor above 500RPM where I am getting positive sign means its consuming power and not helping to generate ? And for high RPMs its not meaningful to generate power ? IS that what you are telling ?

Gert-Jan · November 1, 2018, 05:10

The convention is that you follow the alphabet......

You have to position yourself on the axis of rotation and then look in positive direction of the rotation axis. In your case I think this is along the Global Z-axis. Then you look where the Y- and X-axis are. For your rotation, the Y-axis needs to rotate to the X-axis. This means your angular velocity is negative, because if you go in the alphabet from Y to X, you go backward.
A positive angular velocity would mean that X rotates to Y because in the alphabet X comes before Y, i.e. you move forward.

AS_Aero · November 2, 2018, 04:37

Hi Gert
This explanation kind of gives me an impression that my negative sign in the power is physical and its just the sign and it does mean that power is generating by the turbine till 400 rpm and after that its consuming power ? And as you might have seen the images of the geometry nd my rotor domain setting. I have thickness of 2mm in the positive z direction. And the flow is in the positive X direction and anguar velocity is given negative as my rotor blade is rotating from Y to X or in the anticlock wise direction. And thats how it have to rotate. And hence i have given angular velocity as -100rev/min or -400 rev/min etc... so this make sense for this negative sign right ? So does that mean I just multiply the torque with a negative sign ? Then everything will be meaning ful ?

Gert-Jan · November 4, 2018, 17:58

Yes. Everything is meaning full.

AS_Aero · November 5, 2018, 05:12

Perfect
Thanks a lot Gert. So I dont have to find the Zero Power point right ? I can just plot the RPM vs Power Curve till 400 and then see where I have the maximum power value and take that as my optimum RPM value right ?

Gert-Jan · November 5, 2018, 05:36

If you are not interested in the zero power point, then I think your approach is ok. You should always tune your CFD-calculations to the question that you need to answer.

October 30, 2018, 17:56		#22
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,869 Rep Power: 144	The torque is the area integral of the pressure on the blades, plus the wall shear. In addition, your device has many blades some of which will have a net force in the positive direction and some of which will be pushing in the negative direction. The net torque on the entire blade array is the sum of the contribution of all the individual blades and in your case the individual blade loading will be varying greatly. You can't estimate that from images. You just need to use the torque functions to see the net torque. But you can see that your device has many large separations and some blades pushing in one direction and some the other - in other words blades are working against each other. This means this device is going to have a very low efficiency compared to things like horizontal axis wind turbines or windmills as these devices have all blades pushing in the same direction. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

October 31, 2018, 17:05		#26
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,869 Rep Power: 144	If you want to know which blades are working and which are not then define a torque function on each blade. Then you will see which are working and which are not. You seem hung up on the negative power. We have said many times that this is probably just due to the way you have defined the positive and negative directions, and the negative torque up to 400rpm for 10m/s wind is the device generating power and the positive torque above that is the device loosing power (and would therefore slow down to below 400rpm to be in the generating power region). If this is the case then your device generates power with a negative power from your model. But as has been said many times, you need to check your rotation direction definitions to ensure that this is the case. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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October 31, 2018, 03:42		#23
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	This was just to show the difference between two RPMs and I am still not sure about this negative torque value where it is coming and how. And I thought someone could give me a reasonable answer based on these streamlines why I would have a negative torque in RPM50 and possitive in RPM500. Because as per the explanation from previous thread other person told me that for RPM500 it should be consuming energy to rotate the turbine and wind speed wornt be enough. So am wondering.

October 31, 2018, 04:02		#24
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,922 Rep Power: 28	Don't look at streamlines since they don't drive your rotor. They are a result of pressure differences. So don't solely look at the effect but at the cause......

November 1, 2018, 04:39		#28
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Hi Gert I understood what you are talking about the power sign. But as I mentioned you already, I dont see an evident mistake in the rotation direction I have given. Thats where I wanted you to see the image atleast of my setup so that you can figure out where I went wrong, as you have more experience on this. The thing is I have mentioned many times that my rotor is rotating in anticlockwise direction, that means I have given -RPM in the rotor domain angular velocity. But does this be the reason for bringing the sign of power in negative ? Again it would be really great if you could have a look on the image of the rotor domain setup which I posted long ago in this thread so that you can tell me whats going on actually. And for sure the rotor above 500RPM where I am getting positive sign means its consuming power and not helping to generate ? And for high RPMs its not meaningful to generate power ? IS that what you are telling ?

November 1, 2018, 05:10		#29
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,922 Rep Power: 28	The convention is that you follow the alphabet...... You have to position yourself on the axis of rotation and then look in positive direction of the rotation axis. In your case I think this is along the Global Z-axis. Then you look where the Y- and X-axis are. For your rotation, the Y-axis needs to rotate to the X-axis. This means your angular velocity is negative, because if you go in the alphabet from Y to X, you go backward. A positive angular velocity would mean that X rotates to Y because in the alphabet X comes before Y, i.e. you move forward.

November 2, 2018, 04:37		#30
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Hi Gert This explanation kind of gives me an impression that my negative sign in the power is physical and its just the sign and it does mean that power is generating by the turbine till 400 rpm and after that its consuming power ? And as you might have seen the images of the geometry nd my rotor domain setting. I have thickness of 2mm in the positive z direction. And the flow is in the positive X direction and anguar velocity is given negative as my rotor blade is rotating from Y to X or in the anticlock wise direction. And thats how it have to rotate. And hence i have given angular velocity as -100rev/min or -400 rev/min etc... so this make sense for this negative sign right ? So does that mean I just multiply the torque with a negative sign ? Then everything will be meaning ful ?

November 4, 2018, 17:58		#31
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,922 Rep Power: 28	Yes. Everything is meaning full.

November 5, 2018, 05:12		#32
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Perfect Thanks a lot Gert. So I dont have to find the Zero Power point right ? I can just plot the RPM vs Power Curve till 400 and then see where I have the maximum power value and take that as my optimum RPM value right ?

November 5, 2018, 05:36		#33
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,922 Rep Power: 28	If you are not interested in the zero power point, then I think your approach is ok. You should always tune your CFD-calculations to the question that you need to answer.