# Tower base axial force discrepancy between FAST and Bladed

Hi,

I got big discrepancy of TwrBsFzt between FAST and Bladed. Could anybody give me some suggestions?
The axial force at the tower top is at the same level. The wind is NWP 12m/s. The tower has no point mass placements and the tower is adjusted to have the same mass as that of Bladed.

Thanks
Best regards

Yinping Yang

It’s just a guess, but it looks to me that the towers don’t weigh the same. Turn off all DOFs, set the rpm to 0, and disable aerodynamics (or set air density to zero) and compare axial forces at the top and bottom of the tower. Check the FAST summary file (*.fsm) to see what the tower mass is and compare it to what Bladed reports.

Thanks. I have corrected the tower mass and the results are better. Could you give me some hints that the pitching moments are not in agreements between FAST and Bladed. I calculated the AeroCent according to the rules of the input section of the FAST User’s guide. The wind is NWP 12m/s. I have manipulated the AeroCent to match the results of Bladed. In that case the AeroCent is bigger and does not obey the rules.

Thanks
Best regards

Yinping Yang

I don’t know if it’s still true, but when I did some comparisons to Bladed a few years ago, I found that Bladed did not apply the loads to the deflected position of the blades. They may have fixed that since then.

You didn’t say if you ran this case with rigid blades. If not, try that to see if the codes give the same answers. I don’t remember looking at pitching moment at the time, but I seem to remember good agreement for flap and edge loads when the blades were rigid.

Dear Yinping Yang,

In FAST, the blade-root bending and pitching moments are calculated by integrating along the blade the applied aerodynamic pitching moments + ( radial vector ) x ( applied aerodynamic forces + gravitation forces - inertia forces ). All of these forces, moments, and distances are vectors and are oriented with the blade as the blade deflects. As Marshall said, our understanding is that the modal-based Bladed code does not apply the forces in the deflected state, which will certainly give differences relative to FAST.

I’m not sure I understand your issue with FAST’s AeroCent input parameter.

Best regards,