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QFEM Elastic Center
Hi,
I am currenctly using QFem to calculate the elastic center of my cross section. In the documenation its says, that the elastic center is described in the refrence coordinate system which is located in the pitch axis. I have noticed that you can change the pitch axis in the advanced desgin tab of the blade. I have made some tests by changing the pitch axis location to 0,25 and then calculating the new blade.str file with QFem. X_Ce and Y_Ce dont change and are still given in the original refrence coordinate system which is in 0,5c of the blade. I now manually changed the CoE by taking the old X_Ce and subtracting 0.25.
Am I missunderstanding something or is QFem not concidering the change of the pitch axis position?
Thank you for your time!
Kind regards,
Linus
Hi Linus,
you are correct, thanks for pointing this out and reporting this!
QFEM already accounts for the pitch axis during its calculations, but the pitch axis offset, applied an additional time during the export, was incorrectly shifting the centers back to the half-chord reference.
Your manual correction of subtracting 0.25 is correct for a pitch axis at 0.25c. This has now been fixed and will be included in the upcoming QBlade 2.1.0 release.
Best regards,
David
Hi David,
Thank you for the quick reply!
I have another question and I was wondering if you might have some insight into this. When I run a BEM Simulation the aerodynamic torque of the turbine is consistently lower than in the aeroelastic simulation. For my small wind turbine setup the calculated torque in the BEM Simulation is 425 Nm whereas in the aeroelastic simulation its 460 Nm. I have included all corrections and both simulations share the exact same wind speed, TSR, viscosity and density is the same. The tip deflection of the blade is close to zero (2.1E-02). Why is the torque higher in the aeroelastic simulation? Is it because there is a difference between the unsteady BEM and the Rotor BEM?
Best regards,
Linus
Hi Linus,
I had a closer look at this and found the main reason for the difference.
The steady Rotor BEM still used a legacy BEM expression for the thrust coefficient:
CT = sigma * Cn * (1 – a)^2 / sin(phi)^2
while the unsteady BEM evaluates it from the full section kinematics and geometry:
CT = sigma * Cn * Vrel^2 / Vref^2
For an ideal 2D BEM velocity triangle both expressions are equivalent, but once the actual blade geometry and section velocities are used, small differences appear. Since CT directly determines the induction, this propagated into the torque.
I have changed the steady BEM to use the same Vrel^2 / Vref^2 formulation. With this change, the discrepancy in your case is reduced from about 8% to below 1%.
Also, for a meaningful comparison, the steady Rotor BEM needs to be run with DTU PolyBEM enabled, since the time-domain BEM uses the DTU polynomial BEM induction formulation.
The remaining small difference is expected because the steady and time-domain implementations are still not 100% numerically identical.
Thanks for reporting this!
Best regards,
David
