Dear Jason,
I spent a bit of time recalculating the inertia contribution from the fluid, but I think I have an error in how I calculated the distances from fluid column COMs to the platform COM because my calculated inertia values do not match either simulated solution. However, I also took a quick look at the HydroDyn code to see if I could find what I was doing wrong. I’m wondering if line 427 from Morison.f90 is correct, the formula differs from those on the Wikipedia article as shown below in bold.
Ir = abs(1.0/12.0* rhopiR1R1H (3.0R1*R1 + 4.0HH)) ! radial inertia about node 1
Should it not be:
Ir = abs(1.0/12.0* rhopiR1R1H (3.0R1R1 + HH)) ! radial inertia about node 1
I also ran two v2.5.0 simulations and the results of both fluid-filled and non-fluid-filled matched the non-fluid-filled v2.6.0 quite closely.
Thanks,
Brendan
Dear Brendan,
I believe the equation in HydroDyn is correct because this is the inertia about the node at the end of the cylinder, not the inertia about the center of mass of the cylinder.
Before the improvements to HydroDyn in v2.6, the inertia of the filled fluid in HydroDyn assumed the filled fluid was a thin rod, neglecting the rotational inertia within the cross section. This was discussed in the following forum topic and mentioned in the draft HydroDyn User’s Guide and Theory Manual: AddCLin for OC4 Semisubmersible - #3 by Jason.Jonkman.
If both v2.5 simulations match the non-fluid-filled v2.6 solution closely, then I would guess what happened is that the steel-only inertia in the model with filled fluid was corrected to account for the missing inertia term in HydroDyn. Now that the fluid fill inertia has been improved in HydroDyn within v2.6, the steel-only inertia should be adjusted accordingly to ensure that the overall platform inertia of the steel + ballast is correct. It looks like we should update the NRELOffshrBsline5MW_OC4DeepCwindSemi_ElastoDyn.dat file in the OpenFAST r-test in v2.6 and newer accordingly, once the correct steel-only inertia has been calculated.
Best regards,