Aerodynamic characterisation of a thrust-scaled IEA 15 MW wind turbine model: Experimental insights using PIV data

Fritz, Erik; Ribeiro, André; Boorsma, Koen; Ferreira, Carlos

doi:https://doi.org/10.5194/wes-2024-3

Preprints

https://doi.org/10.5194/wes-2024-3

Preprints

22 Feb 2024

| 22 Feb 2024

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

Aerodynamic characterisation of a thrust-scaled IEA 15 MW wind turbine model: Experimental insights using PIV data

Erik Fritz, André Ribeiro, Koen Boorsma, and Carlos Ferreira

Abstract. This study presents results from a wind tunnel experiment on a three-bladed horizontal axis wind turbine. The model turbine is a scaled-down version of the IEA 15 MW reference wind turbine, preserving the non-dimensional thrust distribution along the blade.

Flow fields were captured around the blade at multiple radial locations using Particle Image Velocimetry. Next to these flow fields, this comprehensive dataset contains spanwise distributions of bound circulation, inflow conditions and blade forces derived from the velocity field. As such, the three blades' aerodynamics are fully characterised. It is demonstrated that the lift coefficient measured along the span agrees well with the lift polar of the airfoil used in the blade design, thereby validating the experimental approach.

This research provides a valuable public experimental dataset for validating low to high-fidelity numerical models simulating state-of-the-art wind turbines. Furthermore, this article establishes the aerodynamic properties of the newly developed model wind turbine, creating a baseline for future wind tunnel experiments using this model.

Received: 22 Jan 2024 – Discussion started: 22 Feb 2024

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Erik Fritz, André Ribeiro, Koen Boorsma, and Carlos Ferreira

Status: closed

RC1:
'Comment on wes-2024-3', Anonymous Referee #1, 28 Feb 2024

This study presents the findings from wind tunnel experiments conducted on a scaled-down version

of the IEA 15 MW reference wind turbine, while ensuring the preservation of the non-dimensional

thrust distribution along the blade. The authors employed the Stereoscopic Particle Image Ve-

locimetry technique to analyze the flow field and successfully acquired a comprehensive dataset

detailing the spanwise distribution of various aerodynamic parameters.

I extend my congratulations to the authors for their diligent work on this study. The literature re-

view is thorough and precise, and the paper is well-organized with clear plots. It is understandable

that the authors encountered challenges with different twist angles for the three blades, stemming

from manufacturing processes. However, what is commendable is that this setback did not deter

the authors. Instead, they utilized this limitation to their advantage and experimentally demon-

strated and validated one of the fundamental assumptions in blade element momentum theory:

that induction can be regarded as a blade-averaged phenomenon.

As a result, I believe this paper falls well within the scope of Wind Energy Science, and would be

an important contribution in the field of wind energy. Nevertheless, I would like the authors to

address the following comments and make suitable changes in the paper.

1: Refer to the Table 2: The scaling factor for the rotor diameter D is 133.33, while for the

root radius it stands at 50. Drawing from my experience, I speculate that the difference

in the scaling factor could stem from the necessity to accommodate electronics within the

rotor root. This adjustment may have disrupted the maintenance of a consistent scaling

factor. However, such intricacies might not be readily apparent to individuals not directly

involved in the experimental process. Hence, I advise to present the

rationale behind the difference in the scaling factor. Furthermore, it is important to address

the potential ramifications of this difference on the ultimate conclusions drawn from the study.

To bolster these assertions, referencing relevant studies would provide additional support to

your arguments.

2: Refer to the line 100: In the study, the authors noted a manual reduction of the chord to 4cm

below r/R = 0.25. However, the term ”manually” lacks clarity regarding its intended mean-

ing. It appears to indicate a deviation from utilizing the scaling law specified in equation 3.

If this interpretation holds true, what alternative method did the authors employ? Fur-

thermore, what factors contribute to their confidence that the results would not significantly

diverge from those obtained by precisely following equation 3? The authors are encouraged

to expound upon their approach, providing support from relevant literature.

3: Upon re-deriving equation 6, I discovered that it was not that straightforward. The deriva-

tion of this equation entails several algebraic steps. Given the significance of this equation in

determining the blade’s twist angle, it would be beneficial to include its derivation in either

the main text or an appendix. Additionally, I recommend using the term ”twist angle” to

define β, as ”pitch angle” typically denotes the rotation of the entire blade at the root along

the blade axis with respect to the rotor plane.

4: Refer to the line 112: The authors are requested to precisely indicate the simulations

from which the parameter values were extracted. Furthermore, it is advisable to provide

insight into the reasons behind selecting those specific papers and elaborate on their relevance

to the current study.

5: Refer to lines 130 and 131: Please specify the value of the time delay and the way the authors calculated it.

6: Suggestion for Equation 14: It is noted that you have presented the equation in vector form.

For mathematical accuracy, it would be more appropriate to use n · (u − uB ) instead of

n(u − uB )^T . This adjustment should also be applied to the third term. While Euclidean

inner products, which are relevant in this context, can be calculated as the matrix product

of a row and column vector, they are distinct concepts in precise mathematical terms.

7: Refer to the figure 12:

Blade 3: If the airfoil maintains a consistent shape across all sections, one would expect

identical lift coefficients (cl) for a given Angle of Attack (AOA). However, a difference is

observed in the initial two points, suggesting other factors at play. It would be beneficial to

explore potential causes for these differences. On a positive note, it is commendable that the

remainder aligns well with the design lift curve.

Blade 2: The observation that the lift coefficient (cl) remains constant despite increasing

AOA, especially at lower angles, is unexpected. This outcome warrants further investigation

to determine the underlying cause, as it does not closely adhere to the anticipated design lift

curve, leading to some reservations about its accuracy.

Blade 1: This blade shares the same issue as Blade 2, with an added discrepancy at a 6°

AOA, where the cl value significantly deviates from the design lift curve.

For Blades 1 and 2, it is plausible that the airfoil’s shape diverges from the standard SD7032

profile, possibly in the curvature at the leading edge. Such variations might also stem from

inadequate resolution in the velocity field analysis.

8: Refer to lines 314 and 315: This scaled-down version of the original wind turbine (WT)

replicates the non-dimensional thrust by design, as it was developed based on thrust simi-

larity principles. However, the scaled-down WT is tailored to a specific thrust configuration,

determined by factors such as blade pitch angle (where pitch angle refers to the angle by

which the entire blade rotates around its axis relative to the rotor plane). Altering the pitch

angle of blades in the original WT would necessitate a redesign of the scaled WT. For in-

stance, consider the minimum function defining the twist angle β (as shown in equation 6).

Changing the inflow angle to the blade through pitch adjustment would consequently alter

the radial twist distribution in the scaled-down WT. It’s important to exercise caution, as

blade shapes and types differ between the original and scaled-down WTs, resulting in varying

Reynolds numbers. Consequently, flow physics, such as flow transition and separation at a

given radial position (r/R), differ at the airfoil level. Therefore, while global aerodynamic

properties like non-dimensional thrust may exhibit similarity, it’s essential to recognize that

local aerodynamics at each radial position between the original and scaled-down WTs may

not necessarily align.

Citation: https://doi.org/10.5194/wes-2024-3-RC1
- AC1: 'Comment on wes-2024-3', Erik Fritz, 25 Mar 2024
  
  Dear reviewers,
  
  The authors would like to thank you for the time and effort that you have dedicated to providing valuable feedback on our manuscript. We have been able to incorporate changes to reflect your suggestions. Please find a point-by-point response to your comments and a version of our manuscript highlighting all made changes in the attached file.
  Kind regards,
  
  Erik Fritz, André Ribeiro, Koen Boorsma, Carlos Ferreira
  
  Citation: https://doi.org/10.5194/wes-2024-3-AC1
RC2:
'Comment on wes-2024-3', Anonymous Referee #2, 06 Mar 2024

The paper describes the design and aerodynamic characterisation of a thrust-scales IEA 15MW wind turbine model. The paper is well written and the single aspects are well motivated. The results are relevant to the community. Nevertheless, there are a few aspects that are not 100% clear to me that the authors should address.

In the introduction the authors mention other scaled experiments with e.g. the DTU 10 MW model turbine. There are also a few experiments with a model turbine scaled down from a 5MW reference turbine (https://doi.org/10.5194/wes-6-1341-2021). In these experiments they also use the method by Herreaz to determine the axial and tangential induction factor along the rotor blade.

Figure 3 can easily be removed since it doesn’t provide any extra information for the rest of the paper.

On page 9 line 150 the authors state that in the post-processing they stich together the results from the pressure and the suction side of the profile to get the total flow around the profile. Are these images averaged over several events or are they temporally highly resolved? In the later case, can the authors say anything about variations in the flow field on the upper and lower side for measurements between single rotations?

Line 200, page 11: The model turbine was running at constant rotational speed. Was that actively controlled or is that given due to the fact that the inflow was constant ? I can image, that there might have been some fluctuations in the rotational speed also due to the fact that the blades were all different. Passing the tower will cause different aerodynamics for each of the blades which could be seen in fluctuations in the rotational speed. Did the authors observe something like that ?

In section 3.1 it is not quite clear if the authors corrected the mean offset on the pitch (twist) for each blade or if they just measured the offset. If they did not correct the mean offset, why not? The authors say that the turbine is equipped with a manual pitch system which should allow the correction of the offset, right?

I do not fully understand why the authors show figure 12. Here they compare the calculated lift coefficient for each blade and compare it to the one from a reference for a specific Reynolds number. Why not going the other way and calculating the normal and tangential forces for each radial position on the blades using the angle of attack, acting velocities and the corresponding lift and drag coefficients from figure 1 for the correct Reynolds number and ad the results to figure 11? That would minimise the effect of the Reynolds number in the end should show how well all methods agree — or maybe I misunderstood the intension of figure 12.

Line 316, page 18, the authors mention that in future research they would like to reduce the impact of blade deflection on the results. In the paper they never mentioned the problem or determined the deflection and the impact on the results. While you mentioned it here, what is the impact of blade deflection on the results presented in the paper?

Citation: https://doi.org/10.5194/wes-2024-3-RC2
- AC1: 'Comment on wes-2024-3', Erik Fritz, 25 Mar 2024
  
  Dear reviewers,
  
  The authors would like to thank you for the time and effort that you have dedicated to providing valuable feedback on our manuscript. We have been able to incorporate changes to reflect your suggestions. Please find a point-by-point response to your comments and a version of our manuscript highlighting all made changes in the attached file.
  Kind regards,
  
  Erik Fritz, André Ribeiro, Koen Boorsma, Carlos Ferreira
  
  Citation: https://doi.org/10.5194/wes-2024-3-AC1
AC1: 'Comment on wes-2024-3', Erik Fritz, 25 Mar 2024

Dear reviewers,

The authors would like to thank you for the time and effort that you have dedicated to providing valuable feedback on our manuscript. We have been able to incorporate changes to reflect your suggestions. Please find a point-by-point response to your comments and a version of our manuscript highlighting all made changes in the attached file.
Kind regards,

Erik Fritz, André Ribeiro, Koen Boorsma, Carlos Ferreira

Citation: https://doi.org/10.5194/wes-2024-3-AC1

Status: closed

RC1:
'Comment on wes-2024-3', Anonymous Referee #1, 28 Feb 2024

This study presents the findings from wind tunnel experiments conducted on a scaled-down version

of the IEA 15 MW reference wind turbine, while ensuring the preservation of the non-dimensional

thrust distribution along the blade. The authors employed the Stereoscopic Particle Image Ve-

locimetry technique to analyze the flow field and successfully acquired a comprehensive dataset

detailing the spanwise distribution of various aerodynamic parameters.

I extend my congratulations to the authors for their diligent work on this study. The literature re-

view is thorough and precise, and the paper is well-organized with clear plots. It is understandable

that the authors encountered challenges with different twist angles for the three blades, stemming

from manufacturing processes. However, what is commendable is that this setback did not deter

the authors. Instead, they utilized this limitation to their advantage and experimentally demon-

strated and validated one of the fundamental assumptions in blade element momentum theory:

that induction can be regarded as a blade-averaged phenomenon.

As a result, I believe this paper falls well within the scope of Wind Energy Science, and would be

an important contribution in the field of wind energy. Nevertheless, I would like the authors to

address the following comments and make suitable changes in the paper.

1: Refer to the Table 2: The scaling factor for the rotor diameter D is 133.33, while for the

root radius it stands at 50. Drawing from my experience, I speculate that the difference

in the scaling factor could stem from the necessity to accommodate electronics within the

rotor root. This adjustment may have disrupted the maintenance of a consistent scaling

factor. However, such intricacies might not be readily apparent to individuals not directly

involved in the experimental process. Hence, I advise to present the

rationale behind the difference in the scaling factor. Furthermore, it is important to address

the potential ramifications of this difference on the ultimate conclusions drawn from the study.

To bolster these assertions, referencing relevant studies would provide additional support to

your arguments.

2: Refer to the line 100: In the study, the authors noted a manual reduction of the chord to 4cm

below r/R = 0.25. However, the term ”manually” lacks clarity regarding its intended mean-

ing. It appears to indicate a deviation from utilizing the scaling law specified in equation 3.

If this interpretation holds true, what alternative method did the authors employ? Fur-

thermore, what factors contribute to their confidence that the results would not significantly

diverge from those obtained by precisely following equation 3? The authors are encouraged

to expound upon their approach, providing support from relevant literature.

3: Upon re-deriving equation 6, I discovered that it was not that straightforward. The deriva-

tion of this equation entails several algebraic steps. Given the significance of this equation in

determining the blade’s twist angle, it would be beneficial to include its derivation in either

the main text or an appendix. Additionally, I recommend using the term ”twist angle” to

define β, as ”pitch angle” typically denotes the rotation of the entire blade at the root along

the blade axis with respect to the rotor plane.

4: Refer to the line 112: The authors are requested to precisely indicate the simulations

from which the parameter values were extracted. Furthermore, it is advisable to provide

insight into the reasons behind selecting those specific papers and elaborate on their relevance

to the current study.

5: Refer to lines 130 and 131: Please specify the value of the time delay and the way the authors calculated it.

6: Suggestion for Equation 14: It is noted that you have presented the equation in vector form.

For mathematical accuracy, it would be more appropriate to use n · (u − uB ) instead of

n(u − uB )^T . This adjustment should also be applied to the third term. While Euclidean

inner products, which are relevant in this context, can be calculated as the matrix product

of a row and column vector, they are distinct concepts in precise mathematical terms.

7: Refer to the figure 12:

Blade 3: If the airfoil maintains a consistent shape across all sections, one would expect

identical lift coefficients (cl) for a given Angle of Attack (AOA). However, a difference is

observed in the initial two points, suggesting other factors at play. It would be beneficial to

explore potential causes for these differences. On a positive note, it is commendable that the

remainder aligns well with the design lift curve.

Blade 2: The observation that the lift coefficient (cl) remains constant despite increasing

AOA, especially at lower angles, is unexpected. This outcome warrants further investigation

to determine the underlying cause, as it does not closely adhere to the anticipated design lift

curve, leading to some reservations about its accuracy.

Blade 1: This blade shares the same issue as Blade 2, with an added discrepancy at a 6°

AOA, where the cl value significantly deviates from the design lift curve.

For Blades 1 and 2, it is plausible that the airfoil’s shape diverges from the standard SD7032

profile, possibly in the curvature at the leading edge. Such variations might also stem from

inadequate resolution in the velocity field analysis.

8: Refer to lines 314 and 315: This scaled-down version of the original wind turbine (WT)

replicates the non-dimensional thrust by design, as it was developed based on thrust simi-

larity principles. However, the scaled-down WT is tailored to a specific thrust configuration,

determined by factors such as blade pitch angle (where pitch angle refers to the angle by

which the entire blade rotates around its axis relative to the rotor plane). Altering the pitch

angle of blades in the original WT would necessitate a redesign of the scaled WT. For in-

stance, consider the minimum function defining the twist angle β (as shown in equation 6).

Changing the inflow angle to the blade through pitch adjustment would consequently alter

the radial twist distribution in the scaled-down WT. It’s important to exercise caution, as

blade shapes and types differ between the original and scaled-down WTs, resulting in varying

Reynolds numbers. Consequently, flow physics, such as flow transition and separation at a

given radial position (r/R), differ at the airfoil level. Therefore, while global aerodynamic

properties like non-dimensional thrust may exhibit similarity, it’s essential to recognize that

local aerodynamics at each radial position between the original and scaled-down WTs may

not necessarily align.

Citation: https://doi.org/10.5194/wes-2024-3-RC1
- AC1: 'Comment on wes-2024-3', Erik Fritz, 25 Mar 2024
  
  Dear reviewers,
  
  The authors would like to thank you for the time and effort that you have dedicated to providing valuable feedback on our manuscript. We have been able to incorporate changes to reflect your suggestions. Please find a point-by-point response to your comments and a version of our manuscript highlighting all made changes in the attached file.
  Kind regards,
  
  Erik Fritz, André Ribeiro, Koen Boorsma, Carlos Ferreira
  
  Citation: https://doi.org/10.5194/wes-2024-3-AC1
RC2:
'Comment on wes-2024-3', Anonymous Referee #2, 06 Mar 2024

The paper describes the design and aerodynamic characterisation of a thrust-scales IEA 15MW wind turbine model. The paper is well written and the single aspects are well motivated. The results are relevant to the community. Nevertheless, there are a few aspects that are not 100% clear to me that the authors should address.

In the introduction the authors mention other scaled experiments with e.g. the DTU 10 MW model turbine. There are also a few experiments with a model turbine scaled down from a 5MW reference turbine (https://doi.org/10.5194/wes-6-1341-2021). In these experiments they also use the method by Herreaz to determine the axial and tangential induction factor along the rotor blade.

Figure 3 can easily be removed since it doesn’t provide any extra information for the rest of the paper.

On page 9 line 150 the authors state that in the post-processing they stich together the results from the pressure and the suction side of the profile to get the total flow around the profile. Are these images averaged over several events or are they temporally highly resolved? In the later case, can the authors say anything about variations in the flow field on the upper and lower side for measurements between single rotations?

Line 200, page 11: The model turbine was running at constant rotational speed. Was that actively controlled or is that given due to the fact that the inflow was constant ? I can image, that there might have been some fluctuations in the rotational speed also due to the fact that the blades were all different. Passing the tower will cause different aerodynamics for each of the blades which could be seen in fluctuations in the rotational speed. Did the authors observe something like that ?

In section 3.1 it is not quite clear if the authors corrected the mean offset on the pitch (twist) for each blade or if they just measured the offset. If they did not correct the mean offset, why not? The authors say that the turbine is equipped with a manual pitch system which should allow the correction of the offset, right?

I do not fully understand why the authors show figure 12. Here they compare the calculated lift coefficient for each blade and compare it to the one from a reference for a specific Reynolds number. Why not going the other way and calculating the normal and tangential forces for each radial position on the blades using the angle of attack, acting velocities and the corresponding lift and drag coefficients from figure 1 for the correct Reynolds number and ad the results to figure 11? That would minimise the effect of the Reynolds number in the end should show how well all methods agree — or maybe I misunderstood the intension of figure 12.

Line 316, page 18, the authors mention that in future research they would like to reduce the impact of blade deflection on the results. In the paper they never mentioned the problem or determined the deflection and the impact on the results. While you mentioned it here, what is the impact of blade deflection on the results presented in the paper?

Citation: https://doi.org/10.5194/wes-2024-3-RC2
- AC1: 'Comment on wes-2024-3', Erik Fritz, 25 Mar 2024
  
  Dear reviewers,
  
  The authors would like to thank you for the time and effort that you have dedicated to providing valuable feedback on our manuscript. We have been able to incorporate changes to reflect your suggestions. Please find a point-by-point response to your comments and a version of our manuscript highlighting all made changes in the attached file.
  Kind regards,
  
  Erik Fritz, André Ribeiro, Koen Boorsma, Carlos Ferreira
  
  Citation: https://doi.org/10.5194/wes-2024-3-AC1
AC1: 'Comment on wes-2024-3', Erik Fritz, 25 Mar 2024

Dear reviewers,

The authors would like to thank you for the time and effort that you have dedicated to providing valuable feedback on our manuscript. We have been able to incorporate changes to reflect your suggestions. Please find a point-by-point response to your comments and a version of our manuscript highlighting all made changes in the attached file.
Kind regards,

Erik Fritz, André Ribeiro, Koen Boorsma, Carlos Ferreira

Citation: https://doi.org/10.5194/wes-2024-3-AC1

Erik Fritz, André Ribeiro, Koen Boorsma, and Carlos Ferreira

Data sets

Supporting data belonging to the publication Aerodynamic characterisation of a thrust-scaled IEA 15 MW wind turbine model: Experimental insights using PIV data Erik Fritz, André Ribeiro, Koen Boorsma, and Carlos Ferreira https://doi.org/10.4121/164890ab-39d7-4af8-8b3c-9e21f789b80a

Erik Fritz, André Ribeiro, Koen Boorsma, and Carlos Ferreira

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Short summary

This study presents results from a wind tunnel experiment on a model wind turbine. Using a non-intrusive measurement technique, the flow around the turbine blades was measured. In post-processing, the blade-level aerodynamics are derived from the measured flow fields. The detailed experimental database has great value in validating numerical tools of varying complexity, which aim at simulating wind turbine aerodynamics as accurately as possible.


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