WISDEM: shear load trend along blade span not maximum at root

Hello everybody,

I am peeking at the code of WISDEM to try to implement blade load calculations in case of storm, and now I am dealing with the outputs of rotor_structure::RunFrame3DD, i.e., wt.rotorse.rs.frame.(F2, F3, M1, M2).

Below is the trend of F2 vs the blade span that I get with the data from the .csv file of a successful optimization made with the standard version of WISDEM (not the one I am modifying):


Now, in my understanding, F2 is mainly the shear force on each blade section, so I was expecting the graph to show the maximum at the blade root, but the F2 load peaks at ~30 m and then decreases towards the blade root.
I checked the code and found the line of RunFrame3DD that computes F2 commented as below (I am using WISDEM 3.10.1):

# shear and bending w.r.t. principal axes
        F2 = np.r_[-forces.Vz[iCase, 0], forces.Vz[iCase, 1::2]]  # TODO verify if this is correct
        F3 = np.r_[-forces.Nx[iCase, 0], forces.Nx[iCase, 1::2]]
        M1 = np.r_[-forces.Myy[iCase, 0], forces.Myy[iCase, 1::2]]
        M2 = np.r_[-forces.Mzz[iCase, 0], forces.Mzz[iCase, 1::2]]

So, can someone explain me whether there is a problem with RunFrame3DD outputs, or I was expecting the wrong trend?

Thank you for your time and attention,
Henny

Hello Henny,
Garrett and I did some digging. I don’t have a definitive answer, but some considerations that hopefully help.
First, we have not find any obvious bug in the code. Second, the loading is not from a storm load as you wrote. Rather, it’s coming from the WISDEM approximation of a gust where the rotor is hit at rated rpm and rated pitch by a three-sigma gust. The steady-state loads from CCBLade are then augmented with gravity (accounting for cone and tilt) and centrifugal loads. The aero loading from this big gust toward blade root are unusual (blade is stalling badly, angle of attack ramps up to 50/60 deg depending on the design) and the combination of these weird loads, twist, and gravity seems to cause that weird pattern in the shear force. Note that we don’t use that shear force anywhere in WISDEM…
I hope this helps,
Best regards,
Pietro

Hello Pietro,
Thanks to you and Garret for the digging.
I understand your point, and it’s important to know that no evident bug was found in the code.
Best regards,
Henny