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Article
Peer-Review Record

Assessment of Oscillating Wings to Deliver Air Mass Flow Under Power and Thrust Constraints

Aerospace 2025, 12(8), 740; https://doi.org/10.3390/aerospace12080740
by Emin Burak Ozyilmaz and Mustafa Kaya *
Reviewer 1:
Reviewer 2: Anonymous
Aerospace 2025, 12(8), 740; https://doi.org/10.3390/aerospace12080740
Submission received: 27 June 2025 / Revised: 6 August 2025 / Accepted: 19 August 2025 / Published: 20 August 2025
(This article belongs to the Special Issue Recent Advances in Applied Aerodynamics (2nd Edition))

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this work, an oscillating wing was employed to deliver air mass flow, and the Support Vector Regression algorithm had been implemented as a machine learning tool to link the oscillation parameters to power and thrust values. The simulation results showed that an oscillating wing is compatible with an axial fan in terms of power requirement and thrust generation. This study is interesting, however, some comments are:

  • In Fig.5, there are significant differences between numerical calculations and experimental test results at some azimuth angles, and the reasons need to be given.
  • What is the basis for selecting the Spalart Allmaras turbulence model? The selection of turbulence model needs to be compared with experimental test results.
  • The degree of influence of different oscillating parameters on propulsion efficiency needs to be analyzed.
  • In Fig.9 requires further analysis to provide the characteristics of the oscillating wing flow field with better propulsion performance.

Author Response

Dear Reviewer,

We thank you for your valuable comments and suggestions. We do believe that our manuscript was enriched after the revision. Please note that our improvements and modifications according to your comments and suggestions were highlighted in green in the revised manuscript. The common comments of the reviewers were highlighted in blue.

Comments 1: In Fig.5, there are significant differences between numerical calculations and experimental test results at some azimuth angles, and the reasons need to be given.

Response 1: In flapping wings, there is a widely used parameter to understand the aerodynamic characteristics: effective angle of attack. In our original manuscript, we did not mention about it. Now, in the revized version, we added a description about it. Briefly, massive flow separations are observed at high effective angle of attack values while low effective angle of attack causes weak flow separations. The motion considered in Figure 5 is a high-frequency oscillation case with also high instantaneous values of effective angle of attack. Therefore, we think that the reason for the difference might be this. By the way, since one also considers the average values over an oscillation cycle, we can say that such differences do not have significant effect on the overall average value. Our these explanations were added in the manuscript in Page 4, Line 138-141 and in Page 8, Line 252-260.

 

Comments 2: What is the basis for selecting the Spalart Allmaras turbulence model? The selection of turbulence model needs to be compared with experimental test results.

Response 2: The Spalart-Allmaras turbulence model is widely used to compute the turbulent solutions for the flows around the oscillating wings. We added 8 references and cited them about the selection of this turbulence model. Briefly, the references state that the accuracy of the Spalart-Allmaras model has been validated against the experimental studies on oscillating wings. Our this explanation was added in the manuscript in Page 4, Line 159-161.

 

Comments 3: The degree of influence of different oscillating parameters on propulsion efficiency needs to be analyzed.

Response 3: According to your suggestion, we analyzed the influence degree of each oscillation variable on the power requirement and the thrust generation. For this purpose, the sensitivity of power and thrust has been investigated based on their correlation with the oscillation variables using a Monte Carlo Simulation. We added a table which gives these correlation values. We also added some comments about the results. Our these additions were in the manuscript in Page 13, Line 380 and Page 13, Line 387-395.

 

Comments 4: In Fig.9 requires further analysis to provide the characteristics of the oscillating wing flow field with better propulsion performance.

Response 4: Since we added a new figure, the figure number for Fig.9 is now Fig.10 in the revised manuscript. We also added Fig.11 as an improvement according to your this comment. Figure 10 gives the flowfields for the optimum oscillation case providing the maximum thrust. Figure 11 that we added shows the minimum power case. Therefore, we are now able to easily compare the maximum-thrust flow characteristics to the minimum-power flow characteristics. Briefly,  we observed that the generated thrust and the required power is directly proportional to the massiveness of the vortex shedding. Our these explanations were added in the manuscript in Pages 14-15(Figures 10 and 11) and in Page 16, Line 429-439.

Best Regards,

M. Kaya and E.B. Ozyilmaz

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, a machine learning approach is used to optimize the parameters of an oscillating wing for a given air mass flow rate under power and thrust constraints, and it is found that the optimized oscillating airfoil can provide similar power requirements and thrust as a conventional axial flow fan, even though their flow characteristics and mechanisms are completely different. This provides a new perspective on the design of axial fans. This paper is of interest and can be published with the following comments modified.

 

  1. Page 1, line 31.Kaya and Elfarra  should add the reference.
  2. Page 1, line 34-35.Why is this sentence in a individual paragraph?
  3. Page 2, line 72.“In their parametric study, Tuncer and Platzer[22]” seems weird, please rewrite this sentence.
  4. Page 3, line 130-131.The description of test model should be put in the methodology.
  5. Page 1~3.The introduction section should be improved because of the unclear structure and logic. Furthermore, it is not clearly organised.
  6. Page 4, section2.2, More about the flow solver used in this work should be detailed, including the governing equation, spatial and temporal scheme, turbulent model ......, as well as what kind of method used for simulating the movement of oscillating wings.
  7. Page 5, section 3. It is not appropriate to use only two meshes when validating a CFD tool. Generally at least three different mesh systems are needed,
  8. Page 5, lines 204-207. The author only presents the pressure coefficient values for the upper surface of the wing. I would like to know why the upper and lower pressure distributions are not listed at the same time? Also, what are the lift and drag coefficients for different grid systems? Please give specific comparison results.
  9. Page 8, line 252. Use an axial fan for comparison. I understand that the B2 fan uses a different blade airfoil compared to the oscillating wings. I am wondering if it makes sense to compare two configurations with different airfoil geometries?
  10. Page 9, lines 268-271. The authors chose a mass flow rate of 15.4 m3/s as a constraint, a required power of 5000-6000 W and a generated thrust of 340 N as an optimization objective. How did you select these parameters? Why did you choose these values?
  11. Page 9, line 297. Please provide all parameter combinations for the 41 CFD analyses, not just a few of them. You can put them in an appendix.
  12. Page 10, line 317. What does parallel processing mean? What kind of technology were you using for the parallel processing?
  13. Page 12, lines 359-368, Figure 9. Please analyze this figure more. It is desirable to give the corresponding flow field of the axial fan for comparison purposes.
  14. Page 14, section 6, Discussion. I would suggest that the author change “Discussion” to “Conclusion”. Because you're summarizing the whole paper, not just the discussion.

Author Response

Dear Reviewer,

We thank you for your valuable comments and suggestions. We do believe that our manuscript was enriched after the revision. Please note that our improvements and modifications according to your comments and suggestions were highlighted in yellow in the revised manuscript. The common comments of the reviewers were highlighted in blue.

Comments 1: Page 1, line 31.Kaya and Elfarra should add the reference.

Response 1: We corrected this mistake that went unnoticed. Thank you for noticing. The new location in the manuscript is in Page 2, Line 71.

 

Comments 2: Page 1, line 34-35.Why is this sentence in a individual paragraph?

Response 2: We corrected this mistake that went unnoticed. Thank you for noticing. The new location in the manuscript is in Page 2, Line 73-74.

 

Comments 3: Page 2, line 72.“In their parametric study, Tuncer and Platzer[22]” seems weird, please rewrite this sentence.

Response 3: We rephrased this sentence with a better English. The new location in the manuscript is in Page 1-2, Line 41-48.

 

Comments 4: Page 3, line 130-131.The description of test model should be put in the methodology.

Response 4: We moved and enhanced the description of the test model in the relevant methodology section. We also added a figure which illustrates the oscillation motion of the wing (Figure 1). Our these modifications were added in the manuscript in Page 3, Line 119-120 and 129-132.

 

Comments 5: Page 1~3.The introduction section should be improved because of the unclear structure and logic. Furthermore, it is not clearly organised.

Response 5: Almost all the Introduction section were rewritten and re-organised. Some sentences were rephrased and some irrelevant statements were removed. Our these modifications start from Page 1 Line 21 to Page 3 Line 116.

 

Comments 6: Page 4, section2.2, More about the flow solver used in this work should be detailed, including the governing equation, spatial and temporal scheme, turbulent model ......, as well as what kind of method used for simulating the movement of oscillating wings.

Response 6: The flow solver were detailed in the revised manuscript. The governing equations and the numerical schemes to solve it were given now, including the method for the oscillation motion. Moreover, we added 8 references and cited them about the selection of the Spalart-Allmaras turbulence model. Our these additions and explanations are in the manuscript in Page 4, Line 144-161.

 

Comments 7: Page 5, section 3. It is not appropriate to use only two meshes when validating a CFD tool. Generally at least three different mesh systems are needed

Response 7: We added the performance of another (coarser) mesh into the grid independency study. The corresponding figure (Figure 4) were replotted. Our these additions are in the manuscript in Page 6, Line 219, Line 223-229 and Page 7, Line 236.

 

Comments 8: Page 5, lines 204-207. The author only presents the pressure coefficient values for the upper surface of the wing. I would like to know why the upper and lower pressure distributions are not listed at the same time? Also, what are the lift and drag coefficients for different grid systems? Please give specific comparison results.

Response 8: The experimental work by Gregory and O’Reilly is commonly used for the validation of CFD solutions. It has been cited almost 500 times in the last 55 years. Unfortunately, they presented the pressure coefficient distribution only on the upper surface of the NACA0012 wing under an angle of attack of 15 degrees. However, in their paper, they published the lift coefficient for the whole wing. Therefore, we added a comparison with their measurement of lift coefficient. We observed that our computed CL value deviates by almost 1% from the experiment. Our these explanations were added in the manuscript in Page 6, Line 221-230.

 

Comments 9: Page 8, line 252. Use an axial fan for comparison. I understand that the B2 fan uses a different blade airfoil compared to the oscillating wings. I am wondering if it makes sense to compare two configurations with different airfoil geometries?

Response 9: In the literature, there is a work on studying the effect of airfoil geometry on the oscillation performance by Ashraf, M.A.; Young, J.; Lai, J.C.S. (Reynolds number, thickness and camber effects on flapping airfoil propulsion. J. Fluids Struct. 2011, 27, 145–160). We cited their work by stating that the thickness and the camber characteristics do not have significant effects on the aerodynamic loads of an oscillating wing. Our this explanation was added in the manuscript in Page 3, Line 120-122.

 

Comments 10: Page 9, lines 268-271. The authors chose a mass flow rate of 15.4 m3/s as a constraint, a required power of 5000-6000 W and a generated thrust of 340 N as an optimization objective. How did you select these parameters? Why did you choose these values?

Response 10: Since we investigate the ability of an oscillating wing to deliver air mass flow and its limit of this ability, we wanted to select an axial fan case which requires high power while providing a moderate pressure rise. Our this explanation was added in the manuscript in Page 9, Line 288-289.

 

Comments 11: Page 9, line 297. Please provide all parameter combinations for the 41 CFD analyses, not just a few of them. You can put them in an appendix.

Response 11: According to your suggestion, we added the full list of the DoE in the Appendix A (Page 18) while maintaining the incomplete list in the manuscript. 

 

Comments 12: Page 10, line 317. What does parallel processing mean? What kind of technology were you using for the parallel processing?

Response 12: The CFD solver that we used allows to partition the flow domain (mesh) into subdomains and to solve each subdomain in a different computer or computer processor or processor core. We added this explanation which describes "parallel processing" in the revised manuscript in Page 12, Line 351-354.

 

Comments 13: Page 12, lines 359-368, Figure 9. Please analyze this figure more. It is desirable to give the corresponding flow field of the axial fan for comparison purposes.

Response 13:  Before replying your comment, please, let us remind that figure number shifted by 1. Therefore, Figure 9 is now Figure 10. Since the mechanism (rotating fan vs. plunging and pitching wing) is totally different which causes the flowfield through the fan fundamentally differs from the flowfield around the wing, we observed that it is very difficult to compare them. However, your suggestion and the other reviewer's comment gave us an idea. We added the flowfield of a low-power and low-thrust case (Figure 11) and compared to the already given flowfield (Figure 10) in the original manuscript. Briefly,  we observed that the generated thrust and the required power is directly proportional to the massiveness of the vortex shedding. Our these explanations were added in the manuscript in Pages 14-15(Figures 10 and 11) and in Page 16, Line 429-439.

 

Comments 14: Page 14, section 6, Discussion. I would suggest that the author change “Discussion” to “Conclusion”. Because you're summarizing the whole paper, not just the discussion.

Response 14: According to your suggestion, we changed the "Discussion" title to "Conclusion". Thank you for this attentive recommendation.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The author has made significant revisions to the paper and recommends its publication

Reviewer 2 Report

Comments and Suggestions for Authors

The authors addressed all my comments well. I think this paper could be published in its current form.

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