Virtual Disk Model for Wind turbine

ashkan.ostovari · June 2, 2015, 19:53

Hi everyone,
I have used BEM method of virtual disk model in star ccm+, and I got some some results of parameters such as Torque or Thrust, but I am confused in getting efficiency and overall performance to evaluate the wind turbine.

Basically, I wonder if we can do an 3-D unsteady analysis using like RANS or Large Eddy to find the Cd and Cl and then use those results in BEM method which is 2-D and steady analysis?

fluid23 · June 3, 2015, 15:36

What your propose is trivial with respect to the virtual disk method. You supply lift and drag. Why would it matter if that was experimental or CFD data (other than the obvious answer, accuracy)? You can get section aero data from any source you like as long as you are comfortable with it.

ashkan.ostovari · June 3, 2015, 16:29

Thank You, In case of evaluation:
1.I dont get how the solver calculates the torque and thrust, by integrating over the surfaces of the blades? how does take the the effect of wind speed and rotational(which is assigned in BEM independently) in to account?
2. Any recommendation on choosing the performance parameters rather than torque and thrust to evaluate efficiency?

fluid23 · June 3, 2015, 17:03

You can think of the BEM in virtual disk as being the same as classical BEM. The difference here is that rather than assuming you have uniform freestream acting along the blade, you actually use the distributed flow properties that results from the navier-stokes solution. The BEM/VD calculations are done outside of the frame work of the navier-stokes equations and applied as a momentum source which then manifests itself in your flow.

The inflow plane that you define is where your freestream flow is coming from. Its divided into annular sections just like your blade so you can capture any non-uniformity in the flow field. So at r/R=0.25 and r/R=0.5 you can have different inflow conditions. Classical BEM theory assumes that inflow is uniform and steady.

Torque and thrust are calculated just like in BEM, by integrating along the blade using the aero coefficents you define and the calculated angle of attack. The main difference between VD and classical BEM is the inflow plane. There are some other small descrepancies, but that is the big one.

Efficency of a wind turbine is given in terms of its power coefficient. Betz says that no turbine can capture more than 59.3% of the kinetic energy in a given flow. So if your power coefficient is above 59.3% you screwed up. Some people occasionally claim to have broken this, but in all cases it can be shown to be a mathematicall error. Even these people who claim that their vertical axis turbine is better than 59.3% are all wrong. They made a math error somewhere. I have done Betz's derivation myself for vertical axis machine and got the exact same asnwer.

That being said, most practical turbines never even get within 80% of the betz limit (so 47.4%).

Hope this helps, I do have tons of wind energy experience so if you have any questions just ask.

ashkan.ostovari · June 3, 2015, 19:15

yeah, that helped a lot, thank you for your time. And I have considered the BETZ limit, but(just to make sure) from what I get from Thrust plot should I export it and integrate it over the blade area? because the part that I chose in the plot and report are the all of blades. and we have just time and thrust parameter in the plot not based on the location on the blades.
I wish I knew how to upload my plot here so I can show you what I mean better.

ashkan.ostovari · June 4, 2015, 10:26

yeah, that helped a lot, thank you for your time. And I have considered the BETZ limit, but(just to make sure) from what I get from Thrust plot should I export it and integrate it over the blade area? because the part that I chose in the plot and report are the all of blades. and we have just time and thrust parameter in the plot not based on the location on the blades.
I wish I knew how to upload my plot here so I can show you what I mean better.

June 2, 2015, 19:53	Virtual Disk Model for Wind turbine	#1
ashkan.ostovari New Member Join Date: Feb 2014 Posts: 5 Rep Power: 12	Hi everyone, I have used BEM method of virtual disk model in star ccm+, and I got some some results of parameters such as Torque or Thrust, but I am confused in getting efficiency and overall performance to evaluate the wind turbine. Basically, I wonder if we can do an 3-D unsteady analysis using like RANS or Large Eddy to find the Cd and Cl and then use those results in BEM method which is 2-D and steady analysis?

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June 3, 2015, 15:36		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	What your propose is trivial with respect to the virtual disk method. You supply lift and drag. Why would it matter if that was experimental or CFD data (other than the obvious answer, accuracy)? You can get section aero data from any source you like as long as you are comfortable with it.

June 3, 2015, 16:29		#3
ashkan.ostovari New Member Join Date: Feb 2014 Posts: 5 Rep Power: 12	Thank You, In case of evaluation: 1.I dont get how the solver calculates the torque and thrust, by integrating over the surfaces of the blades? how does take the the effect of wind speed and rotational(which is assigned in BEM independently) in to account? 2. Any recommendation on choosing the performance parameters rather than torque and thrust to evaluate efficiency?

June 3, 2015, 17:03		#4
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	You can think of the BEM in virtual disk as being the same as classical BEM. The difference here is that rather than assuming you have uniform freestream acting along the blade, you actually use the distributed flow properties that results from the navier-stokes solution. The BEM/VD calculations are done outside of the frame work of the navier-stokes equations and applied as a momentum source which then manifests itself in your flow. The inflow plane that you define is where your freestream flow is coming from. Its divided into annular sections just like your blade so you can capture any non-uniformity in the flow field. So at r/R=0.25 and r/R=0.5 you can have different inflow conditions. Classical BEM theory assumes that inflow is uniform and steady. Torque and thrust are calculated just like in BEM, by integrating along the blade using the aero coefficents you define and the calculated angle of attack. The main difference between VD and classical BEM is the inflow plane. There are some other small descrepancies, but that is the big one. Efficency of a wind turbine is given in terms of its power coefficient. Betz says that no turbine can capture more than 59.3% of the kinetic energy in a given flow. So if your power coefficient is above 59.3% you screwed up. Some people occasionally claim to have broken this, but in all cases it can be shown to be a mathematicall error. Even these people who claim that their vertical axis turbine is better than 59.3% are all wrong. They made a math error somewhere. I have done Betz's derivation myself for vertical axis machine and got the exact same asnwer. That being said, most practical turbines never even get within 80% of the betz limit (so 47.4%). Hope this helps, I do have tons of wind energy experience so if you have any questions just ask.

June 3, 2015, 19:15		#5
ashkan.ostovari New Member Join Date: Feb 2014 Posts: 5 Rep Power: 12	yeah, that helped a lot, thank you for your time. And I have considered the BETZ limit, but(just to make sure) from what I get from Thrust plot should I export it and integrate it over the blade area? because the part that I chose in the plot and report are the all of blades. and we have just time and thrust parameter in the plot not based on the location on the blades. I wish I knew how to upload my plot here so I can show you what I mean better.

June 4, 2015, 10:26		#6
ashkan.ostovari New Member Join Date: Feb 2014 Posts: 5 Rep Power: 12	yeah, that helped a lot, thank you for your time. And I have considered the BETZ limit, but(just to make sure) from what I get from Thrust plot should I export it and integrate it over the blade area? because the part that I chose in the plot and report are the all of blades. and we have just time and thrust parameter in the plot not based on the location on the blades. I wish I knew how to upload my plot here so I can show you what I mean better.