Characterization of Dynamic Stall on Large Wind Turbines

Kim, Hye Rim; Printezis, Jasson A.; Ahrens, Jan Dominik; Seume, Joerg R.; Wein, Lars

doi:https://doi.org/10.5194/wes-2024-31

Preprints

https://doi.org/10.5194/wes-2024-31

Preprints

02 Apr 2024

| 02 Apr 2024

Status: a revised version of this preprint was accepted for the journal WES and is expected to appear here in due course.

Characterization of Dynamic Stall on Large Wind Turbines

Hye Rim Kim, Jasson A. Printezis, Jan Dominik Ahrens, Joerg R. Seume, and Lars Wein

Abstract. This study shows an extensive analysis of dynamic stall on wind turbine airfoils preparing the development of a reduced-order model applicable to thick airfoils (t / c > 0.21) in the future. Utilizing URANS simulations of a pitching FFA-W3-211 airfoil at the Reynolds number of 15 million, our analysis identifies the distinct phases in the course of the evolution of dynamic stall. When the dynamic stall is conventionally categorized into the primary instability transitioning to the vortex formation stage, we suggest two sub-categories in the first phase, and an intermediate stage featuring a plateau in lift prior to entering the full stall region. This delays the inception of deep stall, approximately 3° for a simulation case. This is not predictable with existing dynamic stall models, optimized for low Reynolds number applications. These features are attributed to the enhanced flow attachment near the leading-edge, restricting the stall region downstream of the position of maximum thickness. The analysis on the frequency spectra of unsteady pressure confirms the distinct characteristics of the leading-edge vortex street and its interaction with large-scale mid-chord vortices to form the dynamic stall vortices (DSVs). Examination of the leading-edge suction parameter (LESP) proposed by Ramesh et al. (2014) for thin airfoils under low Reynolds numbers reveals that LESP is a valid criterion in predicting the onset of the static stall for thick airfoils under high Reynolds numbers. Based on the localized separation behavior during a dynamic stall cycle, we suggest a mid-chord suction parameter (MCSP) and trailing-edge suction parameter (TESP) as supplementary criteria for the identification of each stage. The MCSP exhibits a breakdown in magnitude at the onset of the dynamic stall formation stage and full stall, while TESP supports indicating the emergence of a deep stall by detecting the trailing-edge vortex.

Received: 21 Mar 2024 – Discussion started: 02 Apr 2024

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Hye Rim Kim, Jasson A. Printezis, Jan Dominik Ahrens, Joerg R. Seume, and Lars Wein

Status: closed

RC1:
'Comment on wes-2024-31', Anonymous Referee #1, 01 May 2024

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)

Citation: https://doi.org/10.5194/wes-2024-31-RC1
- AC1: 'Reply on RC1', Hye Rim Kim, 08 May 2024
  
  Dear Reviewer,
  
  We would like to appreciate your valuable comments and insight. We will incorporate all comments in the revised manuscript and submit with the final Rebuttal. Ahead of that, we would like to reply to the major concerns you addressed.
  
  A. Concerning the definition of the specific angles, especially the static stall (αss).
  Static stall angle at 15° in this case is defined as the lowest angle where the dynamic curve departs from the static curve. This is a common definition for low Reynolds number applications since they regard the leading-edge stall. As you mentioned, in high Reynolds numbers especially for thick airfoils, there is no complete stall found in this angle of attack. At 15°, the leading-edge region starts partially stalling, resulting in increase in lift compared to the static curve. In our case, the airfoil is completely stalled at the angle of attack of approximately 35°. We would explain the definition of each state more explicitly in the revised manuscript, although we don’t suggest a precise method to provide the criteria for those states. This study focuses on the characterization of dynamic stall in very high Reynolds number, and provides a first estimation based on the characteristics of lift curse, drag curve, suction parameters, and spectra. POD could be a very good method to set the criteria for those characteristic stages. We would consider that for the further studies when we have more parameter studies completed in this application.
  
  B. Definition of MCSP and TESP
  In this numerical study, LESP is calculated as the chordwise projection of leading-edge suction vector at the first 10% of the leading edge as suggested in Deparday and Mulleners (2019) for the evaluation of their experimental data. The MCSP and TESP are calculated in the same manner at 30-40% and 90-100% of the chord, respectively. This is applicable for other profiles as well. As you mentioned, the theoretical LESP is defined as the first Fourier coefficient of the Fourier series since it represents the suction peak at the leading-edge. MCSP and TESP should be represented as combinations of the Fourier coefficients of the higher orders since they are associated to the circulation in different chord positions. This paper addresses the possibility of supporting criteria of higher orders since it has been reported that the critical LESP is not sufficient in case of trailing edge stall. The manuscript will be revised with more explicit explanations.
  
  C. Low turbulence intensity, angle of attack and pitching angle in reality
  The authors are aware of the limited parameter studies in this paper, especially regarding the turbulence intensity. Few recent studies show the great impact of freestream turbulence intensity on the static and dynamic performances (Damiola et al., 2023 and Huang et al.,2020). Before we further investigate on this topic, we have started with low turbulence intensity since many of previous studies characterizing dynamic stall have been reported in very low turbulence conditions as well. Since the goal of this paper is to explore the dynamic stall characteristics in a very high Reynolds number, the authors have taken the low turbulence intensity as the base setup. The future studies will be conducted with different turbulence intensities.
  The sinusoidal motion could be still applicable for wind turbine considering a quasi-steady rotor oscillation induced by fluid-structure interaction. This can present in low angle of attack region with a moderate oscillating amplitude. 20°+/-15° is a very unlike kinematic condition in real wind turbines. However, it is still necessary to understand the characteristics of dynamic still in these extreme cases to develop a comprehensive dynamic stall model in the future.
  The manuscript will be revised accordingly, we appreciate for the comments to improve the paper.
  
  Sincerely
  Hye Rim Kim
  
  Citation: https://doi.org/10.5194/wes-2024-31-AC1
RC2:
'Comment on wes-2024-31', Anonymous Referee #2, 04 Jun 2024

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

Citation: https://doi.org/10.5194/wes-2024-31-RC2
- AC2: 'Reply on RC2', Hye Rim Kim, 06 Jun 2024
  
  Dear Reviewer,
  
  On behalf of the authors, I would like to appreciate your valuable insight, comments, and practical information. We will incorporate all comments in the revised manuscript and submit with the final Rebuttal. Ahead of that, we would like to reply to the comments you addressed.
  
  1. The title: In more precise manner, yes we have looked at only pitching motion of the Q3D domain to explore the dynamic still in high Reynolds number region. We would like to extend our studies into 3D effects in the future. If it is still allowed by WES, we would consider changing the title to similar to “characterization of dynamic stall in pitching motions of the rotor section in large wind turbines”. However, it might be not possible since the preprint is already published.
  2. A table of symbols: This will be added in the revised manuscript.
  3. Fully turbulent assmption: Kiefer et. al. (2022) investigated different Reynolds number of 0.5*10^6, 2.0*10^6, and 5.0*10^6. They found out that for Re=5.0*10^6, the boundary layer is transitioned to turbulence, upstream of the laminar separation point of low Reynolds numbers. That is why we first applied the fully turbulent setup. We would like to apply transition model for our future studies to make sure our assumption. The manuscript will be revised with more explanation.
  4. lift coefficient: Yes, the factor 0.5 is missing, this will be correced in the revised manuscript.
  5. URANS vs. XFOIL comparison: This might help out argument to apply fully turbulent model. XFOIL scripts that free transion occurs at x/c=0.004, which is very near to the leading edge. We will revise the manuscript with this information.
  6. BEM/HGM/BL reference: Thanks you for the additional clarification. We will definitely cite the original paper and revise the parts.
  7. IAG model: I would appreciate if you can send me the recent work of you. We are relatively new for the dynamic stall modelling, more information would help us for the future work. We would like to apply your model for the future studies and the manuscript will be revised referring that.
  
  Sincerely
  Hye Rim Kim
  
  Citation: https://doi.org/10.5194/wes-2024-31-AC2

Status: closed

RC1:
'Comment on wes-2024-31', Anonymous Referee #1, 01 May 2024

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)

Citation: https://doi.org/10.5194/wes-2024-31-RC1
- AC1: 'Reply on RC1', Hye Rim Kim, 08 May 2024
  
  Dear Reviewer,
  
  We would like to appreciate your valuable comments and insight. We will incorporate all comments in the revised manuscript and submit with the final Rebuttal. Ahead of that, we would like to reply to the major concerns you addressed.
  
  A. Concerning the definition of the specific angles, especially the static stall (αss).
  Static stall angle at 15° in this case is defined as the lowest angle where the dynamic curve departs from the static curve. This is a common definition for low Reynolds number applications since they regard the leading-edge stall. As you mentioned, in high Reynolds numbers especially for thick airfoils, there is no complete stall found in this angle of attack. At 15°, the leading-edge region starts partially stalling, resulting in increase in lift compared to the static curve. In our case, the airfoil is completely stalled at the angle of attack of approximately 35°. We would explain the definition of each state more explicitly in the revised manuscript, although we don’t suggest a precise method to provide the criteria for those states. This study focuses on the characterization of dynamic stall in very high Reynolds number, and provides a first estimation based on the characteristics of lift curse, drag curve, suction parameters, and spectra. POD could be a very good method to set the criteria for those characteristic stages. We would consider that for the further studies when we have more parameter studies completed in this application.
  
  B. Definition of MCSP and TESP
  In this numerical study, LESP is calculated as the chordwise projection of leading-edge suction vector at the first 10% of the leading edge as suggested in Deparday and Mulleners (2019) for the evaluation of their experimental data. The MCSP and TESP are calculated in the same manner at 30-40% and 90-100% of the chord, respectively. This is applicable for other profiles as well. As you mentioned, the theoretical LESP is defined as the first Fourier coefficient of the Fourier series since it represents the suction peak at the leading-edge. MCSP and TESP should be represented as combinations of the Fourier coefficients of the higher orders since they are associated to the circulation in different chord positions. This paper addresses the possibility of supporting criteria of higher orders since it has been reported that the critical LESP is not sufficient in case of trailing edge stall. The manuscript will be revised with more explicit explanations.
  
  C. Low turbulence intensity, angle of attack and pitching angle in reality
  The authors are aware of the limited parameter studies in this paper, especially regarding the turbulence intensity. Few recent studies show the great impact of freestream turbulence intensity on the static and dynamic performances (Damiola et al., 2023 and Huang et al.,2020). Before we further investigate on this topic, we have started with low turbulence intensity since many of previous studies characterizing dynamic stall have been reported in very low turbulence conditions as well. Since the goal of this paper is to explore the dynamic stall characteristics in a very high Reynolds number, the authors have taken the low turbulence intensity as the base setup. The future studies will be conducted with different turbulence intensities.
  The sinusoidal motion could be still applicable for wind turbine considering a quasi-steady rotor oscillation induced by fluid-structure interaction. This can present in low angle of attack region with a moderate oscillating amplitude. 20°+/-15° is a very unlike kinematic condition in real wind turbines. However, it is still necessary to understand the characteristics of dynamic still in these extreme cases to develop a comprehensive dynamic stall model in the future.
  The manuscript will be revised accordingly, we appreciate for the comments to improve the paper.
  
  Sincerely
  Hye Rim Kim
  
  Citation: https://doi.org/10.5194/wes-2024-31-AC1
RC2:
'Comment on wes-2024-31', Anonymous Referee #2, 04 Jun 2024

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

Citation: https://doi.org/10.5194/wes-2024-31-RC2
- AC2: 'Reply on RC2', Hye Rim Kim, 06 Jun 2024
  
  Dear Reviewer,
  
  On behalf of the authors, I would like to appreciate your valuable insight, comments, and practical information. We will incorporate all comments in the revised manuscript and submit with the final Rebuttal. Ahead of that, we would like to reply to the comments you addressed.
  
  1. The title: In more precise manner, yes we have looked at only pitching motion of the Q3D domain to explore the dynamic still in high Reynolds number region. We would like to extend our studies into 3D effects in the future. If it is still allowed by WES, we would consider changing the title to similar to “characterization of dynamic stall in pitching motions of the rotor section in large wind turbines”. However, it might be not possible since the preprint is already published.
  2. A table of symbols: This will be added in the revised manuscript.
  3. Fully turbulent assmption: Kiefer et. al. (2022) investigated different Reynolds number of 0.5*10^6, 2.0*10^6, and 5.0*10^6. They found out that for Re=5.0*10^6, the boundary layer is transitioned to turbulence, upstream of the laminar separation point of low Reynolds numbers. That is why we first applied the fully turbulent setup. We would like to apply transition model for our future studies to make sure our assumption. The manuscript will be revised with more explanation.
  4. lift coefficient: Yes, the factor 0.5 is missing, this will be correced in the revised manuscript.
  5. URANS vs. XFOIL comparison: This might help out argument to apply fully turbulent model. XFOIL scripts that free transion occurs at x/c=0.004, which is very near to the leading edge. We will revise the manuscript with this information.
  6. BEM/HGM/BL reference: Thanks you for the additional clarification. We will definitely cite the original paper and revise the parts.
  7. IAG model: I would appreciate if you can send me the recent work of you. We are relatively new for the dynamic stall modelling, more information would help us for the future work. We would like to apply your model for the future studies and the manuscript will be revised referring that.
  
  Sincerely
  Hye Rim Kim
  
  Citation: https://doi.org/10.5194/wes-2024-31-AC2

Hye Rim Kim, Jasson A. Printezis, Jan Dominik Ahrens, Joerg R. Seume, and Lars Wein

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Aug 2024	25	6	2	33
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Short summary

The need of renewable energy, so thus more efficient wind turbines, is ever increasing. Accurate prediction of the performance in the design stage is a necessary. Especially, predicting the dynamic performance of wind turbine in the region where it undergoes highly unsteady flow, is very challenging. We investigated this dynamic performance of an airfoil, which is typical for the mega-structure wind farms, to support the development of more efficient design tools in the future.


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