Characteristics and Effects of Laminar Separation Bubbles on NREL S809 Airfoil Using the γ - R e θ Transition Model

Round 1

Reviewer 1 Report

Recomendation: Major Revision

Comments to the Authors:

Manuscript ID: applsci-912418

Title: Characteristics and effects of laminar separation bubbles on NREL S809 airfoil using the γ­Reθ transition model

Authors: Jang-Oh Mo, Beom-Seok Rho

Overview and general recommendation:

This paper addresses the aerodynamic characteristics of wind turbine airfoil, NREL S809, to maximize wind turbine power and efficiency. The article is interesting, and despite addressing a topic that is not new, it presents new insights, considering the characteristics and the effects of laminar separation bubbles on NREL S809 airfoil for a Reynolds number of 2x10^-6. To analyze the flow structure of laminar separation bubbles, the authors performed numerical simulations with commercial CFD software and for the simulation was used two different conditions: a standard k-e turbulence model and a γ­Reθ transition model coupled with k-w SST turbulence model. The predictions were compared with experimental data and numerical simulations from other authors. The paper is well organized, and the analysis of the results well supports conclusions.

The English language and style paper should be checked. The description of γ­Reθ transition model is too generic and fails to give appropriate insight into the transition model, should include all the relevant equations and values of constants should be added. Equation 1 is wrong and should be corrected.

In lines 286-289 the authors mention: “The steady state computation was performed for approximately 20,000 iterations where the convergence tolerance for all relevant variables was less than 0.001%. Note that the results were taken using the average of the last 2,000 data acquired at each iteration step.”, what means relevant variables? Why was used an average of the last two thousand 2000 iterations for the results if it was considered a steady state condition, there is an oscillation problem with the simulations?

The mesh independence study is mention, and according to table 1 was performed a γ­Reθ transition model coupled with k-w SST turbulence model. For a standard k-e turbulence model has performed a grid independency study? Or the authors consider the same grid for the two turbulence models (standard k-e and γ­Reθ transition models)?

As mention by the authors, the maximum Cl/Cd ratio at a designed AoA is an important parameter in accurately predicting and maximizing the power of the wind turbine blade. The authors compare the predictions of the transition and standard k-ε models with experimental data, however, they do not mention any reference to the result obtained with the Mixed Laminar/Turbulent model. The Cl/Cd value obtained by using the Mixed Laminar/Turbulent (Tables 2 and 3) is 109 at the AoA of 5.13˚, which corresponds to an overprediction 0.3% when compared with to the experimental value, while the transition model results showed underprediction of 4.2%. For the authors, the results obtained with the transitional model are due to with k-w SST turbulence model or a γ­Reθ transition model coupled with k-w SST turbulence model?  The authors should see the manuscript:

• Haci Sogukpinar, and Ismail Bozkurt, “Numerical calculation of aerodynamics wind turbine blade S809 airfoil and comparison of theoretical calculations with experimental measurements and confirming with NREL data”, AIP Conference Proceedings 1935, 020004 (2018); doi: 10.1063/1.5025958 (published by the American Institute of Physics)

 

Minor points

Abstract:

- l15, "fS809" should be replaced by "S809";

- l17-19, the next sentence should be rewritten, the contribution of this work to knowledge should be clear"The simulation results ... wind tunnel.";

- l25-28, the following sentence is repeated: "Futhermore, ... with increasing AoAs."

Introduction:

- l56, “is required” should be replaced by “are required”;

- l90, “modelling” should be replaced by “modeling”.

Numerical Simulation:

- l223 , “modelled” should be replaced by “modeled”;

- l238 , “Figure 2” should be replaced by “in Figure 2”.

Results and Discussion

- l389 , “whereas overpredictions between…” should be replaced by “whereas lift coefficient  overpredictions between...”;

- l422-423 , “the blue solid lines,… the red solid lines.” should be replaced by “he blue lines,… the red lines.”;

- l512-518, the next sentence should be rewritten"The results ... calculations [7].".

For these motives, I cannot accept the proposed manuscript for publication. However, the paper is worth to be published but only after implementing the reviews addressing the comments/questions detailed before.

Author Response

Dear Reviewers:

The manuscript has been rechecked and appropriate changes have been made in accordance with the reviewers’ suggestions. The responses to their comments have been prepared and attached herewith/given below.

We thank you and the reviewers for your thoughtful suggestions and insights, which have enriched the manuscript and produced a better and more balanced account of the research. We hope that the revised manuscript is now suitable for publication in your journal.

Thank you for your consideration. I look forward to hearing from you.

Sincerely,

Jang-Oh, Mo

Author Response File: Author Response.docx

Reviewer 2 Report

The authors carrier out a very interesting work on the effect of laminar- turbulent transition on the aerodynamic and flow characteristics of tool S809 by means of the solution of the Ranse equazione coupled with a transition based turbulence model. The comparison with a fully turbulent model and the esperiments highlighted the fact that transition model, with better results, la a reliable choice to prodict the behavior of this kind of foil and can be used to develop regression models for the enhancement of low order models like BEM. The paper is developed with care and It can be published on the journal. I have few observation to submit to the authors of minor importance. Feel free to answer. 

1) is the grid sensitivity performed with the transition based model? Why have you considered only 0 deg? Grid sensitivity can chance at other angles of attacks, Since the flow conditions are different.

2) what is sensitivity of the transition based model with the grid refinement in terms of laminar separation bubble length and position, and ultimately loads? This point is important in the case esperiments are non provided, however the cfd analyst should be confident on the size of the grid that should be used to develop reliable corrections for low order models. Include a sentence on this aspect in the paper, while you can investigare this aspect in more detail in a future study.  

3) why did you not considered angle of incidence between 6 and 8 deg? According to figure 9 in the paper this range is very interesting, because the behavior of the laminar separation bubble changes. This point should be interesting to stress the potential capability of the transition model to capture this behavior.

Author Response

Dear Reviewers:

The manuscript has been rechecked and appropriate changes have been made in accordance with the reviewers’ suggestions. The responses to their comments have been prepared and attached herewith/given below.

We thank you and the reviewers for your thoughtful suggestions and insights, which have enriched the manuscript and produced a better and more balanced account of the research. We hope that the revised manuscript is now suitable for publication in your journal.

Thank you for your consideration. I look forward to hearing from you.

Sincerely,

Jang-Oh, Mo

Author Response File: Author Response.docx

Reviewer 3 Report

See the attach.

Comments for author File: Comments.pdf

Author Response

Dear Reviewers:

The manuscript has been rechecked and appropriate changes have been made in accordance with the reviewers’ suggestions. The responses to their comments have been prepared and attached herewith/given below.

We thank you and the reviewers for your thoughtful suggestions and insights, which have enriched the manuscript and produced a better and more balanced account of the research. We hope that the revised manuscript is now suitable for publication in your journal.

Thank you for your consideration. I look forward to hearing from you.

Sincerely,

Jang-Oh, Mo

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The paper should be considered for publication in Applied Sciences journal.

Author Response

Thanks for the comments.

Reviewer 3 Report

Several of the questionable points invoked in the first review were solved. However, the most important ones, remained neither changed, nor motivated. In the reviewer opinion, an extra revision is needed.

Comments for author File: Comments.pdf

Author Response

The responses to the comments has been prepared and attached given below. 

 

Author Response File: Author Response.docx