Learning Forum

[Nate K.]

Hello Forum,

I am wondering why I am getting negative moment values for a Darrius type vertical axis wind turbine when there is constant counterclockwise rotation. The Motion History data in Fluent shows constant CC motion, yet the moment plot is cyclic with positive and negative values. See attached pic. 

I am using dynamic mesh and the three airfoils are treated as one unit... the turbine... which is defined to rotate in the 6DOF settings.

By any chance, anyone have any thoughts why the negative moment values exist and how to correct it? 

Also, do the residuals look good? I don't see much of a reduction in value from start to finish (but my impression is that wind turbines don't share this outcome)... although the time interval from high to low cycles does shorten with time (the zigzag is more prominant) in the latter 1/2 of the flow time.

Thanks, Nate

 

 

[Rob]

The residuals aren't good, and unless your cells are 1m in size the time step is almost certainly too big. Aim for a step size such that it takes 5-10 time steps for the flow to cross a cell. Then adjust to hit convergence in 10-15 iterations. Those are rough rules of thumb so are unlikely to be in the text books. 

For the moment, are you plotting on all surfaces of all blades or one face on one blade or.....? 

 

[Nate K.]

 

Hi Rob,

Thanks for replying.

Why do you say the residuals are not good? Do you have experience in simulating vertical axis wind turbines to know first hand or? Just wondering how you know, as I have heard what I mentioned prior… that the residuals may not drop in order of magnitude for certain geometries and that the sharp zig-zig later in flow time is a good sign… are you saying they have not converged? Can you share more about this?

The moment is from all three airfoils as one unit… calling that the turbine… which comes from me selecting the edges of all three airfoils and creating a named selection in Fluent Mesh. Then this feature (not sure the correct term) in Fluent is what I set to rotate... 

 

[Rob]

I've not modelled any full scale VAWTS, but do work with some of the clients who do. Aerodynamics is more often linked to meshing, so I leave it to colleagues. But with a few years of modelling "stuff" they're not too complex. 

Ideally you want to drop the residual below 1e-3 for continuity. Assuming the mesh is good then generally that means reducing the timestep. 

OK, so you're summing the moment on all (6?) faces of all blades. I assume you checked for any area/vector definitions in the moment panel? How is the motion defined? Ie is speed purely a function of the sum of the blade moments? 

[Nate K.]

 

Is that the only criteria, residual dropping below 1e-3, when looking at the residual picture above, that you are using to conclude it has not converged? 

Moment:

I am not sure what I am doing — or what Fluent is doing — regarding summing the moment on the blades… does Fluent automatically do this with the way I have created the “turbine”? And for whatever it’s worth, I have not created blade faces that I am aware of… I simply created a named selection in Fluent Mesh called “Turbine” using the edges of the three airfoils (edges not faces). Then, when opening Fluent, in the 6DOF settings I set “Turbine” to rotate. Or are we talking about the same thing… edge, face… just depends if looking at it as 2D or 3D I suppose…

I have not checked for area/vector definitions… havent heard of this before. 

I am not sure how the speed is defined… in other words, I have not changed anything that I know of… I simply chose a constant wind speed of 8m/s at the inlet of flow area…   

 

[Rob]

OK, you've used 6DOF to calculate the rotation speed. So, the incoming wind will cause the blades to move at a rate defined by the 6DOF solver. That needs to be checked to ensure it makes sense. 

Named Selections move over to Fluent as boundary labels, and these are then picked for the various boundary conditions etc. 

Force reports are based on a vector. I can't remember about moments other than the axis of rotation is important. Coefficients are another one to watch, read the definitions carefully when using those. 

[Nate K.]

The moving on the blades does not seem to be the issue, as the angle of rotation is ever increasing (one way rotation so opposite of what the moment plot is describing), and the facet average-weighted tangential velocity plot also reflects one direction rotation with steady velocity or slight increase. The issue seems to lie with the moment plot... perhaps something to do with the assigning of the "rotor" and its three airfoils (although, if this was the issue, then how can the angle of rotation be in conflict with the moments?), or calculations behind the moment? Seems to be a disconnect in Fluent between the moments and actual rotation...

How I can check how the solver is defined? Is there a way to change just how moments are calculated / defined?

Got the moment axis of rotation down correctly (z axis). So yeah, knowing more about how the moments are based would be helpful. By any chance, can you look into this? 

[Rob]

If the blades are still accelerating you may need to run longer. The sum of the moment for 360 needs to stabilise. 

As I don't know how you've linked force and rotation speed I can't comment. I'm also not able to debug UDFs or use non-public knowledge to assist, or review any files. I can look at pictures. This is covered somewhere in the rules & click-wrap you didn't read when signing up and is to allow Ansys staff to help on here whilst not breaking any US Export Laws. 

Moments will be covered in the manual(s). If you click on "Help" on the relevent panel you'll get taken to the page covering that panel.