|The authors present the implementation of an actuator sector method, allowing to perform aeroeastic simulations of wind turbines at an affordable computational time. The numerical methods used and the methodology employed are described with the necessary detail.|
That is followed by a relative comparison of the performance of the method with the results of other popular approaches. An academic wind turbine is used for the task. To the eyes of the reviewer, that comparison could have been more rigorous, as the limited load cases chosen and the short simulated times makes the formulation of any conclusion very hard. However, the authors stated in their response to the original submission that this first exercise showed the potential of the methods in terms of computational time saving, and the "plausibility" of the obtained results.
Finally, a validation case with a real turbine is presented. That is indeed a very complex task to do (due to the uncertainties involved), and it is not often seen in scientific publications as it normally involves private companies. Nevertheless, the authors openly comment on these issues, giving in general reasonable explanations and highlighting the capabilities of the method. The reviewer misses the presence of other numerical models in this section. However, the authors stated that this was due to computational resources limitations in their response to the original submission.
While the manuscript is clear and comprehensive, there are still a number of major concerns that, in the opinion of the reviewer, should be tackled before publication. Those are detailed below, together with a series of minor comments.
* L67: It is mentioned that the implemented methodology can be seen as an enhancement with respect to Storey (2015). However, this is not further detailed in the manuscript. Is there any novelty, rather than the implementation on a different set of solvers, on the computational methods used?. Wether is the case or not, it should be clearly stated in the text.
* L148: "this can be resolved in the postprocessing of the results by shifting the results in time" -> would this imply a certain level of approximation? In other words, is it a "model" that could be applied in the post-processing step? In that case it would more an "estimation" rather than a "solution" itself.
* L172: "For FAST on its own, the inflow wind option "steady wind conditions" is used.": this should be re-phrased in conceptual terms, rather than as the naming used for the FAST software. Does it simply mean laminar inflow? The reviewer believes that this should have been consistent with the setup of the other methods, for the laminar comparison. So that it is something related to the load case itself, rather than to FAST alone.
* L173: "To evaluate the different methods, at first, a laminar case with the same wind speed over height is considered.": could be interesting to give more details about the specific profile that was used, e.g. relating it to a shear factor.
* L177: " However, no differences in the results ": This could be quantified, in order to be more precise. Maybe the authors can show the relative difference of a targeted quantity, just as an example.
* L188: "The result calculated by FAST coincides with the value, as published by NREL (Jonkman et al., 2009b), based on the same FAST model.": The reviewer wonders why is this comment important. There is no quantification of the relative differences, and from an academic perspective is probably not that relevant. It is assumed that a simulation of the same inflow conditions and with the same code, will lead to the same results. If the comment had to do with software versioning, then it should be clear in the text. If it was more related to potential discrepancies while building the aeroelastic model, the possible origin of those should be mentioned.
* L207: "A turbulent case is calculated as well.": Could be interesting to lift this comment up in the text, while introducing the load cases. Note that several comments on the FAST inflow have been already made at this point, and the reader could lack of context.
* L219: "Also, roughly the same peaks and therefore structures of the flow are present in the ASM results. This implies, that the coupling works in a turbulent environment as well.": The reviewer thinks that there is not enough evidence for such a statement. The phrase "the coupling works" is very vague, as it relies on a qualitative observation. There is room, for instance, for an implementation bug that could eventually shift the loading spectra during the coupling, or that accounts for a bad projection. To comment on the flow structures a qualitative measure should be used.
* L234: " The reference power curve is obtained from stand-alone FAST runs, with a laminar inflow. The FAST turbine model is provided by eno. The calculated reference power curve coincides well with the published power curve of eno (eno energy, 2019).": the reviewer wonders what is the added value of this statement. There is no plot shown, and no discussion on how the power curves were computed from the manufacturer side.
* L360: "The results of the simulations correspond well with the measurement data": In the opinion of the reviewer, there is not enough evidence to support this statement.
* L368: "This can be seen in figures 19 and 20. ": These figures are not properly introduced, and their numbering does not match the text sequence.
* L368: "Apparently, the rotor speed curve at the start of the peak shaver region is slightly different (c.f. figure 20). Therefore, it is only possible to compare loads at either the same rotor speed or the same wind speed.": There is no explanation given for this particular point. While the controller was not provided by the manufacturer, it was previously stated that the power curves matched perfectly.
* Figures A.1, A.2: Could be more useful to switch the x-axis by another variable, such as radius or projected length. The nodes might not be equidistant for every aeroelastic code.
* L33 "with comparatively simple models, like e.g. TurbSim" -> TurbSim seems to be a software, rather than a model. Consider using another word here
* L48 "Here, the use of an ALM, moving meshes and fluid–structure interaction (FSI) lead to very detailed results but also requires a further reduction of the computing time" -> the reviewer found this phrasing a bit cumbersome. Could it be simpler to state that the consideration of FSI implies an increase of the required computing time?.
* L56 "losing" -> should it be "loosing" instead?
* L135 " that occur at the blades" -> experienced by the blades?