Unsteady Aerodynamic Characteristics Simulations of Rotor Airfoil under Oscillating Freestream Velocity

Round 1

Reviewer 1 Report

The paper in review reports the dynamic stall characteristics of the OA209 airfoil under oscillating freestream conditions. Efforts are focused on studying the conditions that enhance or diminish the dynamic stall characteristics of the airfoil. While the paper is quiet interesting to the community, However, there are concerns (listed below) that need to be addressed before publication.

1) Line 31 - The authors have cited work by Ramesh et. al. when talking about research on theoretical methods for dynamic stall. However, that research deals with high-pitch rate motions at low Reynolds numbers without any unsteady trailing-edge separation. Have the authors looked into more recent research by Narsipur, Gopalarathnam, and Edwards ('Low-Order Modeling of Airfoils with Massively Separated Flow and Leading-Edge Vortex Shedding' & 'A time-lag approach for prediction of trailing edge separation in unsteady flow') to draw better parallels with their on-going research into dynamic stall?

2) Line 98 - The used solver (unsteady RANS) may not possess a high enough fidelity to extract such thorough conclusions for the current work. For example, with one of the study being focused on LEV initiation, timing is very important. Also, the smoothing of quantities in the spatial and temporal dimensions by the unsteady-RANS solver may be detrimental to the reliability of the work. Efforts need to be made to validate the results with high-fidelity CFD codes or experimental results.

3) Line 129 - Have the authors considered validating their OFV computations with experimental data from Granlund, Ol, and Jones ('Streamwise Oscillation of Airfoils into Reverse Flow')? While the experiments are for an airfoil undergoing freestream oscillations at a constant angle of attack, it ill provide a better picture on the validity of the CFD method as compared to Issac's theory. I suggest using the AoA = 20 degree case to validate the CFD method.

4) Line 144 - No validation case has been provided for the 3D grid.

5) Figures 7, 9, and 10 - It would be very useful to provide a plot showing the variation in freestream velocity with angle of attack. Most of the discussions being presented will be clearer to the reader.

6) Figure 8 - The x-axis on the plot needs to be shown clearly. It is difficult to figure out the location of the leading- and trailing-edge. Also, font size on the plots needs to be increased.

7) Line 187 - What surface or vortex signature is being used to determine time frame o LEV initiation? Authors are encouraged to look at work by Choudhuri, Knight, and Visbal ('Two-dimensional unsteady leading-edge separation on a pitching airfoil') and Narsipur, Hosangadi, Gopalarathnam, and Edwards ('Variation of leading-edge suction at stall for steady and unsteady airfoil motions').

8) Line 221 - 225 - The statement made by the authors here is confusing. Based on research by Ramesh et. al. and Narsipur et. al., the ability of the leading-edge to sustain attached flow decreases with increase in angle of attack and decrease in freestream velocity. By that logic, the airflow at the leading-edge will tend to separate off more easily at higher AoAs and lower Mach numbers, thereby contradiciting the statement made by the authors in Line 224 ('the airfoil separation near the leading edge of (the) airfoil is restricted').     

9) Figures 17 and 18 - The right-end of the x-axis on the plots are cut-off.

10) Are there any parallels between the suction-peak value and the LESP criterion (by Ramesh et. al.) to determine LEV initiation?

Author Response

Thank you for your cautious and patient works on this manuscript. According to the comments of the reviewers, we have modified the manuscript carefully as follows for the purpose of improvement, and all the modifications in revised manuscript are marked by red color.

Author Response File: Author Response.docx

Reviewer 2 Report

Submitted paper deals with the analysis of unsteady aerodynamic characteristics of rotor airfoil under oscillating freestream velocity. First, introductory part with the state-of-the-art description is provided. Next, the theoretical background on helikoptér rotor aerodynamice and the numerical simulation methods is provided. The methods are chosen and used appropriately. Next, the analyses with results are presented and discussion is provided. Finally, the outcomes are formulated. The very last part includes references and nomenclature.

The paper is not a scientific paper, it presents just standard engineering work. Thus, it has a character of case study (application of the existing state-of-the art methods on the specific problém). Nevertheless, considering this, there is no objection on the content of the paper. The state-of-the art is sufficiently comprehensive and descriptive with a  sufficient number of references. Means and methods are chosen appropriatelly. Results are presented correctly. 

To conclude, the paper is acceptable as a case study, not as a scientific paper.

Author Response

Thank you for your cautious and patient works on this manuscript. According to the comments of the reviewers, we have modified the manuscript carefully as follows for the purpose of improvement, and all the modifications in revised manuscript are marked by red color.

Author Response File: Author Response.docx

Reviewer 3 Report

journal but before publication authors may ask for revision in view of the following comments:

1. In handling the complex equations laid out, the authors argue for a numerical treatment and opt for the method utilized. No reference is given, no discussion is made of the possible limitations, so non-specialist readers would be excluded at this point.
2. Solution convergence study needs to be performed. Mesh sensitivity analysis and error comparison should be performed.
3. For fortifying the introduction section with the new publications, add more relevant works regarding the unsteady analysis of flow around air/hydrofoils (see [R1]).
4. How the data is transferred from background mesh to airfoil mesh and vise-versa.
5. Some figures are not well-plotted. For instance, in figures 6 the end right portion is missing.
6. What is the reason for the choice of the RANS method? As shown in [R2], RANS is not able to predict all flow features as in the DNS flow.
7. More relevant physical discussions need to be added to the text.
8. A nomenclature will help the readability of the text.

References:
[R1]https://www.emerald.com/insight/content/doi/10.1108/HFF-05-2019-0447/full/html

[R2]https://www.sciencedirect.com/science/article/abs/pii/S0960148116311326

 

Author Response

Thank you for your cautious and patient works on this manuscript. According to the comments of the reviewers, we have modified the manuscript carefully as follows for the purpose of improvement, and all the modifications in revised manuscript are marked by red color.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

All technical queries answered. Please proof-read paper for minor spelling and grammatical errors.

Reviewer 3 Report

Authors sufficiently answered my queries.