On the significance of rain droplet slowdown and deformation for leading-edge rain erosion

Barfknecht, Nils; von Terzi, Dominic

doi:https://doi.org/10.5194/wes-2023-169

Preprints

https://doi.org/10.5194/wes-2023-169

Preprints

22 Dec 2023

| 22 Dec 2023

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

On the significance of rain droplet slowdown and deformation for leading-edge rain erosion

Nils Barfknecht and Dominic von Terzi

Abstract. Leading-edge rain erosion is a severe problem in the wind energy community since it leads to blade damage and a reduction in annual energy production in the order of several percent. The impact speed of rain droplets is a key driver for the erosion rate; therefore, its precise computation is essential. This study investigates the aerodynamic interaction of rain droplets and wind turbine blades. Based on findings from the literature and an analysis of the relevant parameter space, it is found that the aerodynamic interaction leads to a reduction in the impact speed. Additionally, the rain droplets deform and break up as they approach the wind turbine blade. An existing Lagrangian particle model, developed for research in aircraft icing, is adapted, extended, and validated for leading edge rain erosion to study the process in more detail. Results show that the droplet slowdown reduces predicted damage toward the tip of the blade by over 50 %. The model indicates that the aerodynamic blade interaction affects small droplets significantly more than large droplets. Due to this drop size dependency, the damage accumulation is shifted towards higher rain intensity events. Additionally, the droplet impact speed is sensitive to the aerodynamic nose radius of the airfoil. Due to this sensitivity and its drop size dependency, the slowdown effect provides interesting levers for erosion mitigation via blade design or operational adjustments. To conclude, the aerodynamic interaction between droplet and blade is non-negligible and needs to be taken into account in erosion lifetime models.

Received: 11 Dec 2023 – Discussion started: 22 Dec 2023

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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Nils Barfknecht and Dominic von Terzi

Status: closed

RC1:
'Comment on wes-2023-169', Anonymous Referee #1, 26 Apr 2024

GENERAL COMMENTS
The paper addresses Leading edge erosion (LEER) of wind turbine blades (WTB). In particular it explores the relative impact velocity between droplet and WTB leading edge as function of blade velocity, droplet size, droplet shape distortion and airfoil geometry. This is of broad international interest to WT operators, manufacturers and academia. It´s a highly relevant scientific topic within the scope of WES. The topic is important for understanding some of the governing parameters of leading edge erosion and the translation from rain erosion test results to field conditions and expected blade life with respect to erosion. The objectives of the paper are clear, and the scientific methods are clearly outlined. The assumptions and analysis seem valid. The presented results are sufficient to support the interpretations and associated discussion. The discussion is relevant and relate to the results and methods. The concept of modelling shape distortion and relative impact velocity as functions of blade velocity, droplet size, and airfoil geometry ad correlating it to erosion damage is new for WTB erosion and thus is the presented data. The authors refer and give credit to earlier related work and clearly indicate their own contribution. The title reflects the contents of the paper. However, it will be more informative if it relates “droplet slowdown and deformation” with “aerodynamic interaction with wind turbine blade airfoil” or something like it. The paper is generally well structured and written in good concise language. The abstract provides a concise and complete summary, including quantitative results. Figures and tables are useful and well explained in the text. Mathematical formulae, symbols, abbreviations, and units are appropriate. However, a list of symbols is highly recommended.
The methodology section contains a lot of background information and references to literature and earlier studies. This should be moved to the introduction. It could be in a section 1.1 “Background. The methodology should explain own work but can refer to the introduction and background section.
A discussion section would be welcome. Discuss your methods. E.g. discuss how the new knowledge affects the models for expected WT blade life (with respect to erosion) based on data from rain erosion test. Velocities, nose radii and droplet sizes may be different in the two situations.
The number and quality of references are appropriate.
There is no supplementary material. The authors state that code and data are available upon request.
A list of contents would enhance overview.

SPECIFIC COMMENTS
L2: up to a few percent.
L115: bubble starts growing: bag starts forming
Figure 6: own data or from literature?
L180-187 and beyond. Should be in background section. Maybe just refer to original work and present results. Is it necessary to derive?.
L205: Can that many decimals be justified?
L209: what is the difference between “free stream velocity” and “slip velocity”?
L323 and beyond: again move to background section.
L390-beyond: is it necessary to derive this, or enough to refer to Verma 2021? And it should be moved to the background section.
P30 and fig.23: What is a typical range of nose radii for the outboard parts of WT blades?

TECHNICAL CORRECTIONS
Figure 3 (and others?) missing statement of permission to reproduce.
L84: shapes: shape
L85: This is process: This process
Table 1: image quality could be better
L120: all resulting droplets: each resulting droplet
L137: error in the (computed) lifetime
L354: The figure: Fig.19

Citation: https://doi.org/10.5194/wes-2023-169-RC1
- AC2: 'Reply on RC1', Nils Barfknecht, 01 Jul 2024
  
  Dear Reviewer,
  We would like to thank you for your time and effort in reviewing our manuscript. We believe that your comments have improved the quality of our work.
  Sincerely,
  Nils Barfknecht and Dominic von Terzi
  
  Citation: https://doi.org/10.5194/wes-2023-169-AC2
RC2:
'Comment on wes-2023-169', Anonymous Referee #2, 13 Jun 2024

The manuscript addresses the issue of rain-induced erosion on wind turbine blades and identifies the lack of accuracy/knowledge for currently available models and experimental data from the literature.

A considerable amount of experimental data has been collected and analyzed to develop a new model (derived from previous models) for rain drop shape and impact velocity on the blade.
The developed model is then used to analyze the impact velocities and subsequent erosion damage on two model wind turbine blades, providing insight into the damage induced at different locations along their radius.
The models are well presented and explained with references to the relevant literature.

The plots are all very clear and so is the overlay text.
The conclusions are well presented and useful for both academia and industry.

I have a few minor comments that the authors should address to improve the quality of their manuscript:
- Line 22: Starting a sentence with a variable name is not recommended (this happens several times in the manuscript, e.g. lines 162-164, line 376, etc.).
- Line 27: "in literature" is missing
- Figure 2: i) "Measure vel. -> specify "relative" vel if it is so ii) free-stream vel are actually airfoil vels?
- Figures in general: have you cleared the copyright issues with the various figures you take from other papers?
- The use of "drop(s)" as a synonym for of droplets makes the text difficult to read (especially since you also use the verb droping). Please correct.
- Line 130: You give DNS as the miracle solution, but DNS deals with fluid dynamics, so you mean multiphase? Please explain.
- Figure 9: Can you avoid putting the numbers over the dots?
- eps" - does the choice of numerical values you use significantly affect your results?
- Line 242: were chosen, (add comma)
- Line 244: OpenFOAM, simpleFoam
- Streamlines: You select some streamlines in front of the airfoil using ParaView, what is the uncertainty associated with this manual procedure?
- Table 4: The units for C's are degrees to the power of -2 ? This is a bit unclear, maybe improve the alpha in degrees part of the caption or maybe better just write deg^-2 ?
- 292: 'Case F' and 'Case G' or 'Case F' and 'Case G
- Figure 13: Why are the parameters of the 5MW not smooth along r? (You address this much later in the text, but it would be nice to see it here as well, and can you justify this philosophy of not interpolating between the airfoils a bit more?)
- Line 299: phi_0 is the diameter, but it would be easier if it was stated (or if there was a nomenclature at the beginning of the paper with all the variable definitions).
- Line 331: "projected area" projected onto what?
- Line 354: i) You've italicized E here as opposed to reading the document i) Wouldn't 7.2 <= Beta <= 10.5 be easier to read?
- Line 369: You might want to better explain why velocity is no longer critical in this case, since you mention that it is quite influential for damage.
- Line 411: You second part -> The
- Line 436: the sentence is confusing, the slowdown factors vary with the diameter of the droplet, so maybe better wording.
- Figure 22 (a): Droplets
- Line 461-462: Consistency in number formatting between 2 and 2.0.
- Line 467: i.e. a factor of five larger -> i.e. five times larger.
- Line 470-473: this is hard to follow, please expend/clarify, help the reader!
- Section 3.3: the title is confusing (distribution (singular) of what? damage? intensities (plural)?
- Fig. 24: caption) 1 meter H of I in mm/hr is a bit confusing?
- Lines 528-529: avoid line return between number percent signs
- Lines 540-545: See my comment on fig. 13.
- Lines 550-554: This is hard to follow, you should better explain how this contradicts Fig. 23 and why.
- Lines 585: start with a number

Citation: https://doi.org/10.5194/wes-2023-169-RC2
- AC1: 'Reply on RC2', Nils Barfknecht, 01 Jul 2024
  
  Dear Reviewer,
  We would like to thank you for your time and effort in reviewing our manuscript. We believe that your comments have improved the quality of our work.
  Sincerely,
  Nils Barfknecht and Dominic von Terzi
  
  Citation: https://doi.org/10.5194/wes-2023-169-AC1
AC3: 'Comment on wes-2023-169', Nils Barfknecht, 01 Jul 2024

Dear Reviewers,
You can find our answers to your comments in the supplement.
Sincerely,
Nils Barfknecht and Dominic von Terzi

Citation: https://doi.org/10.5194/wes-2023-169-AC3

Status: closed

RC1:
'Comment on wes-2023-169', Anonymous Referee #1, 26 Apr 2024

GENERAL COMMENTS
The paper addresses Leading edge erosion (LEER) of wind turbine blades (WTB). In particular it explores the relative impact velocity between droplet and WTB leading edge as function of blade velocity, droplet size, droplet shape distortion and airfoil geometry. This is of broad international interest to WT operators, manufacturers and academia. It´s a highly relevant scientific topic within the scope of WES. The topic is important for understanding some of the governing parameters of leading edge erosion and the translation from rain erosion test results to field conditions and expected blade life with respect to erosion. The objectives of the paper are clear, and the scientific methods are clearly outlined. The assumptions and analysis seem valid. The presented results are sufficient to support the interpretations and associated discussion. The discussion is relevant and relate to the results and methods. The concept of modelling shape distortion and relative impact velocity as functions of blade velocity, droplet size, and airfoil geometry ad correlating it to erosion damage is new for WTB erosion and thus is the presented data. The authors refer and give credit to earlier related work and clearly indicate their own contribution. The title reflects the contents of the paper. However, it will be more informative if it relates “droplet slowdown and deformation” with “aerodynamic interaction with wind turbine blade airfoil” or something like it. The paper is generally well structured and written in good concise language. The abstract provides a concise and complete summary, including quantitative results. Figures and tables are useful and well explained in the text. Mathematical formulae, symbols, abbreviations, and units are appropriate. However, a list of symbols is highly recommended.
The methodology section contains a lot of background information and references to literature and earlier studies. This should be moved to the introduction. It could be in a section 1.1 “Background. The methodology should explain own work but can refer to the introduction and background section.
A discussion section would be welcome. Discuss your methods. E.g. discuss how the new knowledge affects the models for expected WT blade life (with respect to erosion) based on data from rain erosion test. Velocities, nose radii and droplet sizes may be different in the two situations.
The number and quality of references are appropriate.
There is no supplementary material. The authors state that code and data are available upon request.
A list of contents would enhance overview.

SPECIFIC COMMENTS
L2: up to a few percent.
L115: bubble starts growing: bag starts forming
Figure 6: own data or from literature?
L180-187 and beyond. Should be in background section. Maybe just refer to original work and present results. Is it necessary to derive?.
L205: Can that many decimals be justified?
L209: what is the difference between “free stream velocity” and “slip velocity”?
L323 and beyond: again move to background section.
L390-beyond: is it necessary to derive this, or enough to refer to Verma 2021? And it should be moved to the background section.
P30 and fig.23: What is a typical range of nose radii for the outboard parts of WT blades?

TECHNICAL CORRECTIONS
Figure 3 (and others?) missing statement of permission to reproduce.
L84: shapes: shape
L85: This is process: This process
Table 1: image quality could be better
L120: all resulting droplets: each resulting droplet
L137: error in the (computed) lifetime
L354: The figure: Fig.19

Citation: https://doi.org/10.5194/wes-2023-169-RC1
- AC2: 'Reply on RC1', Nils Barfknecht, 01 Jul 2024
  
  Dear Reviewer,
  We would like to thank you for your time and effort in reviewing our manuscript. We believe that your comments have improved the quality of our work.
  Sincerely,
  Nils Barfknecht and Dominic von Terzi
  
  Citation: https://doi.org/10.5194/wes-2023-169-AC2
RC2:
'Comment on wes-2023-169', Anonymous Referee #2, 13 Jun 2024

The manuscript addresses the issue of rain-induced erosion on wind turbine blades and identifies the lack of accuracy/knowledge for currently available models and experimental data from the literature.

A considerable amount of experimental data has been collected and analyzed to develop a new model (derived from previous models) for rain drop shape and impact velocity on the blade.
The developed model is then used to analyze the impact velocities and subsequent erosion damage on two model wind turbine blades, providing insight into the damage induced at different locations along their radius.
The models are well presented and explained with references to the relevant literature.

The plots are all very clear and so is the overlay text.
The conclusions are well presented and useful for both academia and industry.

I have a few minor comments that the authors should address to improve the quality of their manuscript:
- Line 22: Starting a sentence with a variable name is not recommended (this happens several times in the manuscript, e.g. lines 162-164, line 376, etc.).
- Line 27: "in literature" is missing
- Figure 2: i) "Measure vel. -> specify "relative" vel if it is so ii) free-stream vel are actually airfoil vels?
- Figures in general: have you cleared the copyright issues with the various figures you take from other papers?
- The use of "drop(s)" as a synonym for of droplets makes the text difficult to read (especially since you also use the verb droping). Please correct.
- Line 130: You give DNS as the miracle solution, but DNS deals with fluid dynamics, so you mean multiphase? Please explain.
- Figure 9: Can you avoid putting the numbers over the dots?
- eps" - does the choice of numerical values you use significantly affect your results?
- Line 242: were chosen, (add comma)
- Line 244: OpenFOAM, simpleFoam
- Streamlines: You select some streamlines in front of the airfoil using ParaView, what is the uncertainty associated with this manual procedure?
- Table 4: The units for C's are degrees to the power of -2 ? This is a bit unclear, maybe improve the alpha in degrees part of the caption or maybe better just write deg^-2 ?
- 292: 'Case F' and 'Case G' or 'Case F' and 'Case G
- Figure 13: Why are the parameters of the 5MW not smooth along r? (You address this much later in the text, but it would be nice to see it here as well, and can you justify this philosophy of not interpolating between the airfoils a bit more?)
- Line 299: phi_0 is the diameter, but it would be easier if it was stated (or if there was a nomenclature at the beginning of the paper with all the variable definitions).
- Line 331: "projected area" projected onto what?
- Line 354: i) You've italicized E here as opposed to reading the document i) Wouldn't 7.2 <= Beta <= 10.5 be easier to read?
- Line 369: You might want to better explain why velocity is no longer critical in this case, since you mention that it is quite influential for damage.
- Line 411: You second part -> The
- Line 436: the sentence is confusing, the slowdown factors vary with the diameter of the droplet, so maybe better wording.
- Figure 22 (a): Droplets
- Line 461-462: Consistency in number formatting between 2 and 2.0.
- Line 467: i.e. a factor of five larger -> i.e. five times larger.
- Line 470-473: this is hard to follow, please expend/clarify, help the reader!
- Section 3.3: the title is confusing (distribution (singular) of what? damage? intensities (plural)?
- Fig. 24: caption) 1 meter H of I in mm/hr is a bit confusing?
- Lines 528-529: avoid line return between number percent signs
- Lines 540-545: See my comment on fig. 13.
- Lines 550-554: This is hard to follow, you should better explain how this contradicts Fig. 23 and why.
- Lines 585: start with a number

Citation: https://doi.org/10.5194/wes-2023-169-RC2
- AC1: 'Reply on RC2', Nils Barfknecht, 01 Jul 2024
  
  Dear Reviewer,
  We would like to thank you for your time and effort in reviewing our manuscript. We believe that your comments have improved the quality of our work.
  Sincerely,
  Nils Barfknecht and Dominic von Terzi
  
  Citation: https://doi.org/10.5194/wes-2023-169-AC1
AC3: 'Comment on wes-2023-169', Nils Barfknecht, 01 Jul 2024

Dear Reviewers,
You can find our answers to your comments in the supplement.
Sincerely,
Nils Barfknecht and Dominic von Terzi

Citation: https://doi.org/10.5194/wes-2023-169-AC3

Nils Barfknecht and Dominic von Terzi

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Short summary

Rain droplets damage wind turbine blades due to the high impact speed at the tip. In this study, it is found that rain droplets and wind turbine blades interact aerodynamically. The rain droplets slow down and deform close to the blade. A model from another field of study was adapted and validated to study this process in detail. This effect reduced the predicted erosion damage by up to 50 %, primarily affecting smaller drops. It is shown how the slowdown effect can influence erosion mitigation.


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