Round 1

Reviewer 1 Report

Review report

Aerodynamic free-flight conditions in wind-tunnel modelling through reduced-order wall inserts (Narges Tabatabaei, Ramis Örlü, Ricardo Vinuesa and Philipp Schlatter)

General comment

The paper is overall well written, quite interesting, and the scientific method seems sound.

However, I think the authors could specify a bit better the impact of the method. The objective of the paper is to assess the effect of the walls and of the boundary layer on the wind tunnel tests of an airfoil. To find out the best insert to perform a test in the wind tunnel, one needs to do a good set of preparatory simulations to estimate the limiting streamline and the growth of the boundary layer. After this is done, one needs to manufacture the physical inserts and to do the experiments.

Hence, the question that arises is: if one is indeed interested in the  characteristics, couldn’t one just use the cheaper, unconfined CFD simulations rather than performing experiments, with the consequent doubts on the accuracy of the correction introduced via the inserts? I suppose this may be justified in practice when very accurate experiments are needed, or when the experiments are used to investigate other parameters where one is interested in having the aerodynamic performance conditions as close as possible to free flight (e.g. noise, icing characteristics, etc.). However, the authors do not stress this a lot in their motivation. We suggest adding a couple of sentences to explain this better, as it would add to the impact of the paper.

Moreover, there is one part where the authors perform the simulations with the same inserts across different AOA, which indeed gives some improvement over the simple approach with parallel walls. This could be more feasible than designing inserts for every AOA. This could be a good selling point, increasing the applicability of the paper and the generalization of the results.

Suggestions for edits

There are many parts in the paper where I believe you could improve the explanation and help the reader go through the manuscript more swiftly and with less confusion. I have marked them in the PDF, I hope they can help you.

I have suggested to restructure the paper a bit, some figures are presented too late and some information could be linked better (e.g. table 2 and figure 11).

Language

The writing is overall good and clear; however, some typesetting errors (missing spaces) and some verb-subject accordance are to be corrected. You use the articles in a bit incorrect way, to my understanding. Consider using a grammar checker software to avoid these mistakes.

 

Best regards,

the Reviewer

Comments for author File: Comments.pdf

Author Response

The authors would like to thank the Reviewer for her/his comments and thorough examination of our work, and the relevant questions to be addressed. The Reviewer's comments are reported below in black along with our responses in blue text. The changes made in the revised manuscript are highlighted in the revised version of the manuscript. Additional changes were made in response to the other reviewers and are also highlighted in the revised manuscript. Please see the attachment for the point-by-point response. 

Author Response File: Author Response.pdf

Reviewer 2 Report

In my opinion, it is a very good quality paper. I recommend it for publication in its current form.

Author Response

We thank the reviewer for his/her recommendation.

 

Reviewer 3 Report

The manuscript deals with a proposed methodology for correcting aerodynamic results that are typically obtained experimentally through wind tunnel tests. This methodology was developed using numerical simulations based on CFD. I consider that the manuscript can be considered for publication if the authors correctly address the following questions and comments:

• Section 2.2 and figure 2. Please include differences for drag coefficients in drag counts, please also comment in this section the expected experimental values explicitly.
• For which AoA was the convergence analysis done?
• Please perform a Richardson extrapolation on the convergence results.
• Please give the most important details of the configuration of OpenFOAM, for example: which solver was used? Residuals? Discretization order?
• Please give the details of the boundary conditions related to the turbulence model.
• Was the model 2D for the free-flight conditions?
• In the free-flight conditions simulations, it seems like a computational domain with 50 chord diameter is not a as large as needed. Typical values are in the order of hundreds of chords https://turbmodels.larc.nasa.gov/airfoilwakeverif500c.html. This is very important since this simulation give the basic results to which the other simulations are compared to. Please perform a sensitivity analysis for the size of the domain.
• Section 3.1: I assume that the LES simulation was at least 2.5D? please give details on how those results were compared to 2D simulations.
• Please include a comparison of the friction coefficient for the unconfined flow case.
• Was the LES simulation also tripped? If not, please explain how this could be compared to tripped results.
• I do not understand why the AoA used in section 3.2 is different from the one used for the convergence analysis. Please explain.
• How were the different meshes for different AoA generated? Give details.
• In section 3.2 please also include a comparison of forces and pitching moment with respect to experimental data.
• Please include a new subsection 4.3 in which you compare Cm.

Author Response

The authors would like to thank the Reviewer for her/his comments and thorough examination of our work, and the relevant questions to be addressed. In the attached file, the Reviewer's comments are reported  in black along with our responses in blue text. The changes made in the revised manuscript are highlighted in the revised version of the manuscript. Additional changes were made in response to the other reviewers and are also highlighted in the revised manuscript. Please see the attachment on which you may find the list of major changes as well. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear Authors

thanks for the thorough replies to my comments. I am satisfied with the quality of your revision.

Best regards,

the Reviewer

Reviewer 3 Report

The authors have correctly addressed all my comments and questions, so I consider that the manuscript can be published.