Preprints
https://doi.org/10.5194/wes-2022-115
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wes-2022-115
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
18 Jan 2023
Status: a revised version of this preprint was accepted for the journal WES and is expected to appear here in due course.
The Dynamic Coupling Between the Pulse Wake Mixing Strategy and Floating Wind Turbines
Daniel Graham van den Berg
CORRESPONDING AUTHOR
Delft Center for Systems and Control, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands
Delphine de Tavernier
Department of Flow Physics and Technology, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
Jan-Willem van Wingerden
Delft Center for Systems and Control, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands
Abstract. In recent years control techniques such as dynamic induction control (often referred to as "The Pulse'') have shown great potential in increasing wake mixing with the goal of minimizing turbine-to-turbine interaction within a wind farm. Dynamic induction control disturbs the wake by varying the thrust of the turbine over time, which results in a time-varying induction zone. If applied to a floating wind turbine, this time-varying thrust force will, besides changing the wake, change the motion of the platform. This work investigates if the coupling between the Pulse and floater dynamics has an impact on the wake mixing performance of the Pulse. This is done by considering first the magnitude of motions of the floating wind turbine due to the application of a time-varying thrust force and secondly the effect of these motions on the wake mixing. A frequency response experiment shows that the movement of the floating turbine is heavily frequency-dependent, as is the thrust force. Time domain simulations, using a free wake vortex method with uniform inflow, show that the expected gain in average wind speed at a distance of five rotor diameters downstream is more sensitive to the excitation frequency compared to a bottom-fixed turbine with the same Pulse applied. This is due to the fact that platform motion decreases the thrust force variation and thus reduces the onset of wake mixing.
How to cite. van den Berg, D. G., de Tavernier, D., and van Wingerden, J.-W.: The Dynamic Coupling Between the Pulse Wake Mixing Strategy and Floating Wind Turbines, Wind Energ. Sci. Discuss. [preprint], https://doi.org/10.5194/wes-2022-115, in review, 2023.
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Daniel Graham van den Berg et al.
Status: closed
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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RC1: 'Comment on wes-2022-115', Wim Munters, 22 Feb 2023
The authors present a study on the coupling between the dynamic induction pulse wake mixing control and the dynamics of floating offshore wind turbines. The model set up is detailed for enhanced reproducibility and set up choices are motivated based on an appropriate convergence study. Simulations in the frequency domain are supplemented with specific time-domain simulations at identified frequencies of interest. The paper is interesting and I enjoyed reading it. It is clearly and concisely written and credibly supports its research findings with clear figures. Below, I list a few comments which I believe would further improve the quality of the paper.
Main comments
- In the abstract, the authors highlight that the expected gain in average wind speed is more sensitive to excitation frequency for floating than bottom-fixed turbines (this is supported by Figure 9). However, I think an equally of not more important conclusion is that the "gain in average wind speed" (i.e. the enhanced wake recovery) by applying Pulse control is lower for floating than for bottom-fixed turbines due to turbine motions. If the authors agree, this should be added in the abstract.
- In line 111, the authors claim that, "without loss of generality, the NREL 5MW mounted on an OC3 spar-buoy is used". However, in line 179, they also state that "the motion of the nacelle is mainly dependent on the floater design". These statements appear to contradict. Can the authors comment on to which extent their results can be extrapolated to (1) larger turbines representative of current and near-future offshore wind turbines (2) Different floater concepts.
I expect it is difficult to generalize this, and would invite the authors to comment on this in the manuscript. - The authors refer to the work of Munters and Meyers (2018) (I will refer to this as MM18), where the Pulse concept was first identified. However, some important differences that complicate a direct comparison (e.g. on optimal excitation frequencies and impact on wake recovery) should be kept in mind which are not addressed in the paper. It would be useful for authors to comment on these in the paper.
- The work of MM18 considers a turbulent boundary layer inflow, whereas the current work based on Qblade-Ocean appears to consider a uniform and steady inflow (although it is not explicitly mentioned). This significantly impacts natural wake recovery. Please comment on this and potentially mention it as an important area for future work.
- L285: The authors mention that their finding of "at a 5D distance, the wind speed has converged to nearly identical values." aligns with those in Munters and Meyers (2018). However, Fig. 17a in MM18 clearly shows that overall power gains remain quite different for the different St values of 0.125, 0.25, and 0.5 presented here. Therefore, I do not follow this statement, please explain.Â
Â
Minor / typographical comments
- In table 1, the maximum wake length is shown as "100 Rotor Diameters". In the accompanying text, the authors indicate that the wake is cut off either when this length is exceed, or the maximum number of wake elements is reached. Does this setup length of 100 diameters effectively mean that the latter condition is always the most stringent? If not, 100 Rotor diameters appear excessive for the purposes of this paper. Can the authors shortly comment on this?
- Page 7, L 157: Can the authors add a short comment that, as described in MM18, the Pulse only works for freestream turbines, and that, for waked turbines further research is needed?
- Page 11, L216. "the Pulse becomes less effective as the blades are pitching at a too high frequency" add why the high frequency causes an ineffective Pulse, e.g. since the wake roll-up dynamics are not receptive to these high frequencies.
- Figure 4, bottom panel: I assume the ylabel should be "Phase" and not "Gain"
- Page 14, L268: "This section analyzes the wind speed in the wake, another measure for wake mixing", I would suggest a different phrasing, as the wind speed in the wake is the direct product of enhanced wake recovery / wake mixing, not just another measure. Related, I suggest the authors amend the statement at the end of section 4.1 as "This is likely reducing the overall effectiveness of the wake mixing strategy, as investigated in the next section".
- Figure 7 makes mention of a velocity profile for visualization, yet the figure does not show profiles, but a wake visualization and a planar color plot. Can the authors describe what is exactly shown in the figure? (e.g. the locus of wake elements on the left and contours of axial velocity on the right?)
- Some of the captions can be a little more descriptive to aid the reader to distinguish between plots. For example, in Figure 8, can authors please add in the caption that the top panel is at St = 0.125, etc. It is included in the labels, but mentioning this in the caption makes it easier to distinguish.
- The authors comment on computational cost / time a few times throughout their paper, especially when referring to choices in the simulation setup. Can they illustrate this by mentioning an order of magnitude for the QBlade-Ocean frequency and time-domain simulation times?
Citation: https://doi.org/10.5194/wes-2022-115-RC1 - AC1: 'Reply on RC1', Daniel van de Berg, 20 Mar 2023
-
RC2: 'Comment on wes-2022-115', Anonymous Referee #2, 22 Feb 2023
Interesting work! I think the paper provides a useful contribution in assessing the potential for applying pulse wake mixing in floating wind farms. I think the paper does an excellent job of constructing a theoretical explanation of the observed behaviors, which I think really will add to the readers understanding.ÂA first general comment I have is that I might propose to change the framing of the paper, although maybe this is too subtle a difference to matter too much. My thinking was in the introduction section I was understanding an objective of the paper was to look for benefits of implementing pulse on a floating platform, and in that light the conclusion might appear like a negative result. But in my mind an alternative framing could be that, this and a few other papers are interested if it's even possible to do this type of control on a floating platform. And here the conclusion is more positive, it is possible but on floating it's important to account for the coupling dynamics to motion.ÂA second general comment is that I anticipate there will be questions about the loading impacts. Here I think it could be useful also to frame this paper as confirming the conditions under which it is possible to implement this type of control on a floating platform and realize an uplift. Perhaps this won't be true for every type of wind farm control, and then loading analysis can follow on those methods that pass this first screening.ÂSpecific CommentsÂPage 2ÂYaw misalignment is an example of steady-state optimal control. The use of a model, such as FLORIDyn (Becker et al., 2022; Gebraad and van Wingerden, 2014),ÂWouldn't it make sense to refer to FLORIS rather than FLORIDyn if discussing steady-state control design?ÂÂHowever, wind tunnel experiments and full-scale experiments haveshown that the gain of induction control is negligible (Campagnolo et al., 2016; van der Hoek et al., 2019), contrary to what isfound using 60 wake re-directing (Campagnolo et al., 2016). Similar results were found in a full-scale experiment (Fleming et al.,2017).Â2 quick comments, I believe there is some newer work on this topic that might indicate some potential afterall (https://iopscience.iop.org/article/10.1088/1742-6596/2265/4/042032/meta ) . Also, I don't think Fleming 2017 includes a test of induction control.ÂPage 8ÂFigure 2: Recommend you identify some of the symbols in the figure in the legend for quick readability (Theta_Col -> Collective Pitch) St -> Strouhol number)ÂFigure 2: I associate the strouhol number with atmospheric properties, so wasn't sure why there would be a connection to mechanical frequency responses. Are these shown in Fig 2 just to show where these frequencies may, (coincidentally?), align?Citation: https://doi.org/
10.5194/wes-2022-115-RC2 -
AC2: 'Reply on RC2', Daniel van de Berg, 20 Mar 2023
Dear Reviewer,
Thank you for your in-depth comments. Attached you may find our response to your comments and the implemented changes. If answers and/or actions are unclear feel free to contact us.
with kind regards,
Daniel van den Berg.
Citation: https://doi.org/10.5194/wes-2022-115-AC2
-
AC2: 'Reply on RC2', Daniel van de Berg, 20 Mar 2023
Status: closed
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
| : Report abuse
-
RC1: 'Comment on wes-2022-115', Wim Munters, 22 Feb 2023
The authors present a study on the coupling between the dynamic induction pulse wake mixing control and the dynamics of floating offshore wind turbines. The model set up is detailed for enhanced reproducibility and set up choices are motivated based on an appropriate convergence study. Simulations in the frequency domain are supplemented with specific time-domain simulations at identified frequencies of interest. The paper is interesting and I enjoyed reading it. It is clearly and concisely written and credibly supports its research findings with clear figures. Below, I list a few comments which I believe would further improve the quality of the paper.
Main comments
- In the abstract, the authors highlight that the expected gain in average wind speed is more sensitive to excitation frequency for floating than bottom-fixed turbines (this is supported by Figure 9). However, I think an equally of not more important conclusion is that the "gain in average wind speed" (i.e. the enhanced wake recovery) by applying Pulse control is lower for floating than for bottom-fixed turbines due to turbine motions. If the authors agree, this should be added in the abstract.
- In line 111, the authors claim that, "without loss of generality, the NREL 5MW mounted on an OC3 spar-buoy is used". However, in line 179, they also state that "the motion of the nacelle is mainly dependent on the floater design". These statements appear to contradict. Can the authors comment on to which extent their results can be extrapolated to (1) larger turbines representative of current and near-future offshore wind turbines (2) Different floater concepts.
I expect it is difficult to generalize this, and would invite the authors to comment on this in the manuscript. - The authors refer to the work of Munters and Meyers (2018) (I will refer to this as MM18), where the Pulse concept was first identified. However, some important differences that complicate a direct comparison (e.g. on optimal excitation frequencies and impact on wake recovery) should be kept in mind which are not addressed in the paper. It would be useful for authors to comment on these in the paper.
- The work of MM18 considers a turbulent boundary layer inflow, whereas the current work based on Qblade-Ocean appears to consider a uniform and steady inflow (although it is not explicitly mentioned). This significantly impacts natural wake recovery. Please comment on this and potentially mention it as an important area for future work.
- L285: The authors mention that their finding of "at a 5D distance, the wind speed has converged to nearly identical values." aligns with those in Munters and Meyers (2018). However, Fig. 17a in MM18 clearly shows that overall power gains remain quite different for the different St values of 0.125, 0.25, and 0.5 presented here. Therefore, I do not follow this statement, please explain.Â
Â
Minor / typographical comments
- In table 1, the maximum wake length is shown as "100 Rotor Diameters". In the accompanying text, the authors indicate that the wake is cut off either when this length is exceed, or the maximum number of wake elements is reached. Does this setup length of 100 diameters effectively mean that the latter condition is always the most stringent? If not, 100 Rotor diameters appear excessive for the purposes of this paper. Can the authors shortly comment on this?
- Page 7, L 157: Can the authors add a short comment that, as described in MM18, the Pulse only works for freestream turbines, and that, for waked turbines further research is needed?
- Page 11, L216. "the Pulse becomes less effective as the blades are pitching at a too high frequency" add why the high frequency causes an ineffective Pulse, e.g. since the wake roll-up dynamics are not receptive to these high frequencies.
- Figure 4, bottom panel: I assume the ylabel should be "Phase" and not "Gain"
- Page 14, L268: "This section analyzes the wind speed in the wake, another measure for wake mixing", I would suggest a different phrasing, as the wind speed in the wake is the direct product of enhanced wake recovery / wake mixing, not just another measure. Related, I suggest the authors amend the statement at the end of section 4.1 as "This is likely reducing the overall effectiveness of the wake mixing strategy, as investigated in the next section".
- Figure 7 makes mention of a velocity profile for visualization, yet the figure does not show profiles, but a wake visualization and a planar color plot. Can the authors describe what is exactly shown in the figure? (e.g. the locus of wake elements on the left and contours of axial velocity on the right?)
- Some of the captions can be a little more descriptive to aid the reader to distinguish between plots. For example, in Figure 8, can authors please add in the caption that the top panel is at St = 0.125, etc. It is included in the labels, but mentioning this in the caption makes it easier to distinguish.
- The authors comment on computational cost / time a few times throughout their paper, especially when referring to choices in the simulation setup. Can they illustrate this by mentioning an order of magnitude for the QBlade-Ocean frequency and time-domain simulation times?
Citation: https://doi.org/10.5194/wes-2022-115-RC1 - AC1: 'Reply on RC1', Daniel van de Berg, 20 Mar 2023
-
RC2: 'Comment on wes-2022-115', Anonymous Referee #2, 22 Feb 2023
Interesting work! I think the paper provides a useful contribution in assessing the potential for applying pulse wake mixing in floating wind farms. I think the paper does an excellent job of constructing a theoretical explanation of the observed behaviors, which I think really will add to the readers understanding.ÂA first general comment I have is that I might propose to change the framing of the paper, although maybe this is too subtle a difference to matter too much. My thinking was in the introduction section I was understanding an objective of the paper was to look for benefits of implementing pulse on a floating platform, and in that light the conclusion might appear like a negative result. But in my mind an alternative framing could be that, this and a few other papers are interested if it's even possible to do this type of control on a floating platform. And here the conclusion is more positive, it is possible but on floating it's important to account for the coupling dynamics to motion.ÂA second general comment is that I anticipate there will be questions about the loading impacts. Here I think it could be useful also to frame this paper as confirming the conditions under which it is possible to implement this type of control on a floating platform and realize an uplift. Perhaps this won't be true for every type of wind farm control, and then loading analysis can follow on those methods that pass this first screening.ÂSpecific CommentsÂPage 2ÂYaw misalignment is an example of steady-state optimal control. The use of a model, such as FLORIDyn (Becker et al., 2022; Gebraad and van Wingerden, 2014),ÂWouldn't it make sense to refer to FLORIS rather than FLORIDyn if discussing steady-state control design?ÂÂHowever, wind tunnel experiments and full-scale experiments haveshown that the gain of induction control is negligible (Campagnolo et al., 2016; van der Hoek et al., 2019), contrary to what isfound using 60 wake re-directing (Campagnolo et al., 2016). Similar results were found in a full-scale experiment (Fleming et al.,2017).Â2 quick comments, I believe there is some newer work on this topic that might indicate some potential afterall (https://iopscience.iop.org/article/10.1088/1742-6596/2265/4/042032/meta ) . Also, I don't think Fleming 2017 includes a test of induction control.ÂPage 8ÂFigure 2: Recommend you identify some of the symbols in the figure in the legend for quick readability (Theta_Col -> Collective Pitch) St -> Strouhol number)ÂFigure 2: I associate the strouhol number with atmospheric properties, so wasn't sure why there would be a connection to mechanical frequency responses. Are these shown in Fig 2 just to show where these frequencies may, (coincidentally?), align?Citation: https://doi.org/
10.5194/wes-2022-115-RC2 -
AC2: 'Reply on RC2', Daniel van de Berg, 20 Mar 2023
Dear Reviewer,
Thank you for your in-depth comments. Attached you may find our response to your comments and the implemented changes. If answers and/or actions are unclear feel free to contact us.
with kind regards,
Daniel van den Berg.
Citation: https://doi.org/10.5194/wes-2022-115-AC2
-
AC2: 'Reply on RC2', Daniel van de Berg, 20 Mar 2023
Daniel Graham van den Berg et al.
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Short summary
Wind turbines placed in farms interact with their wake lowering the power production of the wind farm. This can be mitigated using so-called wake mixing techniques. This work investigates the coupling between the Pulse wake mixing technique and the motion of floating wind turbines using the Pulse. Frequency response experiments and time-domain simulations show that extra movement is undesired and the 'optimal' excitation frequency is heavily platform-dependent.
Wind turbines placed in farms interact with their wake lowering the power production of the wind...