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The topic and the activity are very interesting and worthy of investigation. The authors performed a complete and in-depth analysis and the results are very well presented with many details.

The reviewer has few major concerns regarding the numerical methodology and the analysis of the results:

• The radius of the domain seems to be small, being just 30 times the airfoil chord. It was demonstrated that a 2D analysis needs more than 100 chords (preferably, 500 chords) to avoid any blockage effects (https://turbmodels.larc.nasa.gov/naca0012_val.html). How do the authors justify this choice?
• How was the 1c spanwise height of the domain chosen?
• URANS-2D FSI simulations are run for 1300 non-dimensional time units each, while the DDES-3D simulations are run for 300 non-dimensional time units. How do the authors can demonstrate that the length simulated intervals are sufficient for capturing all the relevant features and the collected data are statistically significant?
• It is not clear which is the physical phenomena associated to the St=0.132 that leads to comparable range of lock-in between the two approaches
• Finally, the reviewer suggests to specify in the abstract that the airfoil is at 90° incidence

The authors present a very interesting, detailed and well designed numerical analysis to investigate the vortex shedding regimes and lock-in response of a wind turbine airfoil. To this aim two simulation approaches with different levels of fidelity are proposed (URANS 2D and DDES 3D).

Although the considered configuration is very simple, in my opinion the paper proposes interesting findings, among which the novel metrics for the definition of the lock-in region.

The paper is well written, although there are some minor technical corrections and suggestions for improvements that I have proposed in the following. 

- Abstract: lines from 7 to the end are not really suited for an abstract. I suggest moving them elsewhere and put more focus on the most important scientific novelties and findings of the work.

- Introduction: I appreciated the variety and number of cited works. In order to better position their work within the proposed literature survey, I suggest to the authors to better (and explicitly) mention the novelties and findings of this work which had not been touched (or touched differently) by previous works.

- Section 2.3: to better compare computational costs of URANSE and DDES solvers, a table would be more suitable than many lines of text. Moreover, the choice of the quantities of interest for the analysis (max(x), std dev of Cl and mean Cd) should be better explained and motivated. Finally, lines from 202 to 214 (but in general the whole Section 2.3) would be better positioned at the beginning of the numerical results section. 

- Section 3.1.1: one of the reason for the different frequency content observed for Cl in URANSE and DDES simulations should be the number of vortical structures modelled in the different solvers. I think that this aspect should be better investigated by the authors. In this regard, the comparative analysis of flowfield snapshots from both simulations is needed.

As a further general comment, the authors use an in-house solver which is mentioned to be well validated. Nevertheless, often in the paper they cite results from other authors which deal with a similar configuration (e.g. lines 268 onwards for a flat plate) but the authors don't investigate those cases with their solver. Another example is related to the oscillating cylinder case which is mentioned in the introduction. The authors should put some effort in finding in the literature a test case (either numerical or experimental) which is relevant (or simply similar, e.g. a pitching airfoil undergoing dynamic stall) to the VIV problem and which they can address with the proposed solver. This validation task is, in my opinion, fundamental to improve the value of this work. Finally, when mentioning results which can support their present findings, I suggest the authors not simply mentioning them but also including them in the paper by acknowledging the authors. For instance, the work by Lian which is cited in lines 489 and further. Moreover, this specific work would be suitable as a validation test for the present solvers even if the authors mention that Reynolds numbers of Lian case are much smaller than those typical of wind turbine airfoil flows.

My indication is to ACCEPT the paper only after MINOR REVISIONS following the comments listed above.