Numerical Investigation and Optimization of a Morphing Airfoil Designed for Lower Reynolds Number

Round 1

Reviewer 1 Report

The paper describes the activity carried out to design and manufacturing a morphing profile based on a generalized fishbac concept. The introduction reports some of the relevant papers covering this topic, even if the most recent applications of leading edge and trailing edge morphing controls developed inside Cleansky and Cleanaviation projects are missed.

The paper proposed a numerical approach obtained by combining a genetic algorithm with the well known XFOIL code to optimize the shape of the airfoil, by changing the length of the ribs normal to the camber line. This approach is not really new (see for example the works from R.M.Botez). Finally, a CFD validation of the proposed optimal shapes is also reported showing a reasonable correlation. The paper is well written and easily readable. One of the weak points is that to fully investigate morphing concepts it is fundamental to include the structural response. Indeed, any morphing solution require to fight against the structure, to be deformed, especially the skin. In the last part of the paper the authors introduce a simplified structural model, obtained by 3D printing, that unfortunately does not include any skin. In this way, the structural model appears useless since the actual structural deformation is not fully captured. Without a realistic structural model, any morphing solution simply became an aerodynamic optimization exercise.

The authors are requested to discuss these aspects.

Author Response

The paper describes the activity carried out to design and manufacturing a morphing profile based on a generalized fishbac concept. The introduction reports some of the relevant papers covering this topic, even if the most recent applications of leading edge and trailing edge morphing controls developed inside Cleansky and Cleanaviation projects are missed.

Reply: The authors are grateful for the comment. Additional references have been included.

The paper proposed a numerical approach obtained by combining a genetic algorithm with the well known XFOIL code to optimize the shape of the airfoil, by changing the length of the ribs normal to the camber line. This approach is not really new (see for example the works from R.M.Botez).

Reply: True, the employed approach is based on well-proven techniques, but is employed on complete airfoil (not just trailing edge) and involves GA (and not only straightforward gradient methods). However, additional reference has been included.

Finally, a CFD validation of the proposed optimal shapes is also reported showing a reasonable correlation. The paper is well written and easily readable.

One of the weak points is that to fully investigate morphing concepts it is fundamental to include the structural response. Indeed, any morphing solution require to fight against the structure, to be deformed, especially the skin. In the last part of the paper the authors introduce a simplified structural model, obtained by 3D printing, that unfortunately does not include any skin. In this way, the structural model appears useless since the actual structural deformation is not fully captured. Without a realistic structural model, any morphing solution simply became an aerodynamic optimization exercise.

Reply: True, in the current study, only the initial structural studies have been performed, that do not include the skin. For the time being, it is assumed that the skin does not act as load-bearing element (only the camber and the ribs) and that it is made of elastic material, which is appropriate for small-scale UAVs.

The authors are requested to discuss these aspects.

Reply: Additional comments and text have been added and marked in red.

Once again the authors are grateful for the comments and suggestions that helped us to improve the quality of the manuscript.

Reviewer 2 Report

The paper presents a comprehensive study on morphing airfoils, covering various phases of product design including geometric parametrization, aerodynamic optimization, structural analysis, and manufacturing. While the content is intriguing, there are areas where the English language usage could be enhanced for better clarity and coherence. Additionally, the introduction would benefit from incorporating more recent references that align closely with the topic.

A crucial aspect that requires further elaboration is the description of the Computational Fluid Dynamics (CFD) method employed in the study. Specifically, details regarding the use of wall functions or their absence need to be clarified. Strengthening the paper would involve validating the CFD method against experimental data, if available, and comparing the results against a baseline and optimized airfoil to provide deeper insights into the impact of morphing.

Moreover, the conclusion section should encompass a comprehensive discussion of all facets of the research. By addressing these areas, the paper would significantly enhance its contribution to the field of morphing airfoils.

I have attached a PDF containing detailed comments for further improvement, which I believe will elevate the quality and impact of the paper.

Comments for author File: Comments.pdf

see above

Author Response

The paper presents a comprehensive study on morphing airfoils, covering various phases of product design including geometric parametrization, aerodynamic optimization, structural analysis, and manufacturing. While the content is intriguing, there are areas where the English language usage could be enhanced for better clarity and coherence.

Reply: English language has been checked and improved.

Additionally, the introduction would benefit from incorporating more recent references that align closely with the topic.

Reply: Additional, recent references have been added.

A crucial aspect that requires further elaboration is the description of the Computational Fluid Dynamics (CFD) method employed in the study. Specifically, details regarding the use of wall functions or their absence need to be clarified. Strengthening the paper would involve validating the CFD method against experimental data, if available, and comparing the results against a baseline and optimized airfoil to provide deeper insights into the impact of morphing.

Reply: More explanations and comments have been provided, additional references added, etc.

On the mostly used mesh (of approximately 60000 cells), the dimensionless wall distance along the airfoil was prevailingly around 1, and below 2, except in the vicinity of the leading edge where it approached 5, but only in a small portion. For that reason, the value of 5 (as the ultimate upper limit) was mentioned in the manuscript. Additionally, in ANSYS FLUENT, even for k-ω SST model, on coarser meshes wall functions are turned on, whereas on finer meshes, the flow is resolved up to the laminar sublayer (which was the case here).

However, for increased clarity, the sentence has been rewritten.

“… resulting in dimensionless wall distance dominantly below 2 along the airfoil, y⁺ < 2, which was sufficiently fine for resolving the flow adjacent to the wall surfaces.”

Since the method for airfoil definition is original, we don’t really have a baseline model that would be suitable for comparison, and currently, no experimental studies of the flow have been performed. We hope we can do more in this field in the near future.

Moreover, the conclusion section should encompass a comprehensive discussion of all facets of the research. By addressing these areas, the paper would significantly enhance its contribution to the field of morphing airfoils.

Reply: The authors are grateful for the comment. A more comprehensive discussion and conclusions have been added.

I have attached a PDF containing detailed comments for further improvement, which I believe will elevate the quality and impact of the paper.

Reply: The authors have tried to follow all of the instructions and comments from the attached PDF file.

Once again the authors are grateful for the comments and suggestions that helped us to improve the quality of the manuscript.

Reviewer 3 Report

This study introduces an innovative approach to morphing airfoil design, enabling adjustments to camber and thickness through Genetic Algorithm optimization at Reynolds numbers of 50,000 and 100,000, validated via Computational Fluid Dynamics (CFD).

However, the paper lacks clear objectives and direction, with insufficient comparison to existing data.

The structural analysis section is poorly explained and lacks coherence, while the fabricated fishbone airfoil lacks the anticipated morphing capabilities, detracting from its relevance to readers.

Overall, the manuscript suffers from unclear objectives and includes premature or inconsequential information

Author Response

This study introduces an innovative approach to morphing airfoil design, enabling adjustments to camber and thickness through Genetic Algorithm optimization at Reynolds numbers of 50,000 and 100,000, validated via Computational Fluid Dynamics (CFD).

However, the paper lacks clear objectives and direction, with insufficient comparison to existing data.

Reply: The authors are grateful for the comment. Clearer definitions of objectives and outcomes have been added.

The structural analysis section is poorly explained and lacks coherence, while the fabricated fishbone airfoil lacks the anticipated morphing capabilities, detracting from its relevance to readers.

Reply: The part on structural analysis has been improved and extended. Also some comments on the limitations of the assumed approach have been accentuated. 

Overall, the manuscript suffers from unclear objectives and includes premature or inconsequential information.

Reply: The manuscript has been revised accordingly to remedy some of the observed issues.

Once again the authors are grateful for the comments and suggestions that helped us to improve the quality of the manuscript.