This paper presents URANS simulations for a pitching wind turbine profile at high Reynold numbers (>10^6). This paper describes the dynamic stall onset and then proposes a method to better predict the dynamic stall for Blade Element Momentum models. The paper is globally well written, even if some explanations are a bit hard to follow probably because the figures do not fully help the explanations. The study of the dynamic stall onset is not new, as the authors mentioned  in the introduction, and the novelty of this paper could be that they study dynamic stall on wind turbine airfoils and attempt to develop a simple model to predict the dynamic stall onset.

I would have a couple of major concerns, that make me wonder if the author's explanations are solid enough to be reproduced and published:

A. With the amount of information given in this paper, I do not think I would be able to define the specific angles (α_ss, α_*, α_** and α_tds) on another wind turbine profile. For example the static stall is not clear on a thick airfoil. How do the author define a static stall here? Is it a complete stall of the airfoil? For example Braud et al (Study of the wall pressure variations on the stall inception of a thick cambered profile at high Reynolds number, Physical Review Fluids, 2024) have shown in their recent paper that they did not find a complete stall of the airfoil on a wind turbine profile below 25°. (They also highlighted the importance of 3D effects at high Reynolds numbers). 

The definition of α_* as defined by Mulleners and Raffel (The onset of dynamic stall revisited, Exp Fluids, 2012),  are based on POD modes on the vorticity field. It seems that here all these angles lack clear definitions.

B. Similarly, I would not be able to compute the Middle Chord Suction Parameter (MCSP) or (TESP) Trailing Edge Suction Parameter on another wind turbine profile. Whereas the Leading-Edge Suction Parameter (LESP) defined by Ramesh has a physical definition (it is based on the first Fourier term in thin-airfoil theory), I do not see a physical sense to the new MCSP and TESP. I did not understand how they were calculated. I do not think they can be defined in a similar way to the LESP using the inviscid flow theory.

C. The outcomes of this paper are intended for wind turbines. But I am not sure that the cases studied here are relevant to wind turbines. The applied turbulence intensity is 0.01%, while in general it is at least 8-10%. What would be the operating angle of attack on such a profile, and what would the expected variations in angle of attack be? For example the sinusoidal motion 20°+-15° does not seem realistic to me. Some contextualisation may help to appreciate the importance of dynamic stall on a wind turbine blade section.

I have other minor comments:

1. end of page 6: "the conclusions regarding the formation of dynamic stall on future large WTs remain the same". I am not sure to what the authors refer to when they write "the same".

2. page 5: LE, TE, and CFL (and BLM and k in page 6) are used but were not introduced before. I do not know what CFL means.

3. section 2.2 seems more a section for the introduction. The beginning of section 5 may be more appropriate in this section "2. Methodology".

4. Section 2.4: This section might be better used as a nomenclature (if the WES template allows it).

5. Figure 3: It is hard to compare the different plots in figure 3. It may be easier to compare them if they are on the same plot (and probably with the time as x-axis). The phase-average value would probably suffice here to compare the general evolution of the numbers of cell or maximum CFL.

6. Why is the pitching case different for the mesh and time-step studies? (17°+-8° for the mesh study and 17°+-15° for the time-step study). The same pitching case for both studies would probably ease the comparison.

7. Figure 4. What do the arrows mean in figure 4?

8. page 10. What do the author mean with the term "open flow separation"?

9. Figure 6: The angle of attack as x-axis (place on the top of the plot for example) would help to visualise the time and the angle of attack at the same time.

10. Figure 6: Please indicate t_ss, t_*, t_** and t_ds in the x-axis of figure 6 and not just in the legend. It is harder to follow without these specific times in the graph.

11. Figure 6: the colorbar probably represents the pressure coefficient. Could you please mention it on top of the colorbar?

12. Figure 6: The colormap used is divergent, with a white color in the middle which "separates" the blue and red color. But the white value has no signifiant value here. A convergent colormap might be more appropriate here to better visualise the transitions in the pressure coefficients.

13. Figure 6: A Cp of -14 seems a lot to me. I cannot recall such a high absolute value even in simulations. Could the author confirm this extremum please?

14. Figure 7: The instantaneous pressure contours are probably not the best to visualise the vortices described in the text. The z-vorticity contours or the Q-criterion might be more appropriate.

15. Figure 7: Could the author add a colorbar here for the pressure value. It seems to be a different scale to the colorbar shown in figure 6, which uses the same colormap.

16. Figure 7: For each subcaption, it would be good to add the angle of attack and time, when these snapshot were taken, and if they correspond to a salient time such as t_ss, t_*, t_** or t_ds.

17. Page 13: What do the authors consider to be a thin profile and low Reynolds numbers? For example in a sinusoidal pitching airfoil cited in this paper (Deparday and Mulleners, PoF, 2019) or Deparday et al, JFM, 2022 (Experimental quantification of unsteady leading-edge flow separation), it seems there is a similar plateau of Cl but the airfoil is thinner and the Reynolds number lower, which would contradict the conclusions here.

18. Figure 8: I have the same comments about the divergent colormap, and no mention about what the colorbar represents.

19. Figure 8: It seems there is a periodic pattern with the Strouhal number. Could the author confirm this is not an artifact due to the time step of the simulations?

20. I do not understand why BEM is applied here. Did the authors model a rotating wind turbine blade? What is the geometry of the blade then, the rotational speed?

21. I would like to mention to the authors a new model for dynamic stall recently published (Bangga et al, Development and Validation of the IAG Dynamic Stall Model in State-Space Representation for Wind Turbine Airfoils, 2023, https://doi.org/10.3390/en16103994)

The studies are aimed at investigating the “characteristics of dynamic stall on large wind turbines”. I personally find the topic relevant, and the studies are suited for the journal. Despite that, I have a few criticisms which I hope could help improving the quality of the paper:

1.    First, the title is misleading. The paper is focused on wind turbine airfoils, not wind turbine itself. In my opinion it is not appropriate to label it as “wind turbines”. The dynamic stall behavior in wind turbines is much more complex than just pitching airfoils because it also involves strong flexibilities, instability, plunging, heave and all complex 3D flow field. Our group luckily had a chance to study that in:  https://iopscience.iop.org/article/10.1088/1742-6596/2626/1/012026/meta 

Just as an example, but you really could find the characteristics in lots other papers. As far as I can see, the present paper only discusses the aspects on airfoils. For sure, please correct me if I read your paper in a completely wrong manner :)

2.    Minor but could be helpful: I would suggest adding a table of symbols, honestly I had difficulty finding out the meaning of alpha* :)

3.    Why did you use fully turbulent solutions? Any solid reasoning not to use transitional model or to enforce transition at certain locations (like how measurement is usually done)?

I think that the argument that high Re will have a fully turbulent flow is a bit strong opinion, since it will still have transition regime to a certain degree, and it will impact the stall behavior.

4.    I think the normalization for lift is not right, are we missing “0.5” factor? Or is it intended not to have the usual formulation?

5.    You compared URANS with XFOIL, by default XFOIL includes transition modelling, how did you align them?

6.    I agree with the other reviewer that you used the term “BEM” for the engineering model calculations, it is more appropriate to label it as HGM model. This model was developed by Hansen, Gauna and Madsen which was based on the Beddoes-Leishman (BL) dynamic stall model. In short, theoretically this is an incompressible version of the BL dynamic stall model. I think it is not fair not to cite the original author (https://orbit.dtu.dk/files/7711084/ris_r_1354.pdf).

7.    I thank the other reviewer for bringing up our recent work in dynamic stall modelling. We recently developed the IAG dynamic stall model for wind turbine airfoils and have tested it against experimental data of pitching airfoils at various conditions. 

You might observe in our paper that the drawback of underestimating the peaks of the loads for the incompressible BL model (which you observe in your paper) is better solved when using the IAG model. 

The papers are here:

https://wes.copernicus.org/articles/5/1037/2020/

and

https://www.mdpi.com/1996-1073/16/10/3994 

and we have tested the model against BL model on 3 different wind turbines under design load cases in the recent Torque conference. It is not yet published but I can send the paper to you if you are interested to read.

I would be interested to see if some improvements can be made here.

Overall, I have no strong opinions about the paper but I hope my comments can serve as a rough guidance to help improving the quality of your work.

Kind regards,

Galih Bangga

DNV