Hi - I’d like to understand the loads that OpenFAST outputs a little bit better. I’m interested in the outputs of simulations with a fixed foundation (monopile or jacket) - I use SubDyn below the interface and ElastoDyn above. I find that the ElastoDyn tower base loads (TwrBs…) match the SubDyn interface loads (Intf…) and that these loads are different to the SubDyn elastic loads (MaBn[F|M]K…) in the element at the end connected to the interface node. Moreover, I can broadly recover the SubDyn interface loads by resolving the SubDyn elastic loads (MaBn[F|M]K…) in the element at the end connected to the interface node and adding the inertial load associated with the Guyan (interface DOFs) when the number of CB modes is 0 - consistent with the SubDyn interface and ElastoDyn tower base loads being reactions. Tower strain gauges similarly appear to be internal reactions, not necessarily internal strain i.e. M=EI * curvature? Perhaps I’m mistaken. Based on other threads, like Mlife and Fatigue - Wind & Water / Computer-Aided Engineering Software Tools - NREL Forum, it seems as if it’s common to use the ElastoDyn tower loads to derive stresses and calculate lifes etc - is this appropriate? I’d have thought we’d evaluate life using internal strain loads (SubDyn-like elastic) loads rather than reactions. Thanks! @Jason.Jonkman any thoughts?
Dear @Sam.Ramsahoye,
I’m not sure I fully understand your question. I’ll just clarify that in ElastoDyn, the tower-base reaction loads are the loads transmitted to the platform through the tower. The SubDyn member-level loads include only the elastic contribution or inertial contribution to this reaction load. The difference can be explained through a simple linear mass (m), spring (k), damper (c) system cantilevered with applied force (F), with equation of motion (in terms of displacement of the mass, q):
m * qdd + c * qd + k * q = F
where d represents the first time derivative.
The force transmitted from the mass to the base is Fr = c * qd + k q, which is equivalent to Fr = F - m * qdd through equations of motion.
ElastoDyn will compute the latter (Fr = F - m * qdd). The member-level load in SubDyn will only output the elastic contribution (k * q) or the inertial contribution (m * qdd).
I hope that helps.
Best regards,
Dear @Jason.Jonkman ,
Thanks very much for your response and apologies that my original question wasn’t very clear - OpenFAST’s behaviour is clear to me now.
I’m curious, why does the standard appear to be to report Fr = F - m qdd = c qd + k q? My recollection is that Bladed does something very similar. It’s true that this is the load applied to a section but the actual strain in the section is given only by the kq term. In this sense, I find the use of the term strain gauge in referring to F_r type loads to be confusing. There will be times when only the k*q component will be of interest such as when assessing material yielding / fatigue.
When OEMs report turbine interface loads, do you know if the standard is to report reactions (c qd + k q = F - m qdd) or elastic loads (kq)?
Thanks very much for your help!
Best wishes,
Sam
Dear @Sam.Ramsahoye,
I would say that the total load transferred through the member is Fr = c * qd + k q = F - m * qdd is the correct load to output for calculating stress.
The elastic contribution (k * q) is often quite a bit higher than the damping contribution (c * qd), and so, is often a reasonable approximation of Fr. Note that in some cases, such as when damping is defined at the system level (modal damping) rather than at the component level, it is impossible to know the damping contribution at the component level (as is the case in SubDyn), which is why this term is often neglected in the reaction load output.
Also note that the inertial contribution (m * qdd ) is often interesting to quantify, but it is typically not a good approximation of Fr.
FYI: The ElastoDyn calculation of Fr = F - m * qdd is preferred over Fr = c * qd + k q because the former works whether the member (blade, tower) is modeled rigidly or flexibly, whereas the latter only works when flexible.
Best regards,