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DGaden March 1, 2007 15:35

Darreius problems
Hey folks,

I'm having a bugger of a time with a 2D darreius turbine simulation. The solution looks intuitively "okay", except the torque is backwards, predicting power loss instead of power production. Worse yet, I have real experimental data to compare against, and it's almost exactly opposite torque. I've been banging my head on the wall, and triple checked all my calculations. The forces all add up to a backwards torque! There must be a flipped sign in there somewhere, but I can't find it.

Perhaps I don't understand rotating reference frames. How would you do a 2D darrieus turbine? -I created a circular mesh region around the blades and hub -Set it as a rotating domain in the direction the blades should rotate. (When I move timestep forward in post, the mesh moves the way the blades should.) -Set the interface as "Transient rotor / stator"

Am I doing something wrong? Should I do something with a rotating mesh, but not using a "Rotating Reference Frame"? Or is the CFD just that wrong?

-Dave Gaden

opaque March 1, 2007 16:19

Re: Darreius problems
Dear DGaden,

The sign of the force on fluid is opposite to the sign of the force on the wall. From which side are you interpreting the results?


DGaden March 1, 2007 16:55

Re: Darreius problems
Thanks for the suggestion, but it's not that. I'm calculating the forces from the wall:

! $Fx=force("BLADE","X")

The forces I get make sense: the upwind blade experiences a large downstream force; cross-stream forces correspond nicely when the blade is tilted 45 degrees, like a kite.

I think the problem is either: -Something with the rotating reference frame... like "it should be a rotating wall instead of a rotating domain", or -CFD is not good at calculating drag / lift forces.

Johnny March 2, 2007 02:27

Re: Darreius problems
Make sure you have angular velocity signed correctly - CFX uses the right hand rule - thumb in the positive axis of rotation direction, fingers curl in the positive direction.

Sign on your angular velocity will then refer to the direction in which the blades move.

DGaden March 2, 2007 13:59

Re: Darreius problems
Thanks for the suggestion, Johnny. I have used the RHR, and double checked it with manual calculations of x,y forces and their component distances. Everything adds up right... except it's backwards... darnit.

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